1. 1 14.3 Resistance Warm-up Introduction Resistance Ohm’s law Measuring resistance ? Factors affecting resistance Resistors The multimeter Check-point 3 ?

2. 2 14.3 Resistance Warm-up 1Margaret connects identical light...Margaret connects identical light... 2How does wetness affect the...How does wetness affect the...

3. 3 14.3 Resistance Warm-up 1Margaret adds light bulbs in a circuit gradually until bulbs are very dim. Why are the bulbs very dim? Resistance of each bulb becomes large. Voltage across each bulb becomes too small. Current becomes too small. Others:_______________________

4. 4 14.3 Resistance Warm-up 2How does wetness affect the resistance of our body? Increase. No change. Decrease.

5. 5 14.3 Resistance 0Introduction Current passes through some conductors more easily than others. The opposition of a conductor to current is called resistance. Good conductor low resistance Bad conductor high resistance

6. 6 14.3 Resistance 1Resistance Resistance of a conductor is... resistance = Unit: ohms (  ) (1  = 1 V A –1 ) R = V I voltage across conductor current through conductor

7. 7 14.3 Resistance 1Resistance If 1 A flows through a resistor and 1 A resistor 1 V = 1  the voltage across the resistor is 1 V, the resistor has 1 .

8. 8 14.3 Resistance Resistance Question ? Example 3 e.g. 3

9. 9 14.3 Resistance Experiment 14a Ohm’s law Use this circuit to find out the relation between the current passing through the wire and the voltage across it. Simulation Video

10. 10 14.3 Resistance 2Ohm’s law In Expt 14a, the V-I graph obtained is... voltage / V current / A 0 1 2 3 4 0.2 0.4 0.6 0.8 1.0      straight line passing origin V  I (Ohm’s law)

11. 11 14.3 Resistance 2Ohm’s law Voltage across a conductor  current passing it, if the temperature and other physical conditions are unchanged. Ohm’s law: Ohm’s law applies to some conductors only. If a conductor does not obey Ohm’s law, its resistance is still V/I but it changes. Note:

12. 12 14.3 Resistance 3Measuring resistance Measure the current passing a bulb and the voltage across it. + – A V 12 V power supply 12 V 24 W Then by R = V/I, the resistance of the bulb can be found. This method is called voltmeter-ammeter method.

13. 13 14.3 Resistance 4Factors affecting resistance aTemperature For most metals, resistance temperature But for carbon and some semiconductors, resistance temperature

14. 14 14.3 Resistance Experiment 14b How does the resistance of a wire change with the length and the thickness? Effect of length and thickness on resistance Simulation Video

15. 15 14.3 Resistance Wires of the same thickness and material, resistance R  length l 4Factors affecting resistance bLength and thickness of wire (l   R )(l   R ) Wires of the same length and material, resistance R  (cross-sectional area A) –1 (A   R )(A   R )

16. 16 14.3 Resistance 4Factors affecting resistance bLength and thickness of wire l   R  and A   R A   R  R R  l A

17. 17 14.3 Resistance 5Resistors aFixed resistors Resistor = a device with known resistance Fixed resistors as their resistance is fixed. E.g.

18. 18 14.3 Resistance 5Resistors bVariable resistors Rotate the slider to vary the length of the resistance track and hence the resistance. E.g. resistance track movable contact Their resistance can be changed.

19. 19 14.3 Resistance 5Resistors cRheostats They are large-current variable resistors used in school laboratory. Slid the contact to vary the length of wire through which the current flows  change its resistance B A Connect it at A and B  fixed resistor

20. 20 14.3 Resistance 6The multimeter Multimeter can measure current, voltage and resistance. multi-range for measurements red and black probes touch different points of the circuit

21. 21 14.3 Resistance Experiment 14c Use a multimeter to measure the current through, the voltage across and the resistance of a lamp. Using the multimeter Video

22. 22 14.3 Resistance Check-point 3 1When a resistor is replaced...When a resistor is replaced... 2A V-I graph of a bulb is obtained...A V-I graph of a bulb is obtained... 3Voltage across a resistor...Voltage across a resistor... 42 metal wires, P and Q, of the...2 metal wires, P and Q, of the...

23. 23 14.3 Resistance Q1When a resistor is... AHalved to 0.7 V.BRemains at 1.4 V. CSlightly larger DDoubled to 2.8 V. than 1.4 V. An iron nail replaces a resistor. The voltmeter reading is 1.4 V before the replacement. What is the voltmeter reading after replacement? 0.4 A  0.2 A 1.4 V  ? V

24. 24 14.3 Resistance Q2A V-I graph of a bulb... A V-I graph of a bulb is obtained. voltage / V current / A 0 1 2 3 4 1 2 34 1st statement: At V = 0, the resistance of the bulb is 1 . 2nd statement: The slope of the V-I graph is 1 . A

25. 25 14.3 Resistance Q3Voltage across a... Voltage across a resistor  current passing through it. What would happen to the current passing through a resistor if both voltage across it and its resistance are (a)doubled, (increases/decreases/unchanged) (b)halved? (increases/decreases/unchanged)

26. 26 14.3 Resistance Q42 metal wires, P and... The radius of Q __________ be found. 2 metal wires, P and Q, of the same metal have the same length but different radii. The dimension of P is known. Can the radius of Q be found with voltmeter, ammeter and cell? Set up a circuit, with metal wire connected in _______ with the cell and the __________ and in ________ with voltmeter. can series ammeter parallel

27. 27 14.3 Resistance Since R  lA –1, where l is _______ and A is _________________ of the wire, compare R P and R Q and use A = ___ to find the radius of Q. Q42 metal wires, P and... Then by _________, the resistance of P and Q, R P and R Q, can be found. 2 metal wires, P and Q, of the same metal have the same length but different radii. The dimension of P is known. Can the radius of Q be found with voltmeter, ammeter and cell? V = IR length cross-sectional area r2r2

28. 28 14.3 Resistance The End

29. 29 14.3 Resistance Example 3 Resistance When an iron is plugged into the 220 V mains supply, a current of 5 A flows through the heating element. What is the resistance of the heating element? Resistance = 44  R = V I = 220 5

30. 30 14.3 Resistance Return