Grade 9 Academic Science – Unit 4 Physics (Electricity)
Solving Problems with Ohm’s Law (V = IR) - Answers
V=IR
 V = potential difference in volts (V)
 I = current in amperes (A)
 R = resistance in ohms ()
Potential difference (V)
Current (A)
125
2.5
1.3
80
36 000
900
85
120
13.7
960
72
2.8
2.3
700
4.0
175
Resistance ()
50
60
40
0.71
70
27.9
300
0.02
1. Find the resistance of an electric light bulb if a current of 0.80 A flows when the
potential difference across the bulb is 120 V.
150 
2. What is the potential difference across a toaster of resistance 13.7  when the
current flowing through it is 8.75 A?
120 V
3. What current is flowing through an electric baseboard heater with a resistance of
38  when the potential difference across it is 240 V?
6.3 A
4. An iron designed for use at 120 V and 5.0 A is connected to a power source of
240 V. Calculate the current the iron will draw at the higher potential. State what
will happen to the iron.
10 A
5. What potential difference is required to produce a current a 8.0 A in a load having
a resistance of 64 ?
512 V
6. An electric toaster has a resistance of 12 . What current will it draw from a
power supply of 120 V?
10 A
Grade 9 Academic Science – Unit 4 Physics (Electricity)
Solving Problems with Ohm’s Law (V = IR)
Section 13.9 Pages 568-570
V=IR
 V = potential difference in volts (V)
 I = current in amperes (A)
 R = resistance in ohms ()
Potential difference (V)
Current (A)
2.5
80
900
85
120
960
72
2.8
700
4.0
175
Resistance ()
50
60
40
70
300
7. Find the resistance of an electric light bulb if a current of 0.80 A flows when the
potential difference across the bulb is 120 V.
8. What is the potential difference across a toaster of resistance 13.7  when the
current flowing through it is 8.75 A?
9. What current is flowing through an electric baseboard heater with a resistance of
38  when the potential difference across it is 240 V?
10. An iron designed for use at 120 V and 5.0 A is connected to a power source of
240 V. Calculate the current the iron will draw at the higher potential. State what
will happen to the iron.
11. What potential difference is required to produce a current a 8.0 A in a load having
a resistance of 64 ?
12. An electric toaster has a resistance of 12 . What current will it draw from a
power supply of 120 V?
Grade 9 Academic Science – Unit 4 Physics (Electricity)
Ohm’s Law – What Does it mean?
Ohm's Law deals with the relationship between voltage (V) and current (I). The
relationship states:
The potential difference (voltage) across a conductor is
proportional to the current through it.
The constant of proportionality is called resistance (R).
Ohm’s Law is given by V = I R where V is the potential difference between two
points which include a resistance (R). I is the current flowing through the
resistance.
What Does the Definition and Formula for Ohm’s Law Mean?
If V varies (i.e., changes) and R is
…then I must
constant (i.e., does not change),
If voltage changes and current is does
not change,
…then resistance must
If both R and I change,
…then V must
If a light bulb glows brighter and the
current measured in the circuit remains
the same,
…then the voltage must
If R = 50  and the voltage decreases
from 10 V to 5 V,
….then, the current must
An experiment is conducted that varies
voltage. The amps are measured.
With changing voltage, the amps also
change. A graph of voltage versus
current is drawn, and a straight line
relationship is revealed.
Thus,
Grade 9 Academic Science – Unit 4 Physics (Electricity)
Solving Problems with V = E / Q
You can solve problems with a GRASP method.
G = Given
R = Required
A = Analyze
S = Solution
P = Present
(Write down what is given in the problem)
(Identify what you are required to find)
(Write down the formula needed, rearrange the formula if
needed, convert any units if necessary)
(Plug in the given values and solve the problem)
(Write a final statement identifying your solution)
1. A battery converts 72 J of energy into electrical energy. This places 12 C
of negative charge at the negative terminal. What is the potential
difference between the positive and negative terminals of the battery?
2. How much energy can an electric motor produce when it is plugged into a
110 V outlet with a total charge of 50 C passing through the motor?
3. Gasoline powered generators change chemical energy (in gasoline) into
electrical energy. If a generator produces 120 V with a charge of 60 C into
a negative terminal, how much energy is produced?
4. If a generator in Question #3 burned enough gasoline to produce 28.8 kJ,
what would be the charge be at the negative terminal?
Solving Problems with V = E / Q
Answer Sheet
1. A battery converts 72 J of energy into electrical energy. This places 12 C
of negative charge at the negative terminal. What is the potential
difference between the positive and negative terminals of the battery?
V
=E/Q
= 72 / 12
=6
The potential difference is 6 volts
2. How much energy can an electric motor produce when it is plugged into a
110 V outlet with a total charge of 50 C passing through the motor?
E
=VQ
= 110 X 50
= 550
The energy passing through the motor is 550 J.
3. Gasoline powered generators change chemical energy (in gasoline) into
electrical energy. If a generator produces 120 V with a charge of 60 C into
a negative terminal, how much energy is produced?
E
=VQ
= 120 X 60
= 7200
The generator produced 7 200 J of energy.
4. If a generator in Question #3 burned enough gasoline to produce 28.8 kJ,
what would be the charge be at the negative terminal?
28.8 kJ = 28 800 J
V
=E/Q
= 28 800 / 60
= 480
The charge at the negative terminal is 480 V.