SCN1DG
Name: __________________
Current Electricity Review
1) Define:
Property
Definition
Symbol
Unit
Unit
Symbol
Unit Definition
Charge
quantity of elementary charged particles
Q
Coulomb
C
1C = 6x1018 e-
Current
the amount of charge moving per sec.
I
Ampere
A
1A = 1C/s
Energy
the ability to do work
E
Joule
J
1J = 1N x 1m
Potential Difference
the change in energy per charge
V
Volt
V
1V=1J/C
Resistance
the voltage needed to produce a current
R
ohm
S
1 S = 1V/1A
Power
the rate at which energy is used
P
watt
W
1W = 1J/s
2) Describe the following circuit diagram symbols:
switch, battery, resistor, lamp, wire, volt meter, ammeter
3) Which of the following is the correct way to connect a volt meter to measure
the voltage of a light bulb?
The first diagram is correct.
4) Which of the above is the correct way to connect an ammeter to measure the current through a light
bulb?
The second diagram is correct.
5) Which of the following are wired in series and which are
wired in parallel?
parallel, series, series, parallel.
6) What is the best way to increase the voltage of a set of batteries: arrange them in series or parallel?
series
7) If light bulbs are arranged in parallel, which of the following are true:
___F_ a) if one bulb burns out, they all stop
___T_ b) they all have the same voltage as the battery
___F_ c) the voltage of each light adds up to the voltage of the battery
___F_ d) all the lights have the same current as the rest of the circuit going through them
___T_ e) the current of each light bulb adds up to the current coming out of the battery
___T_ f) the lowest resistance load is the most powerful
___F_ g) the highest resistance load is the most powerful
8) If light bulbs are arranged in series, which of the following are true:
___T_ a) if one bulb burns out, they all stop
___F_ b) they all have the same voltage as the battery
___T_ c) the voltage of each light adds up to the voltage of the battery
___T_ d) all the lights have the same current as the rest of the circuit going through them
___F_ e) the current of each light bulb adds up to the current coming out of the battery
___F_ f) the lowest resistance load is the most powerful
___T_ g) the highest resistance load is the most powerful
9) Explain your answers to 7 f & g and 8 f & g in terms of voltage and current.
All of the loads in a series circuit have the same current, only the voltage differs. Since resistance is the ability to
transform energy, the greatest resistance will transform the most.
All of the loads in a parallel circuit have the same voltage (same energy per charge), but the lowest resistance load gets the
most current, so it gets the most energy per second.
10) a) What happens to the current in a parallel circuit if more lights are added? it increases
b) What happens to the current through each light in a parallel circuit if an additional lights is added?
nothing
c) What could happen if too many lights are turned on in parallel circuit?
the main current would get too large and the wire could overheat.
11) What happens to the current in a circuit as more lights are added to it in series? it decreases
12) How are lights and other appliances wired up in a home: in parallel or in series?
parallel
13) In the following diagram, what would happen to the current in different parts of the circuit if:
a) light bulb ‘a’ is unscrewed? all stops
b) light bulb ‘c’ is unscrewed? a and b decrease, others unchanged
c) light bulb ‘f’ is replaced by a copper wire? a and b increase, others unchanged
d) the gap at ‘g’ is closed with a light bulb? a and b increase, others unchanged
e) a new light bulb is added between light bulbs ‘a’ and ‘b’? all decrease
f) list two light bulbs that are in series with each other. a and b
g) list two light bulbs that are in parallel with each other. e and f
h) what will happen to the lights if switch ‘x’ is opened? all current stops
i) what will happen to the lights if switch ‘y’ is opened? all current stops
j) which lights will go off if switch ‘z’ is opened? only e
14) Draw a diagram for a circuit with on battery, one light, and one switch to control the light.
15) Draw a diagram for a circuit with one battery and two lights. Each light must have its own switch and
the lights must be independent: either one, both, or neither light can be on at one time.
16) A light bulb has 30 coulombs of charge moving through it per minute (60 seconds). What is its
current?
I = Q/t = 30C/60s = 0.5 A
17) A battery adds 6 joules of energy to each coulomb of charge that leaves it, what is its voltage?
V = ) E/t = 6J/1C = 6V
18) a) A circuit with a 6 volt battery has one light bulb with a resistance of 2 ohms. What current will pass
through the light bulb?
R=V/I, I = V/R = 6V/2S = 3A
b) What is the power of this light?
P=VI = 6V x 3A = 18W
19) A light bulb has a voltage of 3 volts and a current of 6 amps passing through it. What is its resistance?
R=V/I = 3V/6A = 0.5S
20) If a motor uses 10 amps of current, how much charge moves through it in one minute?
I=Q/t, Q = It = 10A x 60s = 600C
21) What voltage is needed to push 2 amps of current through a hair dryer with a resistance of 4 ohms?
R=V/I, V = RI = 4S x 2A = 8V
22) a) What is the effective resistance of 3 2-ohm resistors wired in series? R = 2+2+2 = 6S
b) What is the effective resistance of 3 2-ohm resistors all wired in parallel?
1/R = 1/2+1/2+1/2 = 3/2;
R=2/3S
23) Calculate the power of each of the three resistors in this circuit.
a) What is the total resistance?
Resistance of lamps “a” and “c”
1/R = 1/1 + 1/2
= 2/2 + 1/2
= 3/2
R = 2/3
Add the resistance of lamp “b”
RT=2/3 + 2 = 2.67
b) What is the current out of the battery?
IT = V/R = 6V/2.67S = 2.25A
c) What current flows through lamp “b?”
2.25A (the same as out of the battery since lamp “b” is on the main circuit loop)
d) What is the voltage of lamp “b?”
Vb = RI = 2S x 2.25A = 4.5 V
e) What voltage must lamps “a” and “c” have?
Va = Vc (since they are in parallel with each other)
Va = 6V - 4.5V = 1.5V (these two lamps get what ever is left over from lamp “b”)
f) What current must flow through lamps “a” and “c?”
Ia = Va/Ra = 1.5V/1S = 1.5A
Ic = 2.25-Ia = 0.75A (we know that the currents in lamps “a” and “c” must add up to the main current)
g) What power does each lamp have?
Pa = VaIa = 1.5V x 1.5A = 2.25W
Pb = VbIb = 4.5V x 2.25A = 10.1W
Pc = VcIc = 1.5V x 0.75A = 1.1W
24) An 20 ohm oven and a 15 ohm kettle are plugged into the same circuit with 6 100-watt light bulbs.
Household voltage always is 120 V
a) What current is passing through each appliance?
P=VI, I = P/V
Ilamp = 100W/120V = 0.83A
I = V/R
Ioven = 120V/20S = 6A
Ikettle = 120V/15S = 8A
b) Will the 15 amp circuit breaker be tripped by these loads?
IT = 6(0.83A) + 6A + 8A = 18.98A. This is greater than 15A, therefore, yes, the breaker will trip.
c) Why is just installing a higher amp circuit breaker not a good way to stop the breaker from tripping?
The wire in the wall is designed for 15A, which is why a 15A breaker was installed. A higher amp breaker would allow a
dangerously large current to go through the wire, overheating it.
25) A 10 ohm heater is plugged into a regular .01 ohm extension cord.
a) What current will flow through the cord and the heater?
The cord and heater are in series with each other. Thus, RT = 10S + 0.01S = 10.01S
I=V/R = 120V/10.01S = 12A
b) How much power will the cord generate in the form of heat?
Pcord = Vcord x I
Vcord = RI = 0.01S x 12A = 0.12V
Pcord = Vcord x I = 0.12V x 12A = 1.44W
26) Why are small or damaged extension cords dangerous for high-power devices?
These cords have higher resistance (either thin cords or cords that have been damaged). This means that they will
generate more heating power. Their resistance will go up, they will become more powerful... you get the picture: fire!
27) a) How fast to charges move through a circuit? 3cm/min
b) How is it that the effect of electric current moves down a wire at the speed of light?
electrons push the electrons in front of them so that the electrons at the far end of the wire start moving right away.
28) What four factors determine resistance and what effect to they have?
length of conductor: longer = more resistance
diameter of conductor: wider = less resistance
temperature of conductor: hotter = more resistance
composition of conductor: different materials have different resistances
29) What is the voltage of household electricity?
120 V
30) What is the difference between AC and DC current?
In DC current, the charges move at a constant speed in one direction with constant voltage at any point.
In AC current, the charges vibrate back and forth over a small distance, the voltage fluctuates as well.
31) If a 60 W light bulb produces 9 W of light power, what is its efficiency?
Ef = 100% x Pused / Ptotal = 100% x 9W / 60W = 15%
32) a) What unit of energy does the electricity utility company use to bill its customers?
kilowatt hours (kWhr)
b) To what number of Joules is this unit equivalent?
1 kWhr is a power of 1000 W for 1 hour (3600 s)
1 kWhr = 1000W x 3600s = 3,600,000 J
c) If a radio uses 25 W of power and is used for 5 hours a day, every day of the year:
i) how much energy will it use per year?
25W = 0.025kW
E = 0.025 kW x 5 hr/day x 365 day/year = 45.6 kWhr/year
ii) what will this amount of energy cost if it was plugged into a wall outlet?
energy costs about $0.08 per kWhr
cost = $0.08/kWhr x 45.6 kWhr/year = $3.65 /year
d) Estimate the cost if the radio had used non-rechargeable batteries instead?
We never did get to this assignment (we will later) so skip this.
e) Calculate the current and resistance of this radio.
Plugged into a wall, V=120V
P=VI, I = P/V = 25W / 120V = 0.21A
R = V/I = 120V/0.21A = 576 S
33) a) What is the function of the circuit breaker (or fuse) in a circuit?
It opens the circuit if the current gets larger than the save value.
b) Draw the circuit diagram for a household circuit with 5 lights and 5 electrical outlets.
Include the breaker box and circuit breaker. Label the hot wire, neutral wire, and ground wire.
c) What is the function of the ground wire?
The ground wire connects the case of a device to the ground. Thus, if the insulation between the hot wire and the
case breaks down, there will be an instantaneous short circuit (no load) and the circuit breaker will trip.
34) a) What is the function of a GFCI outlet on a circuit?
It opens the circuit if there is any difference between the hot and neutral wire current (i.e., if any current is leaking from
the circuit).
b) In what situations are GFCI outlets needed (actually required by the building code)?
Where ever there are electrical devices or outlets near water.
c) How many GFCI outlets are needed on a circuit and where should be placed in the circuit?
Usually only one, if it is the first device in the circuit. Check with the local building codes for professional advice, however.
d) Why do circuit breakers not protect people in the way that GFCI outlets can?
It takes less than one amp to kill a person, which is too small a current to trip the circuit breaker.