OCR AS Physics

Revision Notes

4.3.2 Electrical Energy

Electrical Energy Transfer

  • The electrical power is also defined as the rate of change of work done:

Power equation, downloadable AS & A Level Physics revision notes

  • The work done is the energy transferred
    • The power is, therefore, the energy transferred per second in an electrical component
  • Rearranging the energy and power equation, the energy can be written as:

W = Pt = IVt

  • Where:
    • W = Work done / energy transferred (J)
    • P = power (W)
    • V = voltage (V)
    • I = current (A)
    • t = time (s)

Calculating the Cost of Energy & The Kilowatt-hour (kWh)

  • The power of an appliance is:

The amount of energy transferred (by electrical work) to the device every second

power-rating, IGCSE & GCSE Physics revision notes

The power consumption of an electrical appliance can be found on a label that looks like this. This kettle uses between 2500 and 3000 W of electrical energy

  • This energy is commonly measured in kilowatt-hour (kW h), which is then used to calculate the cost of energy
    • This is used to calculate electricity bills
  • A kilowatt-hour is defined as:

A unit of energy equal to 1 kW of power sustained for 1 hour

  • Or as an equation:

Energy (kW h) = Power (kW) × Time (h)

  • Since the usual unit of energy is joules (J), this is the 1 W in 1 s
  • Therefore:

1 kW h = 1000 W × 3600 s = 3.6 × 106 J

  • Since 1 kW = 1000 W and 1 h = 3600 s
  • To convert between Joules and kW h:

kW h  × (3.6 × 106) = J

J  ÷ (3.6 × 106) = kW h

  • The kW h is a large unit of energy, and mostly used for energy in homes

Worked Example

A cooker transfers 1.2 × 109 J of electrical energy to heat. How much will this cost if 1 kW h costs 14.2p?

Step 1: Convert from J to kW h

(1.2 × 109) ÷ (3.6 × 106) = 333.333 kW h

Step 2: Calculate the price

1 kW h = 14.2 p

333.333 × 14.2 = 4733 p = £47.33

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