Anscircuit 4

Electric Circuits Name:
Electrical Power and Energy

Read from Lessons 2 and 3 of the Current Electricity chapter at The Physics Classroom:
http://www.physicsclassroom.com/Class/circuits/u9l2d.cfm
http://www.physicsclassroom.com/Class/circuits/u9l3d.cfm
MOP Connection: Electric Circuits: sublevel 3
Review:
1. The electric potential at a given location in a circuit is the amount of __potential energy__ per
__charge__ at that location. The location of highest potential within a circuit is at the __+__ ( +, - )
terminal of the battery. As charge moves through the external circuit from the __+__ ( +, - ) to the
__-__ ( +, - ) terminal, the charge loses potential energy. As charge moves through the battery, it
gains potential energy. The difference in electric potential between any two locations within the
circuit is known as the electric potential difference; it is sometimes called the __voltage__ and
represented by the symbol __∆V__. The rate at which charge moves past any point along the circuit
is known as the __current__ and is expressed with the unit __Ampere (amp)__.
The diagram at the right depicts an electric circuit in a car. The rear
defroster is connected to the 12-Volt car battery. Several points are labeled
along the circuit. Use this diagram for questions #2-#6.
2. Charge flowing through this circuit possesses 0 J of potential energy
at point __D__.
3. The overall effect of this circuit is to convert ____ energy into ____ energy. Answer: D
a. electrical, chemical b. chemical, mechanical
c. thermal, electrical d. chemical, thermal
4. The potential energy of the charge at point A is __AA__ the potential energy at B.
a. greater than b. less than c. approximately equal to
5. The + charge gains potential energy as it moves between points __D__ and __A__.
a. A and B b. B and C c. C and D d. D and A e. none of these
6. The + charge loses potential energy as it moves between points __B__ and __C__.
a. A and B b. B and C c. C and D d. D and A e. none of these
7. The rate at which energy is delivered to a circuit by the energy source or the rate at which energy is
consumed by an electrical device is known as the electric __power__.
a. current b. potential c. voltage d. power
8. The unit of electric power is the __Watt__.
a. Ampere b. Volt c. Watt d. Joule
9. Mechanical power (discussed in a previous unit) is the rate at which work is done on an object.
Electrical power is the rate at which work is done on a charge (by the battery) or on an electrical
device (by the charge). In terms of an equation, it is … . (Fill in the numerator and the
denominator.)
Work
time
a. A 60-Watt light bulb uses up __2.16x105__ J of energy when left on for 1 hour (3600 s).
b. A 60-Watt light bulb uses up __8.64x105__ J of energy when left on for 4 hours.
c. A 1500-Watt hair dryer uses up __4.5x105__ J of energy when used for 5 min (300 s).
d. A 120-Watt fan uses up __1.04x107__ J of energy when left on for a day.
© The Physics Classroom, 2020 Page 1

Electric Circuits
10. Substitution of other electrical equations ( I = Q/t and DV = I•R and DV = W/Q ) into the power
equation yields the following three equations.
P = I•DV P = I2•R P = DV2 / R
Use these equations to solve the following problems.
a. Calculate the resistance of a toaster oven if its power is 800 W when connected to a 110-V outlet.
Using P = DV2 / R, rearrange for R: R = DV2 / P. Substitute 800 W for P and 110 V for ∆V. The
answer is 15 " (15.12 … ").
b. Calculate the resistance of the 1000 W microwave oven that gets plugged into to a 110-V outlet.
Using P = DV2 / R, rearrange for R: R = DV2 / P. Substitute 1000 W for P and 110 V for ∆V. The
answer is 12 " (12.1 ").
c. The TI-84 calculator uses four 1.5-V batteries and has a power of 0.0008 W. What is the current?
Using P = I•DV, rearrange for I: I = P / ∆V. Substitute 0.0008 W for P and 1.5 V for ∆V. The answer
is 5.3x10-4 ampere.
11. The following two circuits consist of a power supply, an ammeter (for measuring current), and a
light bulb. Use >, <, and = symbols to compare the electric potential at A to B and at C to D.
Indicate whether the devices add energy to or remove energy from the charge. Finally, fill in all
blanks.
VA > VB
(>, <, or =)
VC > VD
(>, <, or =)
24 VB = VD
(>, <, or =)
12. TRUE or FALSE: A kilowatt-hour is a unit of power. Answer: FALSE. It is a unit of energy.
13. Alfredo deDarke often leaves household appliances on for no good reason (at least according to his
parents). The deDarke family pays 15¢/kilowatt-hour (i.e., $.15/kW•hr) for their electrical energy.
Express your understanding of dollar power by filling in the following table.
Power Rating Time Energy Used Cost
(Watt) (hrs) (kilowatt-hour) ($)
60 Watt Bulb 1 0.060 kW•hr $0.009
60 Watt Bulb 4 0.240 kW•hr $0.036
Ten 60 Watt Bulb 24 14.4 kW•hr $2.16
3
60 Watt Bulb 1.11x10 667 kW•hr $10
7 Watt Night Light 168 (1 week) 1.18 kW•hr $0.176
7 Watt Night Light 8760 (1 year) 61.3 kW•hr $9.20
14. People often claim that an electrical appliance "uses up electricity." Explain what is actually being
"used up" and what becomes of this thing that is being used up.
It is energy that becomes used up by electrical appliances. But a more appropriate way to express it is
to say that the energy is being transformed from one form to another form. For instance, chemical
energy of a battery might be transformed to electrical energy of moving charge and then to the
mechanical energy of a fan.
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Electric Circuits Name:

Electrical Power and Energy

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