Electricity Practice Problems June 17, 2010
Ohm’s Law Practice Problems
In a resistive circuit, Ohm's Law states that: voltage is equal to current times resistance.
Ohm's Law: V = I x R
V = voltage (volts)
I = current (amps)
R = resistance (ohms)
1) A circuit consists of a 10 ohm resistor and carries a 5 amp current. What is the
voltage in the circuit (in volts)?
2) One day, Olive has what she thinks is an ingenious idea to connect her dad‟s
ammeter to a flashlight which uses a 6 volt battery. (An ammeter is a device that
measures the current flow, in amps, in an electric circuit.) She connects the
ammeter and sees that there is a current of 3 amps flowing in the circuit. What is
the flashlight‟s resistance?
3) Olive built the circuit below with a variable-voltage power supply (the circuit‟s 1leftmost circuit element) and a resistor defined by the diagram below.
Using her ammeter (shown by the circuit element with an encircled „A‟), she took measurements of current passing through the resistor. She also used a voltmeter (as shown by the circuit element in the encircled „V‟) to measure the voltage across the resistor. She recorded these numerical values in a table, with the results shown below:
1 Question adapted from http://www.allaboutcircuits.com/worksheets/ohm_law.html on 6/276/09.
0.66 V 0.22 A
1.42 V 0.47 A
2.54 V 0.84 A
3.16 V 1.03 A
4.51 V 1.50 A
5.41 V 1.80 A
5.99 V 2.00 A
7.49 V 2.51 A
Plot the figures on the graph.
What mathematical relationship would you tell Olive that you see between voltage and
current in this simple circuit?
4) A circuit is comprised of a battery and variable resistor, known as a rheostat (see
figure). What happens to the current if resistance is doubled?
a) Current doubles resistor
b) Current halves
c) Current remains the same
d) The current goes to zero
5) Which of the following statements is NOT true?
a) Voltage is inversely proportional to the value of Resistance
b) Voltage is directly proportional to current
c) Current is inversely proportional to Resistance
d) Voltage and Resistance are always proportional
6) Olive‟s family car wouldn‟t start and she helped her father install a new 12-volt
battery, but that did not fix the problem. Olive‟s dad then had the car towed to the
auto mechanic‟s (Olive got to ride in the tow truck!), where the mechanic told her
father that the car would need a new 600 amp start motor. Olive, wanting to show
that she understood about such things, speaks up, and what does she proudly tell
her father and the mechanic that she has figured out about the start motor?
1) V = I x R, or (5 amps) x (10 ohms) = 50 volts
2) V = I x R which can be converted to R = V ? I, so:
(6 volts) ? (3 amps) = (2 ohms)
3) Current is linearly proportional to voltage, or equivalently, that the numerical
value of the voltage is always three times that of the current, or equivalently,
that the resistor has a value of three ohms which makes the current always one-
third of the voltage.
6) 0.02 ohm
Electrical Power Law Practice Problems
Power is the energy being used at a given instant in time and is measured in watts. In an electric circuit, power can be calculated by multiplying the circuit‟s voltage times
P = I x V (Power Law)
P - Power consumed or produced by a circuit
(watts, or equivalently: amp-volts)
7) How much power is consumed by
a) The resistor in problem #1 above?
b) The light bulb in problem #2?
c) The start motor in problem #6?
8) If voltage in household circuits is 120 volts, what is the current flow through a
a) 60 watt incandescent light bulb?
b) 12 watt CFL light bulb (which produces light approximately equal to a 60
watt incandescent bulb)?
c) 1500 watt hair drier?
9) A circuit breaker is a safety device that protects a household electric circuit, such as all the electric outlets in a particular room in a house, from having too much electricity running through it. A breaker‟s function is to stop electric current flow
when the current exceeds some preset maximum as given by the breaker‟s “rating”. What do Olive and her twin sister Penny have to do when they both dry their hair at the same time (assume that all of their house‟s bedroom circuit breakers are rated at
15 amps and all bathroom circuit breakers are rated at 20 amps)?
7 a) 250 watts (50 volts x 5 amps)
b) 18 watts (6 volts x 3 amps)
c) 7200 watts (12 volts x 600 amps)