Mr. Storie
Electricity – Ohm’s Law
10S Science
Ohm’s Law
The complete expression called Ohm's Law identifies the relationship among current, voltage, and
resistance. When a wire is connected to the terminals of a source, a complete path called a circuit is formed.
Electric charge (electrons) can flow through a circuit. A flow of charge is called an electric current. The
higher the electric current in a wire, the more electrons are passing through it. The symbol for current is the
letter (I). The unit in which current is measured is the ampere (A). The ampere, or amp for short, is the
amount of charge that flows past a point per second.
The amount of energy stored in an electron when it is separated by chemical reactions in a cell is called the
electric potential. Voltage is the electric potential (energy) difference between the charges at two points in a
circuit. The unit used to measure electric potential is called the volt (V).
The movement of charges through a circuit is resisted by the atoms and molecules of the conductor, and
objects (loads) in the circuit. The amount of electric potential energy lost by the charges depends on the
resistance of the material they are moving through (how hard it is to move). The symbol for resistance is
(R). The resistance of an object is measure in Ohm’s (Ω).
Ohm's Law states that the voltage (V) in a circuit is equal to the current in a wire (I) multiplied by the
resistance (R).
V
I R
Pot. Difference = Resistance x current
V = I·R
Units: R is Ω (ohms)
V is V (volts)
I is A (amperes)
Looking at the equation triangle above (cover the unit you are trying to find):
The equations for resistance would be:
The equations for current would be:
Resistance = Voltage
Current
Current =
R= V
I
Voltage
Resistance
I= V
R
Example:
If the resistance in a wire is 75 ohms and the voltage is 12 volts, what is the current through the circuit?

Find the correct formula. Plug in the number. Make sure you include the units.
I= V
R
I = 12 V
75 Ω
I = 0.16 A
Mr. Storie
1. What is electric current?
10S Science
Electricity – Ohm’s Law
2. What is resistance?
3. What is voltage?
4. Find the unknown quantity:
a) I = 10 A
R = 1500 Ω
V=?
b) I = ?
R = 200 Ω
V = 240 V
c) I = 15 A
R=?
V = 110 V
Calculating Current
1. What is the current produced with a 9-volt battery through a resistance of 100 Ω?
2. Find the current for the circuit in the schematic below.
3. If the potential difference (Voltage) is 120 volts and the resistance is 50 ohms, what is the current?
4. What would be the current in Problem 3 if the potential difference were doubled?
5. What would be the current in Problem 3 if the resistance were doubled?
Mr. Storie
Calculating Voltage
Electricity – Ohm’s Law
10S Science
6. What voltage produces a current of 50 amps with a resistance of 20 ohms?
7. What voltage produces a current of 500 amps with a resistance of 50 ohms?
Calculating Resistance
8. What resistance would produce a current of 200 amperes with a potential difference of 200 volts?
9. A 12-volt battery produces a current of 25 amperes. What is the resistance?
10. A 9-volt battery produces a current of 2.0 amperes. What is the resistance?
11. An overhead wire has a potential difference of 2000 volts. If the current flowing through the wire is one
million amperes, what is the resistance of the wire?
12. What is the resistance of a light bulb if a 120-volt potential difference produces a current of 0.8 amperes?
Converting Common Units and Conversions:
1000 milliamps (mA) = 1 amp
1000 millivolts (mV) = 1 volt
Changing from milli:
20000 mA
1000
= 20 A
Changing to milli:
5A x 1000 = 5000 mA
5V x 1000 = ______ mV
20000 mV = _________ V
1000
Mr. Storie
10S Science
13. Find the unknown quantity (CONVERT FIRST)
a) I = ?
R = 2000 Ω
V = 20 mV = ________ V
b) I = 25 mA = _________ A
R=?
V = 110 V
Electricity – Ohm’s Law
c) I = 1 kA = ________ A
R=?
V = 50 mV = ________ V
14. How much resistance does a light bulb create if it has a current of 25 mA around it in a 9 V circuit?
15. A heating coil offers a resistance of 2.5 kΩ. What voltage is required so that 1.5 A of current pass
through it?
16. The human body offers a very small amount of resistance (let’s say 1 mΩ for argument). If a lightning
bolt (said to have 1.21 GV of potential), hits you how much current is flowing through your body.