6.2.2

# Thermal Energy Transfer Experiments

Test yourself

## Heat Capacity Experiment

Two ways in which heat transfer changes materials are by changing the temperature of an object and by changing the state of an object.

### Equipment

• This diagram shows the simple electrical heater method of finding the specific heat capacity, c, of a material.

### Equipment 2

• Make sure that the heater is fully submerged into the material and that you check the mass of the block, m, with a balance.
• There should be an ammeter and a voltmeter in the electrical circuit to make sure that the current and p.d. for the heater do not change.
• The thermometer measures the initial temperature, T1, of the material and the temperature after every minute.

### Method

• If you record the temperature at the start and after every minute, you can plot a temperature-time graph.
• The graph will be shallow at first because it takes time for the block to heat the thermometer.
• Once the graph has got to its steepest, you can draw the tangent to the curve.

### Result

• The input power of the heater, IV, must equal mc × gradient.
• So the specific heat capacity can be found by c = IV/mass × (temperature-time graph gradient).
• This result ignores the effect of the apparatus heating the rest of the room. It also ignores the fact that the heating effect is not constant throughout the experiment.

## Method of Continuous Flow

A much more accurate way of finding the specific heat capacity of a fluid is the method of continuous flow.

### Apparatus

• By using apparatus as shown in the diagram, you can measure the temperature at the left hand thermometer, Ta, and at the right hand thermometer, Tb.
• When the electrical heater is switched on, Tb increases until thermal equilibrium is reached.

### Equation

• At this point, the heat energy supplied by the heater per unit time is equal to the sum of the internal energy gained by the water plus the heat energy lost by the water to the surroundings.
• V1 I1 = m1 cwater (Tb - Ta) + H
• Where m1 is the mass of water passing through the apparatus per unit time.

### Second equation

• If the flow rate is changed and the electrical heater parameters changed so that the start and finish temperatures remain constant, then we get a second equation:
• V2 I2 = m2 cwater (Tb - Ta) + H.
• As the temperature profile of the apparatus is the same on both occasions, the heat energy supplied to the surroundings, H will be the same.

### Combination

• This means that equation 1 and equation 2 can be combined to give an expression for cwater
• V2 I2 - V1 I1 = (m2 - m1) cwater (Tb - Ta)
• So cwater = (V2 I2 - V1 I1)/(m2 - m1)(Tb - Ta)
• The advantage of this method over the ‘heater in a beaker’ method is that the heat loss to the surroundings can be factored out. This means you can get a value closer to the true value.

### Types of latent heat

• Most materials have two specific latent heats.
• Specific latent heat of melting (or fusion) for the solid to liquid phase transition.
• Specific latent heat of vapourisation for the liquid to gas phase transition.
• In either case, the amount of heat supplied, $\Delta E = l \times \Delta m$, where l is the specific latent heat and Δm is the mass of the material that changes phase.