Edexcel A Level Physics

Topic Questions

A LevelPhysicsEdexcelTopic Questions9. ThermodynamicsThermal Energy Transfer

Thermal Energy Transfer

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1a
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A student determined the latent heat of vaporisation of a liquid using an electrical heater to boil the liquid in a Pyrex beaker.

The apparatus used is shown below.

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She connected the heater into a circuit and took measurements of the potential difference V and the current I for the heater.

Complete the circuit diagram to show a suitable circuit.

q6a-nov-2020-9ph0-03-edexcel-as-a-level-phy

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1b
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The student monitored the mass of the beaker and the liquid m over the time t for which the liquid was boiling. Her results are plotted on the graph.

q6b-nov-2020-9ph0-03-edexcel-as-a-level-phy

The student used her graph to determine a value for the latent heat of the liquid in the beaker. She concluded that the liquid was pure water.

Liquid Latent heat of vaporisation / MJ kg−1
Pure water 2.26
Weak salt water solution 2.10
Strong salt water solution 2.00

 
Comment on the validity of the student’s conclusion.

V = 20.5 V

I = 10.5 A

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1c
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Explain how this method might be modified to improve the accuracy of the student’s conclusion.

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2a
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A student investigated the rate at which a hot liquid transfers thermal energy to the surroundings. He placed hot water in a Pyrex beaker and measured the temperature of the water using a liquid-in-glass thermometer.

He obtained the following data for the temperature θ of the water at times t. He measured t using a stopwatch.

t / s θ / °C    
0 95    
120 87    
240 81    
360 76    
480 71    

 
temperature of surroundings = 23 °C

Theory suggests that a liquid transfers internal energy to the surroundings at a rate proportional to the temperature difference Δθ between the liquid and the surroundings.

This leads to the expression

straight capital delta theta space equals space straight capital delta theta subscript 0 space e to the power of negative b t end exponent

where b is a constant and Δθ0 is the initial temperature difference.

Explain why a graph of ln Δθ against t should be a straight line.

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2b
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i)
Plot a graph of ln Δθ against t on the grid opposite.

Use the columns provided in the table to show any processed data.

(5)

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ii)
Determine the value of b.

(3)




b = ...................................

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2c
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The student suggested that the experiment would have been more accurate if a temperature sensor and data logger had been used to collect the data.

Assess the validity of the student’s suggestion.

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3a
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A student carried out an experiment to determine the specific latent heat of vaporization of water using the apparatus shown.

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The water in the flask was heated and steam was forced out of the flask and through the glass tubing into the cold water in the insulated cup. The steam condensed as it passed into the cold water.

The initial temperature of the cold water was 18.5°C and the mass of water in the cup was 255.0 g. After steam had been passed through the water for some time the temperature had risen to 26.0 °C and the mass of the water in the cup was 258.3 g.

Calculate the specific latent heat of vaporisation of water.

specific heat capacity of water = 4190 J kg–1 K–1





Specific latent heat of vaporisation of water = ...............................

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3b
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i)
Explain why the water was heated to boiling point and left boiling for a few minutes before the insulated cup of cold water was put in place.

(2)

ii)
Identify a significant source of error in this experiment and the steps that should be taken to minimise its effect on the calculated value of the specific latent heat of vaporization of water.

(2)

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1
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A cup contains 180 g of black coffee at a temperature of 82 °C. 68 g of milk at a temperature of 2.7°C is added to the coffee. An ideal temperature range for drinking coffee is said to be 50°C to 60°C.

Deduce whether the coffee will be within the ideal temperature range when the milk is added.

initial temperature of milk = 2.7°C

specific heat capacity of black coffee = 4.2 × 103 J kg−1 K−1

specific heat capacity of milk = 3.9 × 103 J kg−1 K−1

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2
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Latte is a type of coffee made with hot frothy milk. The milk is heated by pumping steam into it.
Calculate the maximum mass of milk that could be warmed to a temperature of 65°C by absorbing 15 g of steam at 100°C.
initial temperature of milk = 4.0°C
specific heat capacity of milk = 3900 J kg−1K−1
specific heat capacity of water = 4200 J kg−1K−1
specific latent heat of vaporisation of water = 2.3 × 106 J kg−1

Maximum mass = ....................................................................

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3a
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The apparatus shown can be used to determine a value for the specific latent heat of vaporisation of water.

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In one experiment the current in the heater was 8.20 A, and the potential difference across the heater was 230 V.

i)
Show that the power of the heater was about 2 kW.

(2)

ii)
There was 0.655 kg of water in the flask at an initial temperature of 22.5 °C.
The heater was switched on, and the water in the flask was heated to boiling point.

Calculate the minimum time taken for the water to be heated to 100.0 °C.

specific heat capacity of water = 4190 J kg−1 K−1


(3)

Minimum time taken for water to be heated = .......................................................

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3b
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The heater was left on and water continued to boil in the flask. The water was allowed to boil for a few minutes. The conical flask was then placed under the heat exchanger and water was collected in it.
i)   Give a reason why the water was left boiling for a few minutes before the conical flask was put in place.
   

(1)

ii)
Water with a mass of 95.0 g was collected in a time of 125 s.
 
Calculate the rate of energy transfer in the heat exchanger.
 
specific latent heat of vaporisation of water = 2.26 × 106 J kg−1

(3)

 Rate of energy transfer in the heat exchanger = .......................................................

iii)
Discuss your answers to (a)(i) and (b)(ii).

(3)

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3c
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State how the apparatus could be modified to minimise the effect of a significant source of error.

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4a
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A student determined the specific heat capacity of aluminium.
She used an electrical heater to heat an aluminium block and measured the temperature of the block with a digital thermometer.
She connected the electrical heater into a circuit and took measurements to determine the power of the heater.
Draw a circuit diagram of a suitable circuit.

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4b
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The student monitored the temperature θ of the aluminium block over the time t for which the heater was switched on.

Her results are plotted on the graph.

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i)
Determine the specific heat capacity of aluminium.
 
power of heater = 37.5 W
mass of aluminium block = 0.986 kg

(3)

Specific heat capacity of aluminium = .......................................................

ii)
The student looked up the accepted value for the specific heat capacity of aluminium. Using this value, the student predicted that it should have taken 240 s for the temperature of the aluminium block to increase by 10 °C.
 

Explain the difference between the predicted time and the student’s actual observations.



(2)

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5
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A wet handkerchief is dried in 56 s using a hot iron rated at 2400 W.
Determine whether energy is transferred to the water in the handkerchief at a greater rate than it is transferred to the iron.

initial temperature of wet handkerchief = 18°C
initial mass of wet handkerchief = 35.9 g
final mass of dry handkerchief = 18.2 g
specific heat capacity of water = 4.19 x 103 J Kg-1K-1

specific latent heat of vaporisation of water = 2.26 x 106 J kg-1

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