Prescribed practical P3: Heat transfer SA - CCEA

What is the purpose of this prescribed practical?

This practical is about comparing the heat conductivity of different materials by measuring the time it takes heat to travel through a variety of conductors and at least one insulator.

What are the variables?

In this experiment:

• the independent variable is the material of the conducting rod.
• the dependent variable is the time taken for the pin to drop off.
• the control variables are the length and cross section area of each conducting rod, the mass of the drawing pin and the amount of Vaseline used to attach the pin to each rod.

The control variables are kept the same by:

• ensuring each rod is the same length and has the same cross section area.
• ensuring that the same region of each rod is in the Bunsen flame.
• attaching the pin at the end of each rod.
• using the same amount of Vaseline to attach each pin.
• using identical pins.

Remember - these variables are controlled (or kept the same) because to make it a fair test, only one variable can be changed, which in this case is the material of the conductor.

What is the prediction?

Metals will conduct faster than non-metals and some metals will conduct faster than other metals.

What is the justification for the prediction?

Different metals have different properties, for example different density, and so they will not all conduct heat energy at the same rate.

What is the risk assessment for this practical?

What apparatus is used in prescribed practical P3?

A Bunsen burner, tripod, bench mat, identical copper, aluminium, iron and glass rods, a stop clock, four identical drawing pins, Vaseline, a gas lighter, safety glasses, tongs and/or heat proof gloves.

What method is used in carrying out prescribed practical P3?

1. Fix the drawing pin to the end of each rods using identical spots of Vaseline.
2. Arrange the rods carefully on top of the tripod. They could rest on a heat proof mat if necessary.
3. Position the other end of each rod over the Bunsen.
4. Using the gas lighter, light the Bunsen flame and adjust the hole to half open.
5. Start the stop clock.
6. Record the time taken for the wax to melt and the drawing pin to drop off each rod in a suitable table. The fastest time shows the best conductor of heat.

How to avoid errors

Ensure that the Bunsen flame heats the same part of each rod and that each rod is evenly in the flame.

How to record the results

Graph

Draw a bar chart with time in seconds (time /s) on the y-axis and material on the x-axis.

Give the graph a suitable title.

What conclusion can be drawn from prescribed practical P3?

The drawing pin fell off the copper rod in the shortest time followed by the aluminium and then the iron.

The glass rod is last.

The glass rod should be removed from the heat approximately 30 seconds after the pin falls off the iron to prevent the glass from melting.

From this we can conclude that copper conducts heat energy better than aluminium, while aluminium conducts better than iron and glass.

We say that copper has a high thermal conductivity.

This agrees with our prediction.

We can now also conclude that the non-metal, glass, is a very poor conductor of heat.

Evaluation

To ensure the reliability of the investigation the method should be repeated to check that the pins fall off the rods in the same order.

An average time could be calculated.

A way of improving the accuracy of this investigation would be to use temperature probes attached to the end of each rod and connected to a data logger.

The time could then be recorded for each rod to reach the same temperature, for example, 50^oC.

The rods could be heated by placing them into a water bath maintained at 90^oC.

This would help to ensure even heating of the rods.

What is thermal conductivity?

The material that heats the quickest in the above investigation is said to have a high thermal conductivity.

Key facts

Thermal conductivity is a measure of how well a material conducts heat energy when it is heated.

Thermal conductivity is measured in W/m/^oC (watts per metre per degree Celsius).

Some typical values of conductivities are:

The higher the thermal conductivity the better the conductor.

Copper is a good conductor of heat, air is a poor conductor of heat.

Generally, metals have high thermal conductivity and are good conductors; non-metals have low thermal conductivity and are poor conductors.