# 20.4, 20.6-20.7 ```Journey of an Electron through a Circuit
V
+
Top Wire
4 V --
4V
Lose potential
in resistor
Gain
potential in
battery
_
Bottom Wire
0V
Start
End
Power
• Current delivers energy from source to electrical device
• Power delivered to device = change in energy of charges
time interval
P =
 qV
=
t
Units = (C/s)(J/C) = J/s = Watts (W)
• I is the current through the device and V is the voltage across it
• For resistors, use V=IR, I=V/R to get:
P=
P=
(and P = IV)
• Common unit of electrical energy consumption:
kilowatt-hour (kWh)
ex. Calculate the resistance of a 40W
automobile headlight designed for 12V.
ex. A power station delivers 520 kW of
power to a factory through 3.0 ohm
lines. How much less power is wasted
if the electricity is delivered at 50,000 V
rather than 12,000V?
If we have 2 (or more) circuit devices there
are two ways we can wire them.
In series:
Same current goes
through each
device.
In parallel:
Same voltage
across each
device
I=
Req
V
V V
= +
Req R1 R2
V
V=
IReq =
IReq = I (R1 + R2 )
For series:
Req =
Req
For resistors in
parallel:
1
=
Req
Req =
Why is Req smaller than any of the
individual resistances for a parallel
circuit?
Consider two identical resistors (R1=R2)
wired in series in a circuit, as shown below.
The current in the second resistor is:
A) equal to
B) half
C) smaller than, but not
necessarily half
the current through the first resistor
ex. A 60W and a 100W lightbulb are
connected in series across a potential
difference of 120V. Find:
a) the current in each bulb
b) the voltage drop across each bulb
c) the power dissipated in each bulb
ex. A 60W and a 100W lightbulb are
connected in parallel across a
potential difference of 120V. Find:
a) the current in each bulb
b) the voltage drop across each bulb
c) the power dissipated in each bulb
ex. A 1650 W toaster, a 1090 W iron, and a 1250 W
microwave oven are turned on in a kitchen.
a) Find the equivalent resistance of the three devices
b) Will the 20A circuit breaker open?
For the circuit shown in the drawing, what is the
ratio of the current I1 in resistance R1 to the
current I2 in resistance R2?
A)
I1
R1
=
I2
R2
B)
I1
R2
=
I2
R1  R2
C)
I1
= 1
I2
D)
I1
R1
=
I2
R1  R2
E)
I1
R2
=
I2
R1
```