GCSE Standard Questions: Electricity in the Home

Q8. The photo shows a coffee machine. The coffee machine uses an electric element to heat water.

(a) The coffee machine has a metal case.

Why would it be dangerous for the live wire of the electric cable to touch the metal case?

If someone touched the case there is a risk they could get an electric shock (or be electrocuted).

[1 mark]

(b) The power output of the coffee machine is 2.53 kW.

The mains potential difference is 230 V.

Calculate the current in the coffee machine.

Before you use an equation you should change values into

the correct unit - power needs to be in watts - not kilowatts!

P = 2.53 kW = 2.53 x 10³ W

V = 230 V

P = IV

I = P/V

I = 2.53 x 10³/230

I = 11 A

[4 marks]

The power output of the coffee machine is 2.53 kW.

Given that the specific heat capacity of water is 4200 J/kg °C, calculate the mass of water that the coffee machine can heat in 14 seconds.

This requires you to use the specific heat capacity equation - E = mcΔθ

On the right hand side of your page set out what you have been given and change it into the correct unit to use the equation.

E = ?

Δθ = 90 - 20 = 70 °C

m = ? kg

c = 4200 J/kg °C

So, we have two unknowns - the mass and the energy given to the water. We have to use the power value and the time to find the energy:

2.53 kW = 2.53 x 10³J/s

so, in 14 seconds the element provides 2.53 x 10³ x 14 J of energy

E = 35.42 x 10³ J

Now we are in business! We have one unknown and can work that out....

E = mcΔθ

m = E/(cΔθ)

m = 35.42 x 10³/(4200 x 70)

m = 0.12 kg

[6 marks]

(Total 11 marks)