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Specific Heat Capacity and Latent Heat Questions - GCSE standard

Q12. Lily heated water in an electric kettle.

Water has a high specific heat capacity.

(a) Complete the sentence below by choosing answers from this box.

°C
J
kg
s
W

 

The specific heat capacity of a substance is the energy needed to raise the temperature of 1 kg of the substance by 1°C .

[2 marks]

(b) The kettle circuit contains a thermistor which is used to switch the kettle off when the water reaches 100 °C. Draw the correct symbol for a thermistor.

[1 mark]

(c) Given that the resistance of the heating element in the kettle is 15 Ω and the current in the heating element is 12 A, calculate the power of the heating element.

P = IV

V = IR

so, P = I2R

P = 122 x 15

P = 144 x 15

P = 2160 W

[3 marks]

(d) Lily investigated how quickly the kettle could increase the temperature of 0.50 kg of water.

Here is a graph of her results of the investigation:

The temperature of the water did not start to increase until 10 seconds after the kettle box was switched on.

What is the reason for this? ( Tick one box).

Energy is transferred from the surroundings to the kettle.  
The charge flows slowly through the kettle circuit.  
The heating element in the kettle takes time to heat up.
The power output of the kettle increases slowly  

[1 mark]

(e) Describe a method Lily could have used to obtain the results shown in the graph.

This type of question is marked in two ways:

- the examiner looks for relevant points

- but also for a logical sequence...

It is not just a 'tick fest' - but a marker does look to see how many relevant points you have made, then considers how you have strung them together to put your answer into a 'level'

Level 3:

The method would lead to the production of a valid outcome.

All key steps are identified and logically sequenced.

5 - 6 marks
Level 2:

The method would not necessarily lead to a valid outcome.

Most steps are identified, but the method is not fully logically sequenced

3 - 4 marks
Level 1:

The method would not lead to a valid outcome.

Some relevant steps are identified, but links are not made clear.

1 - 2 marks
Level 0:

No relevant information
0 marks

 

Indicative content:

measure the mass of water using a balance or measure the volume of water using a measuring cylinder

measure the initial temperature of the water

pour the water into the kettle

put temperature probe in the water or put a thermometer in the water

switch kettle on

record temperature

measure time with a stopclock

use a time interval of 5 seconds

[6 marks]

(f) The mass of water in the kettle was 500 g.

The temperature of the water increased from 20 °C to 100 °C.

Given that the specific heat capacity of water = 4200 J/kg °C , calculate the energy transferred to the water.

Δθ = 100 - 20 = 80 °C

m = 500 g = 0.50 kg

E = mcΔθ

E = 0.50 × 4200 × 80

E = 168 000 J

[4 marks]

(g) The water in the kettle boiled for a short time before the kettle switched off.

During this time 5.0 g of water changed to steam.

Given that the specific latent heat of vaporisation of water = 2,260,000 J/kg, calculate the energy transferred to change the water to steam.

m = 5.0 g = 0.005 kg

E = mL

E = 0.005 × 2,260,000

E = 11,300 J

[3 marks]

(20 marks total)