Practical: Specific heat capacity of water (electric kettle)

Introduction

In this experiment a temperature sensor is used to measure the rate of temperature rise of a known mass of water heated in an electric kettle. This method is far from ideal so think how you could modify the method to make it more accurate. Before performing this experiment you will need to be familiar with using the datalogger software for recording data and drawing graphs.

Theory

The temperature change of the water, ΔT is related to the heat added, Q by the equation

Q=mcΔT

Now the power of the kettle, P is the heat delivered per unit time Q/Δt so if we divide the first equation by Δt we get

Q/Δt = mcΔT/Δt

So

P = mcΔT/Δt

Method

  • Find out the power of the kettle (it's probably written underneath).
  • Connect the temperature sensor to the computer via the interface as usual.
  • Start the data collection software and choose a reasonable sampling rate.
  • Measure the mass of the kettle then add water and weigh again. Calculate the mass of water in the kettle.
  • Place the sensor in the water and switch on the kettle. Measure the temperature of the water as it gets hot.
  • Use the graph of temperature vs time to find the rate of change of temperature and use this to determine the specific heat capacity of the water.
  • Estimate the uncertainty in m, ΔT/Δt and P then calculate the uncertainty in your value of c.

If using Capstone

  • Switch on the sensor and open capstone.
  • Select the table and graph option.
  • Click hardware set up and select temperature sensor.

Selecting table headers

  • click the header and select temperature
  • click the other header and select time

Set the sampling rate to 10 Hz. This means the computer will record 10 temperatures per second.

Run the experiment and collect data as the water is being heated.

Plotting the line of best fit

You will want to know the gradient of the graph.

  • highlight a relevant bit of the line by clicking the highlight tool and arranging the box so that it encloses the bit you want.

  • Choose linear best fit from the curve fitting options.

Compare your result with the expected result of 4180 Jkg-1K-1
Is your result to big or too small? Try to think why your result isn't the same as the accepted value and adapt your method to get a better value.

Did your modifications give a better result or not?

Extension

You can use the same apparatus to measure the latent heat of vaporisation of water.

  • Measure the mass of the kettle then add about 50ml of water and measure the mass again, calculate the mass of water.
  • Place the temperature sensor in the water and turn on the kettle with the lid open, plot the temperature against time until the water has boiled for about one minute. You may need to hold the switch in so the kettle doesn't turn off.
  • Measure the mass of the kettle to determine the mass of water that has turned to steam.
  • Calculate the rate at which heat is given to the water by using the equation P = mcΔT/Δt using the gradient just before the water boiled and a value of 4180 Jkg-1K-1 for c. How does this compare with the power of the kettle?
  • From the graph determine the time taken to turn the water into steam then calculate the amount of heat used to turn the water into steam.
  • Determine the amount of heat that would be required to change 1 kg of water into steam.
  • Compare your value with the specific latent heat of vaporisation of water.
  • Try to explain why your value is not the same as expected.
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