1© Manhattan Press (H.K.) Ltd Factors affecting resistance of a conductor Physical dimension Material Effect of temperature on resistance

2 © Manhattan Press (H.K.) Ltd Factors affecting resistance of a conductor (SB p.131) Physical dimension

3 © Manhattan Press (H.K.) Ltd Factors affecting resistance of a conductor (SB p.131) Material Different materials have different conducting properties  - resistivity = unit:  m

4 © Manhattan Press (H.K.) Ltd Factors affecting resistance of a conductor (SB p.132) Material Resistivity (  ) /  m Metals: copper gold aluminium 1.7 x x x Semiconductors:germanium (pure) silicon (pure) x 10 3 Insulators: glass perspex polyethylene sulphur ~ ~ ~ ~ Go to Example 4 Example 4 Go to Example 5 Example 5

5 © Manhattan Press (H.K.) Ltd Factors affecting resistance of a conductor (SB p.134) Effect of temperature on resistance 1. Effect on conductors - In solids, atoms vibrate about their equilibrium positions - temperature increases, amplitude of vibration larger - chance of collision with free electrons higher - resistance higher - current lower

6 © Manhattan Press (H.K.) Ltd Factors affecting resistance of a conductor (SB p.134) Effect of temperature on resistance 2. Temperature coefficient of resistance R  - resistance at temperature  = R 0 (1 +   ) Notes: One of the features of temperature coefficient of resistance is that for many metals its value is close to o C –1, which is the reciprocal of 273 o C.

7 © Manhattan Press (H.K.) Ltd Factors affecting resistance of a conductor (SB p.135) Effect of temperature on resistance 3. Effect of temperature on semiconductor Semiconductor - neither good conductor of electricity nor good insulator - at low temperature, pure semiconductors like insulators - at high temperature, no. of conduction electrons increases (resistance decreases) Go to More to Know 3 More to Know 3

8 © Manhattan Press (H.K.) Ltd Factors affecting resistance of a conductor (SB p.135) Effect of temperature on resistance 4. Effect of temperature on superconductor Go to More to Know 4 More to Know 4 An element, an inter-metallic alloy, or a compound that will conduct electricity with zero resistance below certain low temperature (critical temperature) is known as a superconductor.

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10 © Manhattan Press (H.K.) Ltd. Q: Q: A thin-film resistor is made of nichrome of thickness 1 μm. If the resistivity of nichrome is 10 –6 Ω m, calculate the resistances between the opposite edges of such a film of area 1 mm 2, (a) if it is a square, and (b) if it is a rectangle whose length is 20 times its breadth Factors affecting resistance of a conductor (SB p.132) Solution

11 © Manhattan Press (H.K.) Ltd. Solution: (a) The resistance between the edges P and Q is: Return to Text (b) The resistance between the edges P and Q is: The resistance between the edges C and D is: 15.5 Factors affecting resistance of a conductor (SB p.132)

12 © Manhattan Press (H.K.) Ltd. Q: Q: An electricity supply cable consists of a steel core of cross-sectional area 50 mm 2 with six other aluminium conductors of the same cross-sectional area arranged around it. Find the resistance of a 120 m length of the cable. (Resistivities: steel: 9.0 × 10 –6 Ω m; aluminium: 2.5 × 10 –8 Ω m) 15.5 Factors affecting resistance of a conductor (SB p.133) Solution

13 © Manhattan Press (H.K.) Ltd. Solution: Resistance of steel cable (R S ) Return to Text The equivalent resistance of the six aluminium cables is equal to a single aluminium cable of length 120 m and cross-sectional area (6 ×50) mm 2. The resistance R of the electricity cable is given by: 15.5 Factors affecting resistance of a conductor (SB p.133)

14 © Manhattan Press (H.K.) Ltd. Unique features of semiconductors Computer chips, both for CPU and memory, are composed of semiconductors. Semiconductors are widely used in electronic industry. It is because they have a unique atomic structure that allows their conductivity to be controlled by stimulation with electric currents, electromagnetic fields, or even light. This makes it possible to construct devices from semiconductors that can amplify, switch, convert sunlight to electricity, or produce light from electricity. Return to Text 15.5 Factors affecting resistance of a conductor (SB p.135)

15 © Manhattan Press (H.K.) Ltd. Maglev Magnetic-levitation is an application where superconductors perform extremely well. Transport vehicles such as magnetic levitation trains (maglev) can be made to "float" on strong superconducting magnets. The train is propelled forwards using the repulsive and attractive forces of magnetism. Because of the lack of physical contact between the track and the vehicle, the only friction is that between the carriages and the air. Consequently, maglev trains can travel at very high speeds with reasonable low energy consumption and noise levels. Return to Text 15.5 Factors affecting resistance of a conductor (SB p.135)