True or False?

1. True or False? F • The electrons in a circuit move in the same direction as the current. • The current through a component is directly proportional to the voltage if the resistance of the component stays the same. • The resistance in a wire is directly to the thickness of the wire. T F

2. True or False? • In a filament lamp the resistance increases with the voltage. • The electrons lose part of the E.M.F. provided by the battery as they go around the circuit once. • In a thermistor the resistance is higher when the weather is cooler. T F T

3. Resistance in Series and Parallel • Resistance in series circuits • Resistance in parallel circuits

4. Resistance in series circuits Use Ohm’s law to calculate the voltage across the resistors in the next slide. Write your values in the voltmeters and see if they agree with the actual readings.

5. .603 Amps 2.2 Ω 1.5 Ω .897 1.332 Volts Volts

6. What should the voltage across the two resistors be? .601 Amps 2.2 Ω 1.5 Ω 2.231 Volts

7. Equivalent resistance The two resistors in the previous series circuit can be thought of as a single resistor. Work out and write the equivalent resistance using Ohm’s law and the values of current and voltage shown. 0.601 A 2.231 V 3.7 Ω

8. Equivalent resistance In a series circuit the total resistance of the components is the sum of the resistance of each component. So, the equivalent resistance R is found as: R = R1 + R2 Equivalent Circuit Real Circuit R1 R2 R 2.2 Ω 3.7 Ω 1.5 Ω

9. Resistance in parallel circuits Look at the parallel circuit on the next slide and work out the current in the main circuit and through each resistor in the parallel branches.

10. What do you think the current in the main circuit should be? 1.830 1.991 Volts Amps 2.2 Ω 0.832 A 1.716 Volts 1.144 A 1.5 Ω

11. Resistance in parallel circuits The current in the main circuit is the sum of the currents in the parallel branches: I = I1 + I2 I R1 I1 R2 I2

12. Work out the combined resistance in the main circuit with these values of current and voltage 1.830 1.991 Volts Amps 2.2 Ω 1.716 Volts 1.5 Ω

13. Combined resistance Using the values of V and I in the previous slide and Ohm’s law to work out the combined resistance, we find: I R1=2.2 Ω I1 V 1.8 V R = = = 0.9 Ω I 1.99 A R2=1.5 Ω I2 What can you notice about the combined resistance?

14. Combined resistance The combined resistance R = 0.9 Ω is less than R1 = 2.2 Ω or R2 = 1.5 Ω. You can work out the combined resistance of a parallel circuit using these formulae: or What is the combined resistance when the resistors are the same?

15. 8 A 2.5 A 12 W 32 W V V V V V V A W What is the equivalent circuit? 8 A 80 30 2.5 A 12 W 32 W This is a series circuit where the current is 2.5 A (the reading inside the branch). What is the voltage across the two resistors? What is the equivalent circuit? Split the branches in two separate circuits. What is this circuit? A W V = R x I

16. 8 A 2.5 A 12 W 32 W V V V V V V A W What is the equivalent circuit? Now you can deal with this. What is the voltage, current and resistance? 8 A 110 80 30 2.5 A 44 W 12 W 32 W 110 What is the equivalent resistance and the voltage across? A W 20 5.5