2. Chapter Twenty: Electric Circuits
20.4 Resistance and Ohm’s Law

3. 20.4 Resistance
Resistance is the measure of how strongly an object resists current flowing through it.
The relationship between electric current and resistance can be compared with water flowing from the open end of a bottle.

4. 20.4 Resistance
The total amount of resistance in a circuit determines the amount of current in the circuit for a given voltage.

5. 20.4 Resistance of common objects
Every electrical device is designed with a resistor that causes the right amount of current to flow when the device is connected to voltage.

6. 20.4 Resistance
Electrical resistance is measured in units called ohms.
This unit is abbreviated with the Greek letter omega (Ω).

7. How much current does it draw?
Solving Problems
A toaster oven has a resistance of 12 ohms and is plugged into a 120-volt outlet.
How much current does it draw?

8. 20.4 Ohm’s Law
The current in a circuit depends on voltage and resistance.
Ohm’s law relates current, voltage, and resistance with one formula.

9. 20.4 Ohm’s Law
If you know two of the three quantities, you can use Ohm’s law to find the third.

10. Solving Problems Looking for: Given Relationships: Solution
…current in amps
Given
…R = 12 ; V = 120 V
Relationships:
I = V R
Solution
I = 120 V
12 
= 10 A

11. Practice #1
How much current is in a circuit that includes a 9-volt battery and a bulb with a resistance of 3 ohms?

12. Practice # 2
How much current is in a circuit that includes a 9-volt battery and a bulb with a resistance of 12 ohms?

13. Practice # 5
What is the voltage of a circuit with 15 amps of current and toaster with 8 ohms of resistance?

14. Practice # 8
How many ohms of resistance must be present in a circuit that has 120 volts and a current of 10 amps?