8463/1H June 2020

7.​​ An electric kettle was switched on.​​ Figure 10​​ shows how the temperature of the water inside the kettle changed.

i-​​ When the kettle was switched on the temperature of the water did not immediately start to increase. Suggest one reason why.​​ 

[1 mark]

ANSWER

It​​ takes time fo the​​ kettle to heat up.​​ 

ii-​​ The energy transferred to the water in 100 seconds was 155 000 J.​​ 

specific heat capacity of water = 4200 J/kg °C​​ 

Determine the mass of water in the kettle.​​ 

Use​​ Figure 10. Give your answer to 2 significant figures.​​ 

[5 marks]

SOLUTION

E=mc∆θ

m= EC∆θ

Where ∆θ, the change in temperature, is equal to 100-22=78℃.​​ 

m= 1550004200×78

m=0.4731 kg

iii-​​ The straight section of the line in Figure 10 can be used to calculate the useful power output of the kettle.

Explain how.

[3 marks]

SOLUTION

The gradient of the straight section of line is equal to​​ 

∆y∆x=∆θt

Where power is equal to​​ 

P= Et

Pt=E

And, we know that thermal energy required, E is equal to E=mc∆θ. So,​​ the equation becomes​​ 

Pt=mc∆θ

P=mc∆θt

P=mc×gradient