Question 1

A torch uses a battery to light a bulb. Explain how energy is transferred in this circuit, starting from the battery.

Accepted Answer

1. The battery stores chemical energy.
2. This chemical energy is converted into electrical energy in the circuit.
3. The electrical energy is transferred to the bulb.
4. At the bulb, electrical energy is converted into light energy and thermal energy (heat), which are released into the surroundings.

Question 2

State two energy transfers that occur when an electric heater is switched on.

Accepted Answer

1. Electrical energy to thermal energy (heating the element).
2. Thermal energy to the surroundings (through convection and radiation).

Question 3

A lamp is connected to a 12V power supply. If the current flowing through the lamp is 0.5A, calculate the power consumed by the lamp.

Accepted Answer

Power (P) = Voltage (V) x Current (I)
P = 12V x 0.5A
P = 6W

Therefore, the power consumed by the lamp is 6 Watts.  This means the lamp is converting 6 Joules of electrical energy into light and heat energy every second.

Question 4

State the equation that relates electrical power, voltage, and current. Briefly explain what each term represents.

Accepted Answer

Power (P) = Voltage (V) x Current (I)

P (Power): The rate at which electrical energy is converted into other forms of energy (e.g., heat, light). Measured in Watts (W).
V (Voltage): The electrical potential difference across a component. It is the energy required to move a unit charge through the component. Measured in Volts (V).
I (Current): The rate of flow of electrical charge through a component. Measured in Amperes (A).

Question 5

A 6V torch bulb has a current of 0.5A flowing through it. Calculate the electrical energy transferred in 2 minutes.

Accepted Answer

Electrical energy = Voltage x Current x Time
E = V x I x t
E = 6V x 0.5A x (2 x 60)s
E = 360 J

Explanation: The formula E=VIT is used to calculate electrical energy transformed, and the time is converted into seconds before using it in the equation.

Question 6

A heater transfers 7200J of electrical energy when connected to a 240V mains supply for 1 minute. What current flows through the heater?

Accepted Answer

Electrical energy = Voltage x Current x Time
E = V x I x t
7200J = 240V x I x 60s
I = 7200J / (240V x 60s)
I = 0.5A

Explanation: First rearrange the energy equation to solve for current (I=E/VT).

Question 7

An electric heater is rated at 2.0 kW. It is used for 3 hours per day for 30 days. Calculate the total cost of using the heater if electricity costs $0.20 per kilowatt-hour (kWh).

Accepted Answer

**1. Calculate total energy consumption:**
Energy (kWh) = Power (kW) × Time (hours)
Energy = 2.0 kW × 3 hours/day × 30 days = 180 kWh

**2. Calculate total cost:**
Cost = Energy (kWh) × Cost per kWh
Cost = 180 kWh × $0.20/kWh = $36.00

**Answer:** The total cost of using the heater is $36.00

Question 8

Define the kilowatt-hour (kWh). Explain what it measures and what units it relates to.

Accepted Answer

The kilowatt-hour (kWh) is a unit of energy.

It measures the amount of electrical energy used by a 1 kilowatt (kW) device operating for 1 hour.

1 kWh is equivalent to 3,600,000 Joules (J). It relates power (kW) and time (h) to energy consumed.

Electrical energy and electrical power

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