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Thermodynamics

Measuring specific heat capacity and calibrating a thermistor

  • 1 thing to know
  • Papers 2 and 3

Overview

Recall questions on the electrical method and why it reads high, reducing heat losses, and calibrating a thermistor in a potential divider as a thermometer.

What you need to know

Answer each one before you reveal it.

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Numbers

How does an NTC thermistor's resistance change with temperature, and what happens when two blocks of different materials are heated identically?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Two blocks of equal mass but different materials are heated by identical heaters for the same time. Which block reaches the higher temperature?

    Model answer

    The block with the smaller specific heat capacity reaches the higher temperature, because less energy is needed per kilogram for each degree of rise.

  2. 2

    Why does a simple electrical experiment usually give a value of specific heat capacity that is too large?

    Model answer

    Some of the electrical energy supplied is lost to the surroundings and to heating the container, so the measured temperature rise is smaller than it should be for the energy input.

  3. 3

    What two steps would reduce heat losses when measuring a specific heat capacity?

    Model answer

    Lag the block or container with insulation, and keep the temperature rise small so the sample spends less time above room temperature.

  4. 4

    How does the resistance of a negative temperature coefficient thermistor change as it gets hotter?

    Model answer

    Its resistance falls markedly as the temperature rises, because more charge carriers are released.

  5. 5

    How is a thermistor connected so that it produces an output voltage that varies with temperature?

    Model answer

    It is placed in series with a fixed resistor across a supply to form a potential divider, and the output is taken across one of the two components.

  6. 6

    In a potential divider with the output taken across the fixed resistor, what happens to the output potential difference as the thermistor is warmed?

    Model answer

    The output potential difference increases, because the thermistor's resistance falls so it takes a smaller share of the supply voltage.

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