# The specific thermal capacity of lead

##### Class practical

Transferring energy mechanically to lead shot and measuring its temperature rise.

#### Apparatus and materials

For each group of students

Lever-arm or top pan balance (+/- 10g is sufficiently accurate)

Cardboard tube (approx 50 - 100 cm long)

Plastic or cardboard cups

Thermometer

Metre rule

Corks or bungs to fit tube, 2

#### Health & Safety and Technical notes

There is no need to handle the lead shot. However, if it is spilt and is collected by hand (for example), the hands must be washed thoroughly before eating.

#### Procedure

a Weigh about 500 g of lead shot in one of the plastic or cardboard cups. Put a thermometer into the lead shot to find the temperature.

b Seal one end of the cardboard tube with a cork or bung. Put the lead shot into the tube and seal the open end.

c Turn the tube over 20, 40 or 50 times, counting the number. Quickly pour the shot into the cup and measure the maximum temperature again.

#### Teaching notes

1 When the lead is falling take care that it falls vertically rather than sliding along the tube, when friction will come into play. Also a hand should not cushion the bottom of the tube when the shot hits the bottom, otherwise energy will be transferred into the hand. Turning the tube over more times in order to achieve higher temperature rises is not advisable. The longer time and higher temperature differences will allow more thermal transfer to the environment.

2 The energy transferred is equal to the number of falls (n) x mgh and therefore
n x mg x h = m x specific thermal capacity x temperature rise.
Note that there is no need to record the mass of the lead, m.

3 Lead is used because its specific thermal capacity is about 1/30 that of water. It is the thermal capacity per unit mass which is important. (Most metals have approximately the same thermal capacity per unit volume.) It is also an inelastic metal so that the gravitation store of energy of the falling shot is transferred to thermal energy efficiently.

4 When Joule was investigating energy conservation he is said to have measured the temperature at the top and bottom of a waterfall on his honeymoon in Switzerland. This is a useful model of his experiment.

This experiment was safety-checked in January 2006