Ohm's law

Ohm's law is a formula discovered by Ohm which helps us to measure the electric current in a circuit.

Before I show you what the formula is, we need to define the voltage source and the electric resistance. 

What is voltage source?

In the lesson about electric current, we showed that the electric current is a flow of electric charge.

Well, charges don't flow unless it is connected to a voltage source such as a battery. 

Simply put, a voltage source is like an electric pump. For example, a battery is like an electric pump since the positive terminal of the battery will attract electrons or negative charges as already explained in the lesson about electric current. 

Another example of voltage source is a generator. Electric power companies use big generators in order to deliver 120 volts to home outlets.

In summary a voltage source is a device used to move electric current in a circuit. The higher the voltage, the more current will flow in the circuit. Home outlets can deliver 120 volts while a AA battery could deliver 1.5  volts.

voltage symbols

DC stands for direct current such as batteries, solar cells, etc...

AC stands for alternating current such as your home outlets.

What is electric resistance?

While a voltage source causes current to flow in a circuit, an electric resistance will resist the flow of electric charge in a circuit. A device which causes electric resistance is called a resistor.

The electric resistance may depend on the following factors

  • The conductivity of the material 
  • The thickness of the material
  • The length of the material
  • The temperature

Some materials allow charges to flow more easily. For example, copper is a better conductor than iron.

Thin wires have more resistance than thick wires.

Longer wire have more resistance than short wires.

The higher the temperature, the more the conductor will resist the flow of electric charges.

Resistance symbol

Ohm's law formula

Ohm's law shows the relationship between the current, the voltage, and the resistance.

current =
voltage / resistance

I =
V / R

Based on what we saw so far, the electric current (I) depends on the voltage and the resistance.

The unit for the resistance is Ohm.

Notice that the voltage is in the numerator. It makes sense since as the voltage increases, so will the electric current. On the other hand, the resistance is in the denominator. As the resistance increases, the electric current will also decrease. 

1 ampere is the resulting current when the voltage is 1 and the resistance is 1 ohm.

1 ampere circuit

Ohm's law quiz

Take this Ohm's law quiz to see how well you understand Ohm's law. After completing this quiz with 100% accuracy, you will know exactly what the Ohm's law is and how to use it to solve problems. You will not need to use a paper and pencil to complete this quiz.

First, read the lesson about Ohm's law and then take this quiz.

Objective of the Ohm's law quiz:

  • Identify the Ohm's law formula
  • Know what a voltage source is
  • Know what an electric resistance is
  • Use Ohm's law to find the current or the voltage
Test your knowledge with the quiz below:
1. Choose the correct equation for Ohm's law

I = R/V
R = VI
I = V/R
O = RI.

2. An example of a device that is not a voltage source is

Battery
Generator
Voltage relays
Electrical outlets.

3. By analogy, a voltage source is like

a water pump
an air pump
a heart
All the above

4. The voltage of a AAA battery is likely to be

20 volts
2 volts
8 volts
None of the above

5. The job of an electric resistance is to

increase the voltage by resisting the current
decrease the voltage by resisting the current
decrease the current by resisting the voltage
None of the above

6. In which material you will expect the current to flow more easily?

1.5 mm thick and 10 m long
1.6 mm thick and 9.1 m long
1.45 mm thick and 9 m long
1.60 mm thick and 9 m long

7. The positive terminal of a voltage source will attract electrons because

electrons have negative charges.
electrons have positive charges
electrons have positive affinity with the voltage source
electrons have negative affinity with the voltage source

8. What is the current of a circuit flowing through a 50 Ohms resistor and powered by a voltage source of 100 volts?

2 volts
1 A
2 A
Not enough information

9. If the current flowing through a 6 Ohms resistor is 2 A, what is voltage of the circuit?

24 volts
8 volts
3 volts
12 volts

10. An example of DC is a

car battery
home outlet
car alternator
direct current



Score =

I strongly recommend you try your very best to complete this Ohm's law quiz on your own. Resist the urge to see the answers. Review the lesson about Ohm's law again. However, if you still cannot get 100% on your own, click here to see the answers.


Ohm's law word problems