Electric Current/Electric Circuits - Teacher

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7 Οκτ 2013 (πριν από 4 χρόνια και 6 μέρες)

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Electric Current/Electric Circuits

Review/Study Guide


In the last unit we looked at Static
Electricity

and, as the name suggests, the charges were static,
they

were
not moving. In this unit we are looking at moving charges

or electrical current
.


Resou
rces:

Text
book

Chapters 22 and 23
; Chapters

from Hewitt Textbook (copied and made available
in class); Glenbrook HS website (link from the Tutorials page of my website) and the
Colorado PhET Electric Circuits s
i
mu
l
ation.



Electric Current, Voltage an
d Resistance



I
n

order for
a
charge to move, a force must be applied to the charge.
We learned in the last unit that a
charge in an electric field experiences a force. A battery creates an electric field in a conductor and
charged particles (electrons)

move through the conductor. The potential difference (voltage) of a battery
is what makes the charge flow. On the next page is a chart setting out the analogy between water flow
and electric charge flow (electric current) that we discussed in class.

Y
ou can also think of potential
difference as the ‘electrical pressure’ that makes current flow.



The greater the potential difference the more

current will flow. The more resistance to the flow of
current, the less current will flow. This is summari
zed in Ohm’s Law:








or alternately, V= IR

or


R=


When charges flow in only one direction, it is called Direct Current (DC); when charges reverse direction
constantly, it is called Alternating Cu
rrent

(AC)
.
B
atteries produce DC

current
; the electricity in our
homes is AC.


Conventional current is the hypothetical flow of positive charges in a circuit. In fact we know that it is
the negative charges (associated with electrons) that actually move

but the convention established long
ago of showing the direction of current as if it is the positive
charges

that move has remained.


Conversion of
Electrical
Energy

To

Other Forms of Energy


The usefulness of electricity is that it is a convenient source

of energy that we can
easily
convert into
another form of energy
.

The work done to give a charge its electrical potential energy can be transformed
into mechanical energy, heat or light. As a charge moves around a circuit and passes through resistors th
e
charge looses its electrical potential energy. The resistor converts that energy into heat and, in the case of
light bulbs, light. Electric motors can
convert

some of the electrical potential energy into mechanical
energy.


Recall that
p
ower, measured

in

(
watts
)

is the rate of converting energy from

one form to another.

The power
dissipated

(transformed from electrical energy into another form of energy
, heat
) in a resistor
is given
by
. From O
hm’s Law we
know V
=IR so
the power dissipated in a resistor as heat is:


P = I
2
R


T
his is sometimes called


joule heating loss
’ or ‘I
2
R loss’


This is important when we consider transmission of large amounts of electrical energy.
A city’s needs for
elect
rical energy per second (power) often have to be met by transmission of electrical energy over long
distances.
In order to
minimize I
2
R losses
engineers transmit the
electrical
energy
at as low a current
level as possible. P=IV so to transmit the same am
ount of power at low current, the voltage is increased

proportionally
. This is why long
-
distance transmission of electricity is done at high voltage (100,000


500,000 volts).


Calculating the
C
ost of Electrical Energy


We buy electrical energy from the u
tility company.
When calculating the cost of that energy we have to
use the units the utility companies use
. These units a
re not the standard MKS units we usually use.


Recall that

or
.
Solving f
or E, we get E = Pt. Utility companies use the
unit of kilowatt (1000 watts) for
power and the unit of hour for time. So the unit for the energy they sell
us is the kilowatt
-
hr. Review the worksheets where we calculated the cost of operating appliances

for a
day, week, month, year etc.


Circuit Schematics


Rather than sketch images of the components of a circuit (batteries, bulbs, resistors etc.) we use electrical
schematics to draw electric circuits
. See the textbook page 515.




Analyzing Circuits



Series Circuits







-

When resistors are connected in series, the same current goes through each resistor. The current that
exits the battery must return. (It doesn’t go anywhere else and charge
is conserved.)


-

The equivalent resistance
(the resistance of a single resistor that could take the place of all the other
resisters for
a series combination of resistances is R
Eq

= R
1

+ R
2

+ R
3

+ …


-

If you know the voltage applied to a series circuit,
y
o
u can a
pply Ohm’s Law and use R
Eq

to
determine the current in the circuit.


Parallel Circuits






-

When resistors are connected in parallel, t
he same voltage is applied
across

each
resistor. The current
through

each resistor

can be different.


-

The equivalent resistance of a parallel combination of resistors

is


If you have
only two

resistors, a simpler form of the e
quation is:



*Check your work by remembering that the R
Eq

for a parallel circuit is always
less than the lowest

resistance in any of the parallel

branches.


The Loop and Junction Rules


Loop Rule:

Around any closed circui
t, the voltage increases must equal the voltage decreases.

(Remember, for voltage: “What goes up must come down.”)



The Loop Rule
is the result of Law of Conservation of Energy. Think of the skier
who is lifted up the hill by the chair lift gaining
potential energy and then returns to
the starting point by loosing that potential energy t
o

friction. The skier has no net
gain in energy.
S
imilarly, charges have no net gain in electrical potential energy
as
they go around a circuit (the battery increas
es their potential energy, the resistors
convert that potential energy to heat
)
.


Junction Rule: The current going into a junction equals the current leaving the junction.

(Remember
for current

at a junction, “What goes in must come out.”)



The Juncti
on Rule is the result of the Law of Conservation of Charge. The charge
that goes into a junction can’t just disappear.
And c
harge can’t materialize from
nowhere. ‘What goes in must come out.’



Combin
ed

Series and Parallel Circuits


Circuits with
both
s
eries and parallel
resistors can be simplified to find the equivalent resistance for all the
resistors. A good strategy is given on page 544 of your text. By simplifying the circuit to find the
equivalent resistance and applying the loop and junction rul
es, you can analyze circuits

and find the
equivalent resistance for a complex combination of series and parallel circuits. Once you know the
equivalent resistance, you can determine the current in the circuit.
Practice
with the worksheets we have
used in

class or for homework.



Ammeters and Voltmeters



We practiced
using ammeters and voltmeters in our circuits lab.
You may be asked to draw an electrical
schematic showing how you would measure
current through

and
voltage across

resistors in a circu
it.


Hous
e
hold Circuits

and Electrical Safety


Appliances
in household circuits are wired in
parallel
.
As we add
more appliances
to the circuit,
the
equivalent resistance in the circuit drops. This results in increased current in the circuit. Fuses or c
ircuit
breakers
are
used to prevent household wiring from getting too hot
when lots of appliances are operated at
the same time. When the current in the circuit exceeds a preset limit, the
heat melts a small strip of metal
in the fuse or causes a circuit

breaker to open. The current then stops flowing in the circuit.
Be prepared
to sketch a household circuit and to explain why this arrangement for the fuses and circuit breakers helps
protect the home from fires.


Be prepared to explain how much current

can cause death or injury due to electrical shock

and what kind
of electric shock (across the chest) is usually the most dangerous. Explain using Ohm’s Law why wet or
dry skin can make a difference in whether someone receives an injurious electric shock.

Be prepared to
explain basic precautions for working safely with electric circuits.