Edexcel IGCSE Physics- electricity questions*

State the equation linking voltage, current and resistance

Voltage = Current x Resistance

Explain what is meant by the term direct current

Current that passes in one direction only

A hairdryer connected to the mains supply takes a current of 5.5 A. Which of these fuses should be used with the hairdryer? 5A or 7A, and why?

7A because it is the nearest above and fuse value must be more than working current.

The hairdryer has a plastic case so there is no need for an earth wire connection in the plug.

Explain why the hairdryer is still safe to use.

Comment on device- plastic is an insulator so does not conduct - double insulated

Comment on user- no risk of shock

Describe the advantages and disadvantages of a series circuit when used as a heater in a car window

series advantage - fewer wires;

series advantage - lower resistance values;

series disadvantage - one fails, circuit fails;

series disadvantage - no independent control

A light dependent resistor (LDR) can be used as a sensor to detect light intensity. Describe how the resistance of an LDR varies as the light intensity changes.

Bright light low resistance/Dim light high resistance;

Inverse relationship between resistance and light intensity

Non-linear relationship

Describe how an earth wire acts as a safety feature (4 marks)

The earth wire is connected to the metal casing of an object (1), so if the casing becomes live (1), the earth wire provides a low resistance path to earth (1)

Therefore, a large current flows in the earth wire so the fuse melts (1)

This breaks the circuit and the current stops. (1)

Explain how a student can use a thermistor, voltmeter and ammeter to investigate how the resistance of the thermistor varies between 0'C to 100'C

Measure current and voltage at any known temperature (2)

Measure and vary temperatures while taking new readings (2)

Calculate R = V/I (1)

Repeat and average at any stage (1)

Explain how a fuse works (3 marks)

If the current becomes too high (1) then the fuse over heats and melts (1) causing the circuit to break and turning off the appliance (1)

What is the purpose of a fuse

To prevent over heating in the circuits

Resistor Y has a higher resistance than Resistor X. Explain how the number of charged particles passing through the cell each second changes when Resistor X is replaced with Resistor Y.

There are fewer charged particles passing through the cell each second because the current in the circuit is decreased and current is the rate of flow of charge.

Give a reason why a variable resistor would be needed in a circuit if a student is investigating how the resistance of a lamp varies as the current is changed

In order to vary the current/resistance

Explain why the thin piece of metal moves away from the metal rod (Gold Leaf Experiment)

Due to the friction causes by the rubbing between the powder and the plastic (glass rod and cloth), the powder is now charged. This charge is then transferred from the metal rod to thin piece of metal as it is able to be conducted. Since the metal rod and thin piece of metal have the same charge, they repel.

Explain how a thermistor can be used to vary the current in a circuit (3 marks)

Resistance of thermistor changes (1) as it decreases as temp increases - inverse relationship. (1) Therefore, current increases as temp increases (1) because if resistance decreases current is able to increase

What happens to a resistor when an electric current passes through it? How is this useful in toasters

Energy is transferred to the resistor, which heats the resistor. Toasters contain a coil of wire with a high resistance. When a current passes through the coil, it's temperature increases so much that it can give off infrared (heat) radiation, which cooks the bread.

Why does current in a resistor result in the electral transfer of energy and an increase in temperature, and how can this be used in a variety of domestic contexts

Resistance causes transfer of electrical energy to heat energy. Some components are designed to have a high resistance to make sure this happens, for example electrical heaters that have lots of resistors to ensure a high resistance so a lot of heat is produced.