Question

This filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light energy. a) How much energy is wasted? b) In what form is the energy wasted? c) What is the efficiency of the bulb?

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Answer 1

Answer:

The efficiency of a device, such as a lamp, can be calculated:

efficiency = useful energy out ÷ total energy in (for a decimal efficiency)

or

efficiency = (useful energy out ÷ total energy in) × 100 (for a percentage efficiency)

The efficiency of the filament lamp is 10 ÷ 100 = 0.10 (or 10%). This means that 10 per cent of the electrical energy supplied is transferred as light energy (90 per cent is transferred as heat energy).

The efficiency of the energy-saving lamp is 75 ÷ 100 = 0.75 (or 75 per cent). This means that 75 per cent of the electrical energy supplied is transferred as light energy (25 per cent is transferred as heat energy).

Note that the efficiency of a device will always be less than 100 per cent. Occasionally the power is shown in W instead of the energy in J. The equations are the same – just substitute power for energy:

efficiency = useful power out ÷ total power in (for a decimal efficiency)

or

efficiency = (useful power out ÷ total power in) × 100 (for a percentage efficiency)