10 Electricity Labs

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7 Οκτ 2013 (πριν από 4 χρόνια και 7 μήνες)

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1

CH 215 EXPERIMENT # 1

Ohm's Law

Purpose:

To study the relationship between electrical current, voltage, and resistance,
known as Ohm's Law.

Introduction:

See Technical Physics, 3rd Ed., Ch 21.2

Materials:

-

dc circuit board

-

D dry
-
cell batteries

(1.5 V)

3

-

-

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-

-

-

-

-

-

㈠敡捨

Schematic:

Procedure:

1. Construct the circuit as illustrated in the schematic (circuit diagram).

2. Connect the voltmeter in parallel across the resistor. (Zero the voltmeter before use).

3. Conn
ect the milliammeter in series to complete the circuit between the resistor and
point "a", so that all three batteries are included in the circuit.

NOTE:

Set or connect the meters so that when the switch is closed the meter needle will
not deflect off sca
le, otherwise the meter may be damaged. A safe practice is to initially
set the meter on its highest scale and then change the setting to more sensitive scales to
obtain the highest scale deflection without going off scale. If you are in doubt, check with

your instructor before closing the switch.

ALSO NOTE
: Make sure the polarities of the meters are as shown by the + &
-

signs on
the circuit diagram. Follow the convention for conductor colours, i.e., black is for
electron flow into a component (black is

live) and red is for electron flow out of a
component (red is the return conductor). If a meter is connected backwards, the needle
will deflect off scale below zero !

4. While holding the switch closed, read and record V (volts) and I (milliamps).

2

5. Move

one end of conductor # 9 from postion "a" to position "b", so that only 2 batteries
are now included in the circuit, and repeat Step 4.

6. Move one end of conductor # 9 from postion "b" to position "c", so that only 1 battery
is now included in the circui
t, and repeat Step 4.

7. Repeat Steps 4 to 6 for the other 2 resistors, each time recording corresponding pairs of
values for V and I . Record all values in a table.

8. For each pair of values of Vand I, calculate R in ohms (i.e., ratio
) and record it in
Note:

Convert I from milliamps to amps before calculating R.

9. Obtain a colour code for resistors from your instructor and determine (and record in
your report) the manufacturer's stated resistance value for each resistor. C
alculate and
report the difference between the manufacturer's resistance value and your calculated
average

resistance for each of the 3 resistors. Assume the colour code value is correct.

10. On the same sheet of graph paper, plot graphs of V versus I for
each resistor (You
should obtain 3 separate lines
-
one for each resistor). Plot I on the horizontal axis and
label each line.

11. Calculate the slope of each line (

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Questions:

1. State in words the exact meaning of Ohm's Law (refer to the equation for Ohm's Law).

2. Did your resistors exhibit "ohmic resistance"? Explain why or why not.

3. How do voltage and current vary with respect to each other for a given resist
ance?

4. Calculate the power (in watts) dissipated (used) by each resistor at the 3 different
voltages. (9 calculations in all). Use the formulas: P = V

Refer to course outline and follow, as closely as possible, the format
described for lab reports.

Reminders.... as per the course outline:

-

Late reports will be docked 2 marks per day.

-

You must attend and complete a lab in order t
o submit a lab report.

-

On the first page of your report, include along with your own name, the name(s) of

-

However, each student must submit his or her own lab report
.

3

CH 215 EXPERIMENT # 2

Series & Parallel Circuits

Purpose:

T
o study the relationship between electrical current, voltage, resistance, and
power in series and parallel circuits.

Introduction:

See Technical Physics, 3rd Ed., Ch 22.1

Materials:

-

dc circuit board

-

D dry
-
cell battery (1.5 V)

3

-

switch

-

3 different ½ watt resistors

-

dc multi
-
range voltmeter

-

dc multi
-
range milliammeter

-

alligator clips (4 straight & 6 right angle)

-

black and red insulated conductors with banana jacks

㈠敡捨

-

㜠獴牡楧桴⁳灲楮朠捯湤7

Part A:
Schematic for a Series Circuit

Procedure:

1. Construct the circuit as illustrated in the series circuit schematic.

2. Connect the voltmeter in parallel across each resistor a
nd across the battery and read
the voltage drop in each case. Be certain to close the switch while reading the voltage
drop across the battery.

3. Connect the milliammeter in series to complete the circuit and read the current.

NOTE:

Set or connect the me
ters so that when the switch is closed the meter needle will
not deflect off scale, otherwise the meter may be damaged. A safe practice is to initially
set the meter on its highest scale and then change the setting to more sensitive scales to
obtain the hi
ghest scale deflection without going off scale. If you are in doubt, check with
your instructor before closing the switch.

ALSO NOTE
: Make sure the polarities of the meters are as shown by the + &
-

signs on
the circuit diagram. Follow the convention for

conductor colours, i.e., black is for
electron flow into a component (black is live) and red is for electron flow out of a
component (red is the return conductor). If a meter is connected backwards, the needle
will deflect off scale below zero !

4

4. Using
1
, R
2
, R
3
, and R
S
.
Also calculate Rs using the formula: R
S

= R
1

+ R
2

+ R
3
.

Report the % difference between the 2 values of Rs calculated.

5. Using the resistor colour codes, determine the manufactur
er's resistance value of each
resistor and, assuming these values to be correct, calculate the % error of each of your
calculated resistor values.

6. Using the power formula: P = V

I, calculate the power dissipated (in watts) by each
resistor and the sum o
f these 3 values (i.e., the total power loss across the circuit).

Also calculate the total power loss across the circuit using :

P = Vs

Is.

How do these compare?

Part B:
Schematic for a Parallel Circuit

1. Measure the

voltage drop and current across R
1
, R
2
, R
3
, and across the battery (whole
circuit). Be sure the switch is closed when reading the voltage drop across the battery.

2. Using the Ohm's Law formula and your meter readings, calculate R
1
, R
2
, R
3
, and R
P.

3. Als
o calculate R
P

using the formula:
. Compare the 2 values for R
P
.

4. Using the resistor colour codes, determine the manufacturer's resistance value of each
resistor and, assuming these values to be correct, calculate the % error of each
of your
calculated resistor values.

5. Using the power formula: P = V

I, calculate the power dissipated (in watts) by each
resistor and the sum of these 3 values (i.e., the total power loss across the circuit).

Also calculate the total power loss across th
e circuit using :

P = Vp

Ip.

How do these compare?

Questions:

1. Write a formula for power loss in series & parallel circuits in terms of P
1
, P
2
, & P
3

2. Discuss the sources of error in your experiment & in which way they affect results.

5

CH 215 EX
PERIMENT # 3

WHEATSTONE BRIDGE

Purpose:

To construct a Wheatstone bridge circuit and use it to determine the resistance
of unknown resistors.

To compare the calculated resistances against values obtained using an
industrial Wheatstone bridge.

Introduction:

See Technical Physics, 3rd Ed., Ch 22.4 and course notes.

Materials:

-

dc circuit board

-

D dry
-
cell battery (1.5 V)

-

switch & galvanometer

-

24

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Ⱐ☠㈰〠

r

-

m整敲⁳瑩捫…⁴慰e

†††††
-

㜮㔠

Ⱐ㈰

Ⱐ☠㄰〠

-

-

㌠敡捨

Schematic

for a Wheatstone bridge circuit

Procedure:

1. Cut a 1 meter length of resistance wire and tape it to a non
-
conducting (wooden) meter
stick as described by your instr
uctor.

2. Construct the circuit as illustrated in the Wheatstone bridge circuit schematic.

3. Note that the 200

㐮⁉湩瑩慬汹Ⱐ畳攠愠㈴,

3

and 1 of the 3 unknown resistors as R
x
.

5. Adjust the position B of the red lead along the length of the resist
ance wire while
simultaneously closing the circuit switch and the #2 key on the galvanometer until there is
no current flow through the galvanometer (branch AB), i.e., zero deflection of the
galvanometer needle. In this position the bridge is balanced.

Note:

Do not allow the galvanometer needle to deflect beyond full scale.

6

6. Calculate the unknown resistance using the Wheatstone equation:

7. Replace R
3
with the 51

x
).

8. Repeat steps 5 to 7 for the other 2 unknown resistors.

9. For each of the 3 resistors determine the arithmetic average of your 2 calculated
resistance values. .

Report all resistance values to 1 decimal place
.

Use the resistor
colour c
odes to check that your measurements and calculations are correct.

10. Obtain the true resistance of each of the unknown resistors by measuring them on the
industrial Wheatsone bridge and report the % difference between these true values and
each average
resistance as determined using the bridge circuit which you made.

11. Using the industrial Wheatsone bridge determine the resistance in ohms (to 3 sig figs)
of 1 additional unknown resistors as provided by the instructor and include these in your
lab repor
t.

Questions:

1. Calculate the values of L
1

and L
2

(in cm) that would be obtained on your Wheatsone
bridge if R
3

= 20

x

= 400

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7

CH 215 EXPERIMENT. # 4

RESISTIVITY & WIRE GAUGES

Purpose:

I. To determine the resistivity of resistance wires.

II. To become familiar with common domestic/industrial electrical

wires.

Introduction:

See Technical Physics, 3rd Ed., Ch 21.3.

Materials:

-

dc circuit board

-

D dry
-
cell battery (1.5 V)

-

switch

-

200

-

m整敲⁳瑩捫…⁴慰e

-

-

-

-

0
-
㈵⁭m⁭楣牯i整敲

-

-

-

㌠敡捨

Schematic

for a resistivity circuit

Procedure for Part I:

1. Cut a 1 meter length of
fine

resistance wire and tape it to a non
-
conducting (wooden)
meter stick as described by your instructor.

2. Construct the circuit as shown in the schematic with the voltmeter conn
ected in parallel
and the milliammeter connected in series (with correct polarity).

3. Note that the 200

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-
m⸠

8

9. Also
measure the current, voltage, length and diameter of 2 other
larger

resistance
wires as provided by your instructor and calculate their resistivity (in

-
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≷楲攠条畧敳∩⸠

Procedure for Part II:

1. With the aid of th
e micrometer, measure, to three decimal places, the diameter of the
following conductors:

a) # 22 AWG wire (solid, single conductor, orange sheathed)

b) # 14/2 AWG cable (solid copper, 2 conductor + ground, white sheathed)

c) # 12/2 AWG cable (solid copper
, 2 conductor + ground, red sheathed)

d) # 10/3 AWG cable (solid copper, 3 conductor + ground, white sheathed)

e) # 8/3 AWG cable ( stranded copper, 3 conductor + ground, white sheathed)

2. Construct a table, and report the following data for the conducto
rs measured in Step 1:

AWG #, measured diameter in mm, calculated diameter in mils, Brown & Sharpe

(B & S) diameter in mm (see text and/or CRC Handbook), difference and % difference of
the measured diameter from the B & S diameter.

3. Table I lists the
AWG # of 4 cables and their typical domestic applications. Copy this
table into your report and include the allowable ampacities (I) from the Ontario Hydro
electrical safety code (column 4 for NMD 7 cable). Calculate the maximum power, in
watts, obtainable

through each conductor.

Table I
. Electrical cable applications

Application

Voltage

(V)

AWG

#

allowable

ampacity

(A)

Maximum

Power

(W)

light and plug circuits

110

14

kitchen and utility room circuits

110

12

electric dryer circuit

220

10

electric range (oven) circuit

220

8

Questions:

1. Report the American Wire Gauge (AWG) number, also called the Brown & Sharpe

(B & S) gauge number of each of the 3 resistance wire diameters measured in the lab.

2. Report the diameter in mm and mils of

each of the following AWG gauge numbers:

40, 30, 20, 10, 1, 0, & 0000.

3. If the diameter of a certain length of a wire is doubled, what happens to its electrical
resistance? (use the formula for resistivity to calculate this)

4. What is the smallest gra
duation of the micrometer? (include units)

5. If the diameter of a wire lies between 1 and 2 mm, what is the maximum number of sig
figs obtainable when measured with a 0
-
25 mm micrometer?

9

CH 215 EXP. # 5A

CHEMICAL CELLS

Purpose:

To study the zinc
-
co
pper cell.

Introduction:

See Technical Physics, 3rd Ed., Ch 21 Supplement and your Chem text.

Materials for the zinc
-
copper cell:

-

dc milliammeter

-

dc voltmeter

-

alligator clips (2 straight)

-

zinc and copper electrodes

-

150 mL beakers

2

-

-

-

㈠敡捨

-

2
O, tap H
2
O, 0.25 M ZnSO
4
, 0.25 M CuSO
4
,
1N H
2
SO
4

Schematic

for the zinc
-
copper cell:

Procedure (A):

1. Polish a zinc and a copper metal strip with emery cloth to ensure good electrical
contact with the solutions.

2. Place t
he Zn strip in a
round

beaker containing about 75 mL of 0.25 M CuSO
4
solution and place the Cu strip in a second
round

beaker containing about 75 mL of 0.25
M ZnSO
4
solution. After 3 minutes, inspect each strip for evidence of a chemical reaction
and rec

3. Remove and clean the metal strips for the next portion of this experiment. Return the
ZnSO
4
solution to the instructor but retain the CuSO
4
solution.

4. Clamp the zinc and copper strips into the electrode clamp and construct the c
ircuit as
shown in the schematic (with correct polarity), except initially, place no solution in the
rectangular

beaker and disconnect one of milliammeter conductors. Read the voltage
alone, then connect the milliammeter and read both voltage and current.

Be sure that the

10

electrodes do not make electrical contact with each other during any of the readings
in this procedure and record any change in the appearance of the electrodes after
each set of readings is taken.

5. Disconnect one of the milliammeter con
ductors, pour about 75 mL of distilled water
into the beaker, and, when the voltage stabilizes, read the voltage alone. Then reconnect
the milliammeter and read both voltage and current.

6. Rinse the electrodes with tap water before proceeding to the next
step.

7. Repeat Steps 5 & 6 using tap water and 0.25 M CuSO
4
, (in that order), in place of
distilled water.

8. For the CuSO
4
solution, note the appearance of
both

electrodes after the cell has been
connected through the milliammeter for at least 5 minutes.

9. Replace the CuSO
4

solution in the correct stock bottle when completed and carefully
clean both electrodes before proceeding to the final portion of this experiment.

10. Place about 75 mL of 1N H
2
SO
4
solution in the rectangular beaker, insert the Zn and

Cu electrodes, and connect a voltmeter. After 3 minutes read the open circuit voltage and
record any evidence of a chemical reaction at both electrodes.

11. Connect a milliammeter and after 3 more minutes, read the current and voltage and
record any evid
ence of a chemical reaction at both electrodes.

12. Complete the following table and copy it into your lab report:

CELL

OPEN CIRCUIT

CLOSED

CIRCUIT

ELECTROLYTE

Voltage (V)

Voltage (V)

Current (mA)

air

d. i. water

tap water

0.25 M CuSO
4

1 N H
2
SO
4

Questions:

1. Write half reactions for the electodes with CuSO
4
as electrolyte and identify these as anode or
cathode, oxidation or reduction, and positive or negative polarity. Include the half cell potential
(emf) for each. Also write a com
bined redox equation for the total cell reaction and report the
theoretical cell emf. See the CRC handbook (indexed under "electrode potentials") or see the

3. Explain the appearance of both electrodes after removing from t
he CuSO
4

electrolyte.

4. Explain the chemical activity (by means of a balanced chemical equation) at the Zn and Cu
electrodes in the H
2
SO
4
solution with and without the milliammeter connected.

5. Explain the difference between the voltages measured on the

open circuits and the voltages
measured on the closed circuits. (explain in terms of the equation V =

-

I

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11

CH 215 EXP. # 5B

CHEMICAL CELLS

Purpose:

-
acid secondary cell.

Introduction:

See Technical Physics, 3rd Ed., Ch 21 Supplement and your Chem text.

-
acid storage
cell:

-

dc milliammeter

-

dc voltmeter

-

dc ammeter

-

dc variable power supply

-

alligator clips (4 straight)

-

2

-

㌰⁭䰠扥慫敲L

-

-

-

㐮㔠丠4
2
SO
4

-

black and red insulated conductors with banana jacks

㌠敡捨

-

4
, & 0.25 M CuSO
4

Schematic

-
acid storage cell:

half reactions:

half reactions:

anode: 2H
2
O

O
2

+ 4H
+

+ 4e
-

anode: Pb + SO
4
-
2

4
(s) + 2e
-

cathode: 2H
+

+ 2e
-

H
2

PbO
2

+ SO
4
-
2

+ 4H
+

+ 2e
-

4
(s) + 2H
2
O

anode: PbSO
4
(s) + 2H
2
O

2

+ SO
4
-
2

+ 4H
+

+ 2e
-

cathode: PbSO
4
(s) + 2e
-

Pb + SO
4
-
2

Procedure (B):

1. Polish the lead electrodes with emery cloth to ensure good electrical contact with

the
alligator clips and with the solution and insert the electrodes into the beaker.

2. Carefully pour about 20 mL of H
2
SO
4

solution into the beaker.

12

CAUTION:

Wear safety goggles throughout this experiment. H
2
SO
4

solution is
corrosive. In the event of
contact with skin or eyes, immediately flush the area with
copious amounts of cold water and notify your instructor.

3. Connect only the voltmeter and record the cell voltage before charging.

4. Now construct the complete circuit as shown in the schematic

for
charging

(with
correct polarity). Be sure that the power supply is unplugged, turned off, and set to the 6V
maximum setting during the circuit assembly.

4. With the voltmeter set to the 10V maximum position and the ammeter set to the 3A
maximum positi
on, turn on the power supply to a setting of 3
-
4 V and allow the cell to
charge for 5 minutes. Record the charging voltage (from the voltmeter) and charging
current. Record the activity at each electrode.

Be sure that the electrodes do not make
contact wi
th each other during any part of this procedure.

5. After the 5 minutes charging, shut off the charger and immediately disconnect the leads
from the charger. Note and record the appearance of each electrode.

6. Read the cell voltage and then rearrange the
ammeter connections and install the
milliammeter as shown in the
discharging

schematic.
Initially, set the milliammeter to
the short position.

Now connect the two leads which were disconnected from the
charger. Note the behavior of the ammeter and voltmete
r as the connection is made.

7. When the ammeter reading has dropped, turn the milliammeter from the short position
to one of the scales which is appropriate for the current of the discharging cell.

8. When the cell current has dropped to 5 mA, record the
closed and open circuit voltages
as accurately as possible.

9. Again close the circuit and allow it to completely discharge. Note and record the
appearance of both electrodes after discharging.

10. Rearrange the circuit connections for charging and rechar
ge the cell again for 3
minutes. Disconnect the charger, record the cell voltage, and again note and record the
appearance of the electrodes.

11. Discharge your cell completely, dispose of the H
2
SO
4

directions, rinse and w
ipe clean both electrodes and the beaker. Ensure that your work
area is wiped clean before leaving.

Questions:

1. Write out each half cell reaction for the lead
-
acid battery (6 in all), and the standard
electrode potential for each reaction proceeding in t
he direction as written. Identify these
as anode or cathode, oxidation or reduction, charging or discharging, and positive or
negative polarity. Also write a combined redox equation for the total cell reaction for the
charging and discharging of this batt
ery and report the theoretical cell emf. Show which
direction is charging and which is discharging.

2. Compare the colour of both electrodes after charging, and after discharging to the
colour of PbO
2

and PbSO
4

listed in the CRC handbook.

3. Distinguish be
tween a primary and secondary cell.

13

4. Auto mechanics measure the specific gravity of H
2
SO
4

-
acid
batteries as an indication of the degree to which a battery is charged or discharged.
Stockel's auto service and repair manual gives the fo
llowing data:

EXTENT OF
CHARGE

s.g.

% /wt

Molarity

Normality

Fully charged

> 1.26

75 % charged

1.22

50 % charged

1.17

discharged

< 1.08

Copy this chart into your lab report and complete the missing information. This can be
obtai
ned from the CRC handbook, indexed under "density of aqueous solutions". Briefly
explain why the s.g. of this battery indicates degree of charge.

14

CH 215 EXPERIMENT. # 6

ELECTROMAGNETISM

Purpose:

To construct and study a solenoid.

Introduction:

See Technical Physics, 3rd Ed., Ch 23.2 and chapter supplement (p485).

Materials:

-

plastic drinking straw

-

6 m of AWG # 30 lacquer coated wire

-

5
-
7 cm steel nail

-

8 cm

ㄠ捭⁳瑲楰⁯⁡汵楮畭⁦潩

-

☠獣楳獯

-

-

-

-

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Schematic for Solenoid:

Procedure:

1. Cut a 4 cm piece of a plastic st
raw and
a 6 m length of motor wire.

2. Tightly coil the wire around the straw in
one direction (either clockwise or counter
clockwise) leaving about 15 cm free at
each end and tape the loose end.

3. Rub off the lacquer coating at each end
of the wire wit
h emery cloth to expose the
bare conductor and connect one end to an
ammeter and the other end to the nail.

4. Complete the connection of the circuit
as shown in the schematic by connecting
the ammeter to a dc power supply. The 8
cm strip of aluminum foil
is held by an alligator clip and is electrically connected to the
other terminal of the power supply. During this step keep the power supply turned off.

5. Set the power supply to its 6 V maximum setting, turn on the switch, and adjust the

4 V. Incline the solenoid at about a 20
-
50

潩氮

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15

Questions:

1. Explain the oscillating motion of the core.

2. Draw a sketch of your complete solenoid cir
cuitd showing the direction of
conventional current flow over the coil, polarity, and the north and south poles of your
solenoid.

3. Calculate the number of turns (N) on your coil using: N =

where C (circumf
erence) = 2

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solenoid

in meters).

5. Assuming a relative permeability (K
m
) of 8000 for the steel solenoi
d core, calculate the
magnetic field strength of your solenoid. Also calculate the magnetic field strength of
your coil with an air core. Report both values in units of Tesla's and Gauss.

Use the formula for solenoids:

6. By means

of a simple sketch and in words, explain the difference between a relay and a
solenoid. Show the direction of current flow through the coils and external circuits and
the north and south poles. List 2 applications of each.

7. Answer question 17 on p482.

(see Fig 23.23 on p483).

8. Complete all the following calculations and
copy these into your lab report in the
same way as they are shown below

Be careful. Students often confuse the wire length, co
il length, and coil diameter values
in their calculations and loose marks carelessly here.

k = 8000,

o

= 4

10
-
7
,

m

= ........................,

wire length = ................. m

coil diameter (d) = .................. cm = ........................
m

solenoid (coil) length (L) = .................... cm = ........................ m

circumference (C) =

d = ........................ m

# turns (N) =

= ........................ turns

turn ratio (N/L) = ....................
.... turns/m

o

= ........................ T = ........................ G

m

= ........................ T = ........................ G

16

CH 215 EXPERIMENT. # 7

TRANSFORMERS

Purpose:

To construct and study transformers.

Introduction:

See Technical Physics, 3rd Ed., Ch 24.5 and chapter supplement.

Part A:

Constructing and testing a step down transformer:

Materials:

-

2.2 m #22 lacquer coated wire

-

ac analog voltmeter

-

4.8 m # 26 lacquer coated wire

-

ac digital voltmeter

-

7
cm section of iron bar

-

variable ac power supply

-

emery cloth, scissors, tape measure

-

straight alligator clips

-

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Schematic for Step Down Transformer :

Procedure:

1. Cut a 2.2 m length of # 22 gauge
lacquer coated wire and tightly wrap a 2
-
3
cm section of the iron bar with about 50
close loops of the wire, wound in only one
direction (clockwise or counter
clockwise). Count and record the e
xact
number of turns used and leave about 10
cm of wire loose at each end. This is the
inner secondary winding.

2. Cut a 4.8 m length of # 26 gauge
lacquer coated wire and wrap this tightly
around the inner winding with 100 loops (wound in only one directi
cm free at each end. Count and record the exact number of turns used. This is the outer
primary winding.

3. Rub off a 1
-
2 cm section of the lacquer coating at the 4 wire ends with emery cloth to
expose the bare conductor. Connect the

ends of the larger gauge inner (secondary)
winding to the digital voltmeter and connect the ends of the smaller gauge outer (primary)
winding to the ac power supply and in parallel to the analog ac voltmeter. Polarity of
connections need not be considered

for ac operation.

Note: The power supply must be turned off and set to its minimum output setting
during hookup.

4. Before turning on the power, set the digital voltmeter to the ac 2 V maximum setting
and the analog voltmeter to the ac 2.5 V maximum sett
your settings and connections before you turn on the power supply.

5. Turn on the ac power supply and adjust the output to 0.20 V on the primary winding
and record the induced voltage of the secondary winding on the digital
voltmeter.

17

Continue adjusting primary voltages and recording secondary voltages so as to complete
the following table (Table A). Record all readings to 2 sig figs.

N1 =

N2 =

N1/N2 =

coil temperature:

V
1

(primary)

V
2

(secondary)

V
1
/V
2

(cool, warm,
or hot)

0.20

0.50

1.0

1.5

2.0

Part B:

Testing an industrial transformer.

Materials:

A commercial 115 V transformer is used in place of a student
-
transformer. The electrical connections are the same as in Part A.

Procedure:

1. C
onnect the transformer as shown in the schematic for Part A, initially with the
primary (115 V) side connected to the power supply. Set the supply voltage as shown in
the following table (Table B
-
1) and record the induced voltage in the secondary.

Nominal
V1 = 115 V

Nominal V2 = V

Nominal V1/V2 =

V1

V2

V1/V2

1.0

2.0

4.0

6.0

8.0

2. Reverse the transformer connections so that the low voltage side of the transformer is
now connected to the power supply and is now the primary windi
ng and the high voltage
side is connected to the digital voltmeter and is now the secondary winding. Repeat Step
1 and complete the following table (Table B
-
2).

Nominal V
1

= V

Nominal V
2

= 115 V

Nominal V
1
/V
2

=

V
1

V
2

V
1
/V
2

1.0

2.0

4.0

6.0

8.0

STEPDOWN

STEPDOWN

STEPUP

18

Questions:

1. Copy the 3 tables into your lab report and label them Table A, B
-
1, and B
-
2.

2. Complete the tables by calculating V
1
/V
2
. Theoretically, this should be equal to the
turn ratio N
1
/N
2
.

3. Compare the actual turn ratio in the t
ransformer you made with the calculated turn
ratio. Explain how "stray flux" and "eddy currents" cause the discrepancy.

4. For the transformer which you made, comment on its temperature at various primary
voltages. What causes the heating of the transforme
r and how can this be minimized?

5. Explain how "joule heat" losses (described in the chapter supplement) are sometimes
desirable as with "induction heating". Give applications for each.

6. What secondary voltage would be measured if a dc power supply w
ere used in place of

7. How can a dc primary current be used to induce current in a coil? Where is this used in
an automobile? (see p502)

8. Calculate the turn ratio of your commercial transformer.

19

CH 215 EXPERIMEN
T. # 8

CAPACITORS

Purpose:

To construct and study capacitors.

Introduction:

See Technical Physics, 3rd Ed., Ch 20.1, 20.3, and 20.5.

Part A:

Fabricating a tubular capacitor:

Materials:

-

waxed paper

-

high impedance dc voltmeter

-

aluminum foi
l

-

dc power supply or 9V battery

-

straight alligator clips

-

Diagram for Fabricating a Tubular Capacitor:

Procedure:

1. From a kitchen roll dispenser, tear off

a strip of waxed paper which will be about the
size of a standard sheet of paper (ca. 20

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2. From a kitchen roll dispenser, tear off a strip of aluminum f
oil which be about the same
size as the waxed paper. Cut the foil in half so that one half
-
sheet will fit into the folded
waxed paper. Trim off or fold back about 2 cm from 2 adjacent edges of the foil. Do this
to both sheets of the foil.

3. Place one shee
t of foil inside the folded waxed paper, as shown, so that the foil
protrudes at only one end. Lay the second sheet of foil over top and set
-
back, as shown.
The two sheets of foil must not be in contact.

4. Roll the sheets into a tube in the direction whic
h will keep the top layer of foil on the
outside of the tube. Be certain that the two sheets of foil are not in contact. Prevent the
tube from unrolling by securing it with a clothes peg.

20

5. Connect the leads from a 10 V dc power to the 2 foil surfaces and

charge the capacitor
for 1
-
2 seconds.

6. Disconnect the power supply leads and connect the leads from a high impedance dc
voltmeter to the 2 foil surfaces of the charged capacitor and observe the voltage reading
of the capacitor.

7. Repeat the charging
and discharging sequence several times and record the maximum
voltage held by the capacitor and average time taken for the capacitor to discharge.

Part B:

Studying an RC Circuit:

Materials:

-

dc power supply

-

high impedance dc voltmeter

-

10 cm

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-

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-

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-

㄰〠

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-

-

-

Schematic for Charging and Discharging an RC circuit:

Procedure:

1. Multimeter (voltmeter) Setup:

a) Leads on the multimeter are connected as follows. Connect the black lead to the "com"
(common
) terminal of the meter and the red lead to the "V
-

∠瑥牭楮慬⸠

2. Power Supply Setup:

a) With the pow
er supply unplugged and the voltage adjustment dial set to its minimum
position (completely counter clockwise), connect the voltmeter leads to the power supply
leads with correct polarity (red to + and black to

⤮)

-

21

3. Assembling the Circuit Board and Circuit:

a) Mount the 2 black binding posts and the red binding post on
the pegboard as explained

b) Secure the 1 M

䘠敬散瑲潬e瑩挠捡灡捩瑯爠扥瑷敥渠瑨攠c

Note: All electrolytic capacitors are polarized. They must be connected with correct
polarity (neg. to neg. and pos. to pos.) or they may explode when charged in reverse.

d) Connect the

voltmeter in parallel across only the capacitor (i.e., across the middle
-
black and the end
-
red binding posts as per the diagram). Read the voltage of the capacitor
before charging. It must be 0 V. If the voltage before charging is not 0 V, discharge the
c
apacitor as per the discharging schematic.

e) Connect the dc power supply,
still unplugged and

with correct polarity

as per the
diagram. Do not disturb the V
-
adjuster dial setting. Again check that the capacitor voltage
reads 0 V and discharge if necessary
.

4. Charging the Capacitor and Measuring the RC Time Constant:

a) Calculate the time constant (

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-
㔠瑩t攠

䍡汣畬慴攠瑨攠癯汴慧攠數灥捴敤 V
calculated
) for the 1
-
5 calculated time constants using:

calculated voltage during charging :

V = V
max

(1
-

e
-
t/RC
)

b) With safety glasses on, and stopwatch zeroed and ready, plug in the power supply and
immediate
ly start the timer. Record the capacitor voltage at the calculated times of 1
-
5
time constants. If you miss recording the voltage at the correct time, record the voltage
and the time as close as you can to the correct time.

c) Leave the power on and the ci
rcuit charging while preparing for the next steps. The
capacitor voltage should stabilize at the 10.0 V you originally set on the power supply
-
dial has not been moved. Record the final maximum voltage and
the time at which you read t
his.

5. Discharging the RC Circuit:

a) Examine the discharging circuit diagram. Note that the power supply is replaced by a
conductor (jumper wire of another colour insulation). This jumper will allow the RC
circuit to discharge the capacitor according to
the equation:
V = V
max

e
-
t/RC

-

The RC time constant for discharging will be the same as for charging.

b) Calculate the voltage expected at times of 1
-
5 time constants for discharging.

c) When ready to measure the capacitor dischar
ge rate, insert one end of the jumper wire
into the left (outer), black binding post and then carry out the following steps quickly and
in sequence:

-

unplug the power supply.

22

-

loosen the red binding post by unthreading the knurled nut slightly

-

di
sconnect the power supply lead from this red post and retighten the nut

-

insert the free end of the jumper wire into the red post and start timing

Note:
Follow the above sequence. Do not delay in disconnecting the power supply lead
from red post even tho
ugh the unit is unplugged. The capacitor will immediately begin
discharging through the power supply leads if they are left connected.

d) Record the capacitor voltage at the calculated times for all five time constants. If you
miss recording the voltage
at the correct time, record the voltage and the time as close as
you can to the correct time.

-

Repeat the charging and discharging sequence at least once but be sure to recheck the
voltage output setting on the power supply and

to completely discharge the circuit to 0 V
before charging.

DATA AND RESULTS:

Charging:

Initial V = 0.0 V

V
max

= _____ V

Time

Voltage

#

's

seconds

min.
-
sec

V
calculated

V
measured

1st run

V
measured

2nd run

2

3

4

5

final

23

Discharging:

Initial V = V
max

= _____ V

Time

Voltage

#

's

seconds

min.
-
sec

V
calculated

V
measured

1st run

V
measured

2nd run

2

3

4

5

final

Calculations and Questions:

1.

Complete and copy the tables into your lab report and label them appropriately.

2. Choose one of your data sets, i.e., either the first run or the second run, and plot a graph
of voltage (V) versus charging time (in units of

⤮⁄牡

max
.

3. For the same run, plot another graph of voltage (V) versus discharging time (

⤠慮搠

max
.

3. On the second graph, also plot the sum of [V
(charging)

+ V
(discharging)
] vs. time .

Comm
ent on how this does and should compare with the V
max

line.

4. List 3 types of fixed capacitors. (see p 419)

5. Answer question # 22 on p426 and problems # 33
-
35 on p427.

24

CH 215 EXPERIMENT. # 9

TROUBLE SHOOTING WITH MULTIMETERS

Purpose:

To use a m
ultimeter for trouble shooting.

Introduction:

An important tool for locating and repairing electrical/electronic problems
is the multimeter (also called a VO meter, i.e., volts
-
ohms meter). It can be used to
measure ac & dc voltage, ac & dc amperage, and
resistance. The circuitry of the dc
voltmeter and dc ammeter has already been studied (Ch 23.6).

Note:

In general, the black test lead remains connected to the "com" (common) terminal
of the multimeter while the red test lead is moved between the other te
rminals depending
upon the function selected.

Ammeters:
Ammeters are connected in series in a circuit in which current is to be
measured as illustrated.

Ensure that the range selector is set at a
range whose maximum amper
age exceeds
the expected reading. If in doubt, select the
highest range and work downwards for
greater accuracy. Current is measured on a
live circuit.

Work carefully. Avoid touching any live
wiring.

Select an ac or dc setting as required.
Ensure proper po
larity in dc circuits. Most
ammeters have 2 terminals for the red lead;
a low range terminal for up to 2 A and a high range terminal for up to 10 A.

Voltmeters:

Voltmeters are connected in parallel across a component as illustrated and
measure voltage drop
.

Polarity is important in dc circuits especially
with analog meters where polarity reversal
may harm the meter. Digital meters
generally indicate a + voltage reading for
correct polarity and a
-

revers
ed polarity.

As with ammeters, ensure that the range
selector is set at a range whose maximum voltage exceeds the expected reading. If in
doubt, select the highest range and work downwards for greater accuracy. Voltage is
measured on a live circuit. Be car
eful.

Note that if an ac voltage is measured on a dc voltage meter range, the voltage will read
zero volts. A voltmeter should read 0.0 volts when its test leads are shorted together.

25

Ohmmeters:

An ohmmeter consists of a galvanometer, a variable resistor,

and a battery
all connected in series as shown.

When the ohmmeter's leads are open, no current flows through the galvanometer and the
needle rests on the left hand side of the scale which is calibrated as infinite resist
ance, i.e.,

. The leads are then connected together to short the circuit and the needle undergoes
full scale deflection to the right which is calibrated as zero ohms, i.e., 0

䄠穥牯z

s

= R
1

+ R
2

. Thus the
current through the galvanometer and the resultant meter deflection is intermediate
between the extremes and is read from the calibrated scale.

Note:

An ohmm
eter is always used with no external voltage applied to the component. If
the component is live, the meter may be damaged.

The following steps should be followed for measuring resistance:

1. Disconnect the voltage from the component.

2. Disconnect the co
mponent from the circuit unless this will not affect the
resistance.

3. Touch the test leads together and use the zero adjust knob to zero the meter.

4. Select the proper range setting. If unsure, start with the highest scale.

s to the component. Polarity is not important. Be careful

An ohmmeter is not an accurate instrument but it is quick and convenient and is sufficient
f
or most purposes. A Wheatstone Bridge measures resistance accurately.

26

Part A: Tests with a voltmeter

Materials:

-

1.5 V & 9V batteries

-

multimeter & various resistors

-

ac step down transformer

-

ac to dc rectifier & step down transformer

-

strai
ght alligator clips

-

㈠敡捨

1. Battery Testing:

Measuring the terminal voltage of a battery while no current is being drawn is a
quick method
of determining the charge level of a battery. A fully charged battery usually
displays a terminal voltage above its nominal (nameplate) voltage whereas a discharged
battery usually displays a terminal voltage below its nominal voltage. A voltage measured
w
ithout current draw is, however, not always reliable. A weak battery may show a high
terminal voltage under no load but be quite weak when current is drawn. Recall that the
internal resistance of a battery must be considered.

Fully charged 1.5 V AA, C, & D

batteries as well as 9 V transistor batteries should
maintain 90 % or better of their nominal voltage even while about 0.1 A of current is
being drawn. If the voltage drops below 90 %, the battery is weak or spent.

Procedure:

1. Set the multimeter to its

dc volts range and connect the meter, with correct polarity, to
the battery. For each of the batteries supplied by your instructor, measure and record the

2. Use the nominal voltage of the battery and Ohm's Law to calculate

the series resistance
needed to draw 0.1 A from each battery.

3. With meter connected to battery as in step 1., connect the appropriate resistor in series
with the battery but in parallel with the meter, as shown in the diagram above.

4. Measure and recor
d the terminal battery voltage while current is being drawn. Measure
this quickly and do not leave the resistor connected to the battery as this will drain the
battery in short order.

5. Complete the following table.

27

battery

size

brand

name

nominal

voltag
e

V under

resistor

value (

)

V under

% of
nominal
V

strong

or weak

Other Voltage Tests:

1. Measure and record the voltage output of a 120 V ac to 15 V dc adapter on both an ac
volt ran
ge and a dc volt range. Note the polarity for the dc voltage measurement. State

2. Set an ac step down transformer to 6 V ac using the voltage selector dial on the
transformer. Measure and record the output voltage on both the ac

and dc voltage ranges
of your meter. State which is correct.

Part B: Tests with an Ohmmeter:

1. Measure and report the resistance of the resistors supplied by your instructor and
complete the following table.

colour band

sequence

nominal

resistance (

)

m敡獵牥搠

)

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28

example, auto ignition wires are manufa
ctured with a resistance of about 6k

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-
㌠3

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-
ㄸ欠

.

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#

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)

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㌮3

㄰〠睡瑴⁷桩

㐮4

㘮6

㜮7

㠮8

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-

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ㄱ⸠

ㄲ1

ㄳ⸠

ㄴ⸠

ㄵ1

ㄶ⸠

ㄷ⸠

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-

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29

21. On the auto ignition coil, measure the resistance across the
-

and + primary circuit
terminals. Also measure the resistance across the

+ primary terminal and the secondary
coil tower terminal.

22. Measure and record the resistance of your body by tightly gripping the ends of the
ohmmeter's test leads a) with dry fingers, b) with wet fingers

Questions:

For each of the following, state br
iefly but specifically, how you could safely determine
the required information. Be sure to indicate whether the power would be on or off, the
component would be connected or disconnected, and the setting of the multimeter, i.e., dc
volts, ac volts, dc amp
s, ac amps, or ohms.

a) Determine if a household 120 V duplex is live and which prong is the hot one.

b) Determine if an extension cord has a broken wire within its insulation.

c) Determine if a heating element in an electric dryer is burned out.

d) Deter
mine if the metal body of an appliance is properly grounded to the u
-
shaped
prong of its plug.

e) Determine if a motor winding is shorting to the metal body of the motor.

f) Determine if a circuit breaker is tripping unnecessarily, i.e., at low current in
the circuit.

g) Determine if the cover plate fastening screw is grounded.

30

CH 215 EXPERIMENT # 10

DIODES

Purpose:

To study the function and applications of diodes.

Introduction:

See Technical Physics, 3rd Ed., Ch 26.1 and 26.2.

Part A:
Checking

a diode with a multimeter.

Procedure:

1. Set the multimeter to the diode check position and connect the multimeter leads to one
of your diodes with correct polarity (foreword biased, i.e., black, negative meter lead to
ca
thode
-
banded side of diode and red, positive meter lead to anode
-
other, positive side of
diode) and record the voltage reading on the meter. A properly functioning silicon diode
will show a small voltage drop of less than 1V when foreword biased and OL (ov
when incorrectly connected (reversed biased).

2. Check your other 3 diodes and record all foreword biased voltage readings.

Part B:

Constructing a bridge circuit (full wave rectifier):

Materials:

-

variable ac power supply

-

dc and ac voltmeter

-

4 silicon diodes (e.g. 1N4003)

-

510

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1

-

-

-

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Diagram for Constructing a Diode Bridge Circuit:

31

Procedure:

1. Mount the 6 binding posts on the peg board as shown in the diagram, spacing them so
that the 4 diodes and the resistor can be securely held in the p
osts.

2. Insert the diodes with polarity as shown. Note that the side of the diode with the
colored band is the negative terminal (cathode) and corresponds to the straight line
portion of the diode symbol. The arrow in the diode symbol points in the direct
ion of
conventional current flow (positive to negative). Reversed biased diodes may be
damaged.

3. Insert the resistor into its binding posts and connect the resistor binding posts to the
rectifier bridge circuit with 2 short lengths of # 22 gauge wire.

4
. Connect the leads from an ac power supply to the bridge as shown.

5. Connect the leads from the voltmeter to the power supply and adjust the power supply
until the terminal voltage reads 10.0 VAC using the meter's 20 VAC maximum range.

6. Change the volt
meter to the 20 VDC maximum range and read dc voltage across the
resistor as shown

7. Record your VAC input measured and VDC output measured in row 1 of Table 1 along
with the VDC calculated. The voltmeter reads VAC
rms

and VDC
avg
. Calculate the
VDC
avg

as
follows:

VAC
peak

=

2

rms

VDC
avg

= 2

peak

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avg

= VAC
peak

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Part C:

Constructing a half wave (single diode) rectifier:

Materials:

-
same as for Part B.

Diagram for Constructing a Half Wave (single diode) Rectifier:

32

Procedure:

1. Alter
the connections in your circuit as shown. The dotted lines represent disconnected
conductors. This circuitry connects only 1 diode in series with the resistor thus creating a
half wave rectifier.

2. Repeat step 5
-
2 of Table 1.

Table 1:

VAC
rms

measured (V)

VAC
peak

calculated (V)

VDC
avg

calculated (V)

VDC
avg

measured (V)

1

2

Part D:

1. Use the diode check on your multimeter to determine the foreword biased voltage of an
LED as provided by your in
structor and report this value in your report.

Calculations and Questions:

1. Complete and copy Table 1 along with other data into your lab report and label them
appropriately.

2. Draw a diagram of a full wave bridge circuit rectifier as shown in this la
b and show the
polarity of the ac power supply and direction of conventional current flow through every
branch of the circuit by means of arrows. Draw the diagram again with opposite polarity
on the ac supply and again show the direction of conventional cu
rrent flow through every
branch of the circuit.