Direct Current (DC) Circuits Lab

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

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Direct Current (DC) Circuits Lab


Name:

Group Partners:

Date:


Objective:

In this lab, we will investigate the behavior of direct current (DC) electrical circuits. We
will study the flow of electrical current in a circuit from the cell, through the wires,

and
through various combinations of light bulbs and resistors.

By the end of the lab, you should be able to:



identify the components of a circuit;



understand the relationships that occur between the components;



be able to use all variables associated w
ith Ohm’s Law (
V, I, R
).


Background:

Before we start the lab, we will talk about electrical charge, voltage, current and
resistance, to make sure that you are familiar with these concepts and terms, and the units
we use to measure them.

Electrical circuit
s usually have a power (energy) source such as a cell or battery, and one
or more resistors such as a light bulb. A circuit is a path along which electrons can flow.
If the circuit is closed, there is a continuous flow of current until the power source i
s
exhausted (finished). Current does not flow in an open circuit. A circuit in which there
is a single pathway is known as a
series

circuit whereas a circuit that has multiple (more
than one) possible paths is known as a
parallel
circuit.

Resistors provi
de resistance to the flow of current in a circuit. Many resistors obey
Ohm’s Law (
V = IR
), which states that the current
I

through a resistance
R

is
proportional to the voltage
V

across the resistor.


Apparatus:

You should have the following equipment in
your setup:



One 1.5V cell



One cell holder



Set of ten “alligator
-
alligator” leads



Two small light bulbs (2.5V, 0.3A)



Two light bulb holders



One digital multimeter to take current and voltage readings



Assorted resistors (5)


Procedure:

This lab is designed t
o have several sections, each of which builds upon the work of the
previous sections.


1.

Look at the various components you have in your experimental apparatus and make
sure that you can identify each of them in the list above.


2.

Make a simple circuit using

the cell (and its holder), a light bulb (and its holder), and
some of the connecting leads.

a). What happens to the light bulb?

_____________________________________________________________________

______________________________________________________
_______________

b). Draw a circuit diagram representing your circuit:










Conductors and Insulators (Non
-
Conductors):

3.

Add another lead to your circuit but do not connect two of the alligator clips together
(leave the circuit open). Now, look around
you for three objects that you can connect
the alligator clips to, and connect one to each side. Try to find at least one that is a
metal.

a). Does the lamp light for each object or not?

Object 1: ________________, does the bulb light or not?: ______
_______________

Object 2: ________________, does the bulb light or not?: _____________________

Object 3: ________________, does the bulb light or not?: _____________________

b). What do you think this indicates about whether your test objects conduct
electricity or not?

_____________________________________________________________________

_____________________________________________________________________

_____________________________________________________________________


Simple Circuits and Resis
tance:

4.

Now, make a simple circuit using the cell (and its holder), one of the 20


(ohm)
resistors, and some of the connecting leads. (The 20


resistors will have a red ring
followed by two black rings (and a gold and red ring) painted on them.) Your circ
uit
should look like your circuit from Section 2, but with the resistor replacing the light
bulb.

a). Draw a circuit diagram representing your new circuit:









b). We will now use the multimeter to measure the voltage across the resistor, and
the cur
rent passing through it. In order to measure the voltage
across

a component,
we need to place the two probe tips on either side of the component


we connect the
multimeter in
parallel
. See if you can measure the voltage across the resistor. In
order to

measure the current
through

a component, we need to place the multimeter
so that it is part of the current path


we connect the multimeter in
series
. See if you
can measure the current through the resistor.

c). Fill in the table below, measuring the vo
ltage across the 20


resistor, and the
current through it. Use Ohm’s Law (
V = IR
) to
calculate

the resistance from these
two values, and then measure it directly using the resistance option on your
multimeter.


Resistor

Measured
voltage (V)

Measured
cur
rent (A)

Calculated
resistance (

)

䵥a獵牥搠
牥獩s瑡湣e (

)









Resistors in Series and Parallel:

5.

Next we return to studying light bulbs. Reassemble your simple circuit from Section
2 with a cell and a single light bulb. Try to remember how brightl
y the bulb is
shining.

a). Now add a second identical bulb in
series.

What do you observe about the light
intensity (brightness) in each bulb compared to a single bulb?

_____________________________________________________________________

b). What happe
ns if you remove one of the light bulbs from its holder?

_____________________________________________________________________
c). Instead of adding the second bulb in series, add it in
parallel

to the first. What do
you observe about the light intensity
in each bulb compared to a single bulb?

_____________________________________________________________________

d). What happens if you remove one of the light bulbs from its holder?

_____________________________________________________________________


Eff
ective (Equivalent) Resistance:

Resistors in Series

6.

We look now at two resistors in series. Take two 20


resistors and make a circuit
with them connected in series.

a). Draw a circuit diagram representing your new circuit:









b). We can describe th
e effective resistance of two resistors R
1
and R
2

in series using
the equation: R = R
1
+ R
2
. Measure the effective resistance R across
both

resistors
using the resistance option on your multimeter and note the value below:

________________________________
_____________________________________

c). Calculate the expected value for R from the equation above and note the value
below:

_____________________________________________________________________

d). Is the effective resistance (R) of two resistors in s
eries greater or smaller than the
individual resistances?

_____________________________________________________________________


Resistors in Parallel

7.

We will now look at two resistors in parallel. Take two 20


resistors and make a
circuit with them conne
cted in parallel.

a). Draw a circuit diagram representing your new circuit:









b). We can describe the effective resistance of two resistors R
1
and R
2

in parallel
using the equation: 1/R = 1/R
1
+ 1/R
2
. Measure the effective resistance R across
both

resistors using the resistance option on your multimeter and note the value below:

_____________________________________________________________________

c). Calculate the expected value for R from the equation above and note the value
below:

____________
_________________________________________________________

d). Is the effective resistance (R) of two resistors in parallel greater or smaller than
the individual resistances?

_____________________________________________________________________


8.

Finally,
take a “mystery” resistor (it should have rings that are coloured
either

brown;
black; brown; gold
or

orange; black; brown; gold; red). Choose any method of
measurement
except

the resistance option on your multimeter to determine its value.
Once you have

your answer, ask a member of the Physics Emasondosondo team if
you are correct!