Materials such as wood, glass, cork, plastics and fabrics are bad conductors, the handles of some saucepans are made of wood or plastic. Cork is used for tablemats.
Metal objects below body temperature feel colder than those made of bad conductors because they carry heat away faster from the hand – even if all the objects are exactly the same temperatures.
Also in liquids like water, the forces between the particles are generally weaker than in solids. So particles do not push and pull each other so much – they simply have less to do with each other. Liquids are usually not very good at thermal conduction.
Air is one of the worst conductors (best insulators). This is why houses with cavity wall (i.e. two layers of bricks separated by an air space) and double glazed windows keep warmer in the winter and cooler in the summer.
Materials which trap air, e.g. wool, felt, fur, feathers, polystyrene, fibreglass, are also very bad conductors. Some of these materials are used as ‘lagging’ to insulate water pipes, hot water cylinders, ovens, refrigerators and the walls and roofs of houses. Others are used to make warm winter clothes like fleece jackets. ‘Wet suits’ are worn by divers and water skiers to keep them warm. The suit gets wet and a layer of water gathers between the persons body and the suit. The water is warmed by body heat and stays warm because the suit is made from an insulating fabric, e.g. neoprene, a synthetic rubber.
If I wanted to work out the thermal conductivity of each of the materials I am using to find out which of them would keep the water hotter for longer I could use this equation:
Rate of Heat loss = Q/ t =λAdT/dx
We can find λ By rearranging the above λ = Q/dtA *dx/dt
Where
λ = Thermal conductivity
Q = Rate of heat loss
dx = distance (thickness of cup)
dT= temperature gradient (T1- T2)
T1= temperature inside the cup
T2= temperature outside the cup
I would not be able to work this out because I do not have all the measurements needed, and it would not be very precise because the equation only considers conduction, but in my experiment other factors were also affected.
Nevertheless, I already know from textbooks etc that the thermal conductivity values for the materials I will be using are as follows:
- Aluminium: 238 W/m.k
- Ceramic: 226-284 W/m.k
- Paper: 0.1-0.4 W/m.k
- Polystyrene: 0.122-0.193 W/m.k
This shows that polystyrene is a better insulator than paper because it conducts less heat, paper is better than ceramic, and ceramic is better than aluminium.
Convection:
Convection of heat only occurs in liquids and gasses. Convection occurs when the more energetic particles move from the hotter region to the cooler region taking their energy with them. Convection is the flow of heat through a fluid from places of higher, temperature to places of lower temperature by movement of the fluid itself.
Convection is the usual method, by which heat travels through fluids, steams of warm moving fluids are called convection currents. They arise when a fluid is heated because it expands, becomes less dense and is forced upwards by surrounding cooler, denser fluid which moves under it. We say ‘hot water (or hot air) rises’. Warm fluid behaves as a cork released under water: being less dense, it bobs up.
Radiation:
Radiation is the flow of heat from one place to another by means of electromagnetic waves.
Radiation is a third way in which heat can travel but whereas conduction and convection both need matter to be present, radiation can occur in a vacuum; particles of matter are not involved. It is the way heat reaches us from the sun.
Radiation is emitted by al bodies above absolute zero and consists mostly of infrared radiation but light and ultraviolet are also present if the body is very hot.
Some surfaces absorb radiation better than others dull black surfaces are better absorbers of radiation than white shiny surfaces. Reflectors on electric fires are made of polished metal because of its good reflecting properties.
Some surfaces also emit radiation better than others do when they are hot. Dull black surfaces are a better emitter of radiation than shiny ones.
The cooling fins on the heat exchangers at the back of a refrigerator are painted black so that they lose heat more quickly. By contrast, saucepans etc Which are polished are poor emitters and keep their heat longer. In general, surfaces that are good absorbers of radiation are good emitters when hot.
Apparatus:
For this investigation, I will need the following apparatus:
- A metal cup
- A ceramic cup
- A paper cup
- A polystyrene cup
- Thermometer
- Kettle
- Stop watch
- Measuring cylinder
Method:
For this experiment, I will make sure everything is done with safety and fairness. Throughout the whole experiment all Lab rules must be followed, to make sure the experiment is fair I will test each cup twice. The apparatus should be kept the same for each experiment to ensure all results are taken without any advantages or disadvantages. Everything in the experiment should be kept the same apart from the cup, which I will change for each experiment
- I will boil some water in a kettle
- Measure 100ml of boiling water in a measuring cylinder
- Pour the water in the metal cup
- Use a thermometer to measure the temperature, wait for it to go down to 85.
- Start the stopwatch and record the temperature of the water every minute for 15 minutes.
- Repeat this twice for each of the cups.
- Record the results in a table then draw a graph for each one.
Results:
The results I gained from the experiment are shown in the table below:
Conclusion:
From my first graph, I can see that all the materials followed as predicted, except the aluminium cup. The polystyrene cup showed the best insulation and the ceramic cup showed the worst according to the graph. I can find many reasons for the unusual trend in the aluminium cup as follows:
- The aluminium cup had a smaller surface area than the others did, therefore in proportion the heat loss would be less.
- The aluminium cup had a smaller diameter at the top therefore; less heat was lost due to convection.
- The colour of the aluminium cup was darker than the other cups (all the other cups were white) and so less heat was lost due to less radiation.
Overall, the aluminium cup had had more disadvantages than the other cups
To work out the gradient of the lines in the first graph I drew lines tangent to the curves and worked out the slope as follows:
Initial gradient = (85-45)/3=13.3 oC/min
Gradient of second part =(75-45)/14=2.1 oC/min
This indicates that the rate of heat loss reduced after the first two minutes this is because the heat loss due to convection is higher at high temperatures.
The second graph shows the rate of heat loss. I used Microsoft excel to work out the rate of heat loss for every reduction in temperature using the following equation:
Q= mc (T1- T2)
Where:
Q= heat loss
M= mass of water (100ml/1000=0.1kg)
C= specific heat capacity of water (4200J/(kg oC)
It can be seen from this graph that the rate of heat loss is much higher for the first few minutes, the same as graph one after that it was steadily reducing again due to convection.
Evaluation:
The experiment was not very accurate due to the following reasons:
- The cups did not have the same wall thickness’ therefor there was a different rate conduction. The cups with the thicker walls had more advantage because they lost less heat.
- The cups did not all have the same surface area. Therefore, the cups with larger surface area had less advantage because they had a higher rate of conduction.
- The cups did not have the same size opening at the top. Therefore, more heat was lost due to convection, on the cups with wider openings at the top.
- The cups were not all the same colours. The metal cup was darker than the other three cups, as they were all white, so the metal cup lost less heat.
This experiment could have been improved if we had tried to use similar cups. We could have used a metal cup that was painted white so that the radiation on all the cups would have been the same. We could have tried to use cups with the same surface area.
The results were not very reliable because it was not a fair test, and they are not good enough to draw a firm conclusion, because they did not come out exactly as I had expected, but I still wouldn’t change my method, I would only try to use more similar cups.