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Lesson: Chapter 10 — Power and energy in electrical appliances

Putting the chapter together 2 of 3

The exercise

Worked answers to exercise 10.1.

Question 1

A water pump driven by an electric motor has a power of 750 W.

  1. On a 230 V supply, what current does the motor take?
  2. Besides kinetic energy, name another energy the motor produces.

The answer

  1. I = P ÷ V = 750 ÷ 230 ≈ 3.26 A
  2. Heat (and some sound).

Question 2

A torch bulb is marked 2.5 V, 0.3 A.

  1. What is its power?
  2. If it turns 42% of its energy into light, what happens to the rest?

The answer

  1. P = 2.5 × 0.3 = 0.75 W
  2. The other 58% becomes heat.

Questions 3 and 4

Car lamps and a motorcycle bulb

  1. A car's two headlamps are 50 W each and its two rear lamps 10 W each. How much energy do they all use in half an hour?
  2. A 12 V motorcycle bulb takes 2 A. How much energy does it use in 15 minutes?

The answer

  1. P = 2 × 50 + 2 × 10 = 120 W; E = 120 × 1800 = 216 000 J
  2. E = VIt = 12 × 2 × 15 × 60 = 21 600 J

Question 5

Protection in the home

  1. Name two devices in a household circuit that protect the people living there.
  2. Explain what each one protects against.
  3. What should be done to protect appliances during lightning?

The answer

  1. The RCCB (trip switch) and the MCBs (or fuses).
  2. The RCCB cuts the whole house off when current leaks or someone gets a shock; an MCB cuts its own circuit off when it is overloaded, preventing fires.
  3. Unplug radios, televisions and similar appliances, and avoid using electricity — the RCCB does not protect from lightning.

Question 6

Paying for electricity

  1. What unit is the energy measured in for billing?
  2. How many joules is that unit?
  3. The first 60 units cost Rs 7.50 each and the next 30 Rs 10.00 each. What is the bill for 75 units a month?

The answer

  1. The kilowatt-hour (kWh).
  2. 3.6 × 106 J
  3. 60 × 7.50 = Rs 450.00; 15 × 10.00 = Rs 150.00; total Rs 600.00

Question 7

A day's units

  1. A 1500 W immersion heater runs ½ hour a day; three 40 W lamps are lit 3 hours a day and two 60 W lamps 2 hours a day. How many units are used a day?
  2. At the rates of question 6, what is the bill for a 30-day month?

The answer

  1. Heater 1.5 × 0.5 = 0.75; 40 W lamps 3 × 40 × 3 = 360 Wh = 0.36; 60 W lamps 2 × 60 × 2 = 240 Wh = 0.24; total 1.35 kWh a day
  2. 1.35 × 30 = 40.5 units, all in the first band: 40.5 × 7.50 = Rs 303.75

Question 8

Heating water, and a tripped MCB

  1. A hot plate or an immersion heater can heat water. Which is more efficient?
  2. Explain why.
  3. Why do immersion heaters use a three-pin plug instead of a two-pin one?
  4. An MCB cut off a circuit when an appliance was switched on. Give two possible reasons.

The answer

  1. The immersion heater.
  2. Its element sits in the water, so all the heat it makes goes into the water; a hot plate loses much of its heat to the air.
  3. Its metal case is joined to earth, so if current leaks to the case it flows to earth, the RCCB trips, and no one is shocked by touching it.
  4. The appliance draws more current than the circuit's rating (an overload, for example too many appliances at once), or there is a short circuit in the appliance or its cable.