Section 5 -Electricity

• ?=RA/L (Resistivity equation rearranged)

  Find Rho ρ
• Explain why the efficiency of electrical power transmission is improved when conventional wires are replaced with superconducting wires. (1 Mark) Hint : think about what a superconductor is

  The efficiency of electrical power transmission is seem to be improved due to zero power lost. This is because superconducting wires do not have a resistance at transitional temperatures therefore there is no resistance. No resistance shows there would be no power lost.
• What is the emf - Definition (I Mark)


  An emf is the potential difference into a circuit from a power supply when there is no current passing through the cell or open circuit.
• State and explain what happens to the voltmeter reading when the temperature of the thermistor is decreased (2 Marks)

  As the temperature of the thermistor decreases, the resistance of the thermistor would increase. This would increase the voltmeter reading (Potential difference).
• State and explain what happens to the reading on the voltmeter if the intensity of light incident on the LDR increases.

  The reading on the voltmeter would decrease as the resistance of the LDR has decreased. This is due to light being more incident on the LDR which decreases the resistance.
• Draw a graph showing how the voltage across an LED changes as its forward current varies between 0 mA and 30 mA. The graph must include axis labels and a clear scale.
• Explain this observation : A second voltmeter, of resistance 5.0 kiloohms is connected in parallel with the voltmeter shown in the diagram. The reading on the very high resistance voltmeter falls as a consequence. Hint : Draw a diagram to help explain

  The total resistance has decreased. This means the total current would increase causing the voltage reading to decrease.