Energy and heating
As well as transferring the capacity for doing work. from one store to another, energy is transferred or transmitted from place to place. As it moves through a substance, energy is transmitted by conduction, convection or radiation.
Conduction
The aluminium base of a pan, the copper in the wires from a plug and the stainless steel of a kettle are all An electrical conductor is a material which allows an electrical current to pass through it easily. It has a low resistance..
Key fact: A conductor is a material that allows Energy stored in all materials, including energy due to the motion of particles and the chemical bonds between them. to be transmitted through it easily.
Extended syllabus content: Particles and conduction
If you are studying the Extended syllabus, you will also need to know how thermal conduction works on an atomic level. Click 'show more' for this content:
All metals are good conductors.
When one end of a metal rod is put into a fire, the energy from the flame makes the Electrically charged particle, formed when an atom or molecule gains or loses electrons. in the rod vibrate faster.
Since the ions in the solid metal are close together in a The regular arrangement of atoms in a metal., this increased Repeated movements back and forth (about a fixed point). means that they collide with neighbouring ions more frequently. Energy is passed on through the metal by these collisions, transmitting the energy.
More frequent collisions increase the Per unit time or 鈥榩er second鈥. For example, if 2,000 J are transferred over a period of 10 s, then the rate of transfer is 200 J/s or 200 W. This value is the power rating. of transfer.
Liquids and gases
The particles of liquids are further apart than solids. This means that most liquids are poor conductors of heat because the frequency of collisions is reduced. The particles of gases are even further apart and so even less conduction occurs.
The role of free electrons
Some of the electrons in a piece of metal can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now positively charged metal ions.
When the free electrons absorb heat energy, they gain kinetic energy and move much faster. As they move through the metal, they crash into the metal ions. Some of their kinetic energy is absorbed by the ions and they vibrate faster, and with greater amplitude. We see this as a rise in temperature of the metal.
This process is very much faster than conduction caused by just passing vibrations from atom to atom. So, conduction in metals is faster than in non-metals. Metals are good conductors of heat energy because they contain free electrons.
Cooking pans are made of metal. Energy is transferred to the food by conduction through the metal base and sides of the pan. Many pans have plastic or wooden handles through which the energy is less likely to be conducted.
Insulating houses
When trying to keep houses warm, the best choices are materials that are poor conductors, such as brick, wood, plastic and glass. A house built of conducting materials like copper would be very cold to live in because energy would be able to leave the house easily.
Image caption, A metal bar is heated up
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Conduction experiment 1: Thermal conductors
There are a number of factors that affect how the capacity for doing work. flows through an object. A very important factor is what the object is made from.
Comparing conductivities
The conductivity of materials can be compared by examining the time taken to transmit energy through them. A fan of rods made of different materials can be heated at one end with the same flame. Whichever rod gets hottest first at the other end is the best conductor. The material that heats the quickest is said to have a high A measure of how well a material conducts energy when it is heated..
Key fact: Thermal conductivity is a measure of how well a material conducts energy when it is heated.
There are many solids that conduct thermal energy better than thermal insulators but do so less well than good thermal conductors.
Extended syllabus content: Thermal conductivity values
If you are studying the Extended syllabus, you will also need to understand the conductivity values of different materials. Click 'show more' for this content:
Some typical values of conductivities are:
| Material | Thermal conductivity, in watts per metre per degree Celsius (W/m/掳C) |
|---|---|
| Copper | 400.0 |
| Glass | 1.0 |
| Brick | 0.75 |
| Air | 0.024 |
This means that 400 joules (J) of energy will flow per second through a cubic block of copper (1 m 脳 1 m 脳 1 m) when the temperature difference between its sides is 1掳C.
Most liquids and gases are poor heat conductors - see the previous section about particles and conduction.
Question
Referring to the table of conductivities above, why is it better to have a window made of two layers of glass with a layer of air trapped between them?
Answer
Both glass and air are insulators because they have low thermal conductivities. The layer of air has the lowest thermal conductivity and reduces the overall conductivity of the window unit. Since air and glass are both transparent, people can still see through the window.
Conduction experiment 2: Thermal insulators
There are different ways to investigate methods of insulation. In this practical activity, it is important to:
- make and record measurements of temperature and time accurately
- measure and observe the effect of different materials as thermal insulators
- use appropriate apparatus and methods to measure the effectiveness of different materials as thermal insulators
Aim of the experiment
To compare the effectiveness of different materials as thermal insulators.
Method
- Place a small beaker into a larger beaker.
- Fill the small beaker with hot water from a kettle.
- Put a piece of cardboard over the beakers as a lid. The lid should have a hole suitable for a thermometer.
- Place a thermometer into the smaller beaker through the hole.
- Record the temperature of the water in the small beaker and start the stopwatch.
- Record the temperature of the water every 2 minutes for 20 minutes.
- Repeat steps 1-6, each time packing the space between the large beaker and small beaker with the chosen insulating material.
- Plot a graph of temperature (y-axis) against time (x-axis).
Results
| Time (mins) | No insulation (掳C) | Material 1 (掳C) | Material 2 (掳C) |
|---|---|---|---|
| 0 | 鈥 | 鈥 | 鈥 |
| 2 | 鈥 | 鈥 | 鈥 |
| 鈥 | 鈥 | 鈥 | 鈥 |
Analysis
Plot all of the curves on the same axes. This will make the materials easier to compare.
This graph shows:
- The curve which takes the longest time for the water temperature to drop (the shallowest) should be the material which is the best insulator.
- The temperature falls quickly at high temperatures and slowly at low temperatures.
- When the beaker is at a high temperature, there is a big difference between the temperature of the beaker and the temperature of the surrounding air. This means there is a high Per unit time or 鈥榩er second鈥. For example, if 2,000 J are transferred over a period of 10 s, then the rate of transfer is 200 J/s or 200 W. This value is the power rating. of transfer.
- When the beaker is at a lower temperature, there is less difference between the temperature of the beaker and the temperature of the surrounding air. This means there is a lower rate of transfer.
Convection
Convection is the flow of heat energy from a region of high temperature to a region of low temperature by movement of a fluid.
Convection only occurs in fluids (liquids and gases).
Convection in liquids
Convection in a liquid can be seen by putting a crystal of potassium permanganate in a beaker of water and gently heating it with a Bunsen flame.
- Heat is initially transferred through the glass wall of the beaker by The transfer of heat through a material by transferring kinetic energy from one particle to another..
- The water in the region of the Bunsen flame is heated.
- It expands, becomes less dense and rises.
- It is replaced by the cooler, denser water which surrounds it.
- This water is in turn heated, expands becomes less dense and rises.
- The process continues, a convection current is set up and heat is transferred through the liquid.
Convection currents can be seen in lava lamps.
The wax inside the lamp warms up, becomes less dense than the liquid, and so rises.
Convection in gases
The two-chimney apparatus
A convection current in air can be demonstrated using the apparatus shown.
- A splint is lit and blown out so it is smoking
- When the smoking splint is held in air the smoke rises
- The splint is then held over both chimneys in turn as shown above
- When the splint is held above the candle the smoke rises
- When the splint is held above the other chimney the smoke is drawn down the chimney. It then passes across the horizontal section and up through the chimney above the candle
What is happening?
- The candle flame heats the air around it
- The hot air expands and increases in volume
- The density of the air decreases, and it floats upwards, rising through the chimney
- Cooler air is sucked in through the other chimney to replace the rising, warm air 鈥 a convection current has been set up
- The smoke from the smouldering splint shows the path of the cool air
Applications
Early coal mines were ventilated with fresh air using a similar method. Two shafts were dug down from ground level to the mine and a fire was lit beneath one of them. As hot air rose upwards, fresh, clean, cool air was sucked down the other shaft and across the coal mine. However, lighting a fire in a coal mine caused safety issues, so this method is not used today.
Question
Can you explain why, when a piece of card is set over the chimney that is not above the candle, the candle quickly goes out?
Answer
The rising warm air above the candle is not replaced by any air. Oxygen does not reach the flame and so it goes out.
A convection current from a radiator
- Air close to the radiator is heated.
- It expands, becomes less dense and rises.
- It is replaced by the cooler, denser air which surrounds it.
- This air is in turn heated, expands becomes less dense and rises.
- The process continues, a convection current is set up and heat is transferred through the air and hence through the room.
- A radiator heats mainly by convection 鈥 not by radiation.
Convection and weather
The heating of the Earth鈥檚 surface and atmosphere by the Sun causes convection in the atmosphere and oceans, producing winds, clouds and ocean currents.
For example, as the Sun heats the Earth鈥檚 surface, the air above it heats up, expands and rises. This air can continue to rise, cooling as it does so, forming fluffy, cauliflower-shaped cumulus clouds.
Sea breezes and land breezes
Temperature differences at the Earth's surface occur where there are different surfaces such as land and sea.
- During the daytime, along the coast, the land heats up quicker than the sea water
- Air above the land is heated, expands and begins to rise
- The rising air is replaced by cooler air drawn in from the surface of the sea
- This is why it can be cooler by the coast while further inland the temperature is higher
In the evening and at night the reverse happens:
- Land cools down more quickly than the sea
- Air above the sea is heated, expands and begins to rise
- The rising air is replaced by cooler air drawn off the land
Radiation
Thermal radiation is the transfer of heat energy from a region of high temperature to a region of low temperature by infrared radiation.
All objects emit radiation.
Unlike conduction and convection, radiation takes place without the need of any particles.
Because no particles are involved, radiation can work through the vacuum of space.
This is why we can still feel the heat of the Sun even though it is 150 million km away from the Earth.
Absorbing radiation
When an object absorbs radiation, it heats up and its temperature will rise.
You feel the heat of the Sun when you absorb the infrared radiation from it.
If a cloud passes in front of the Sun, you absorb less infrared, and it feels cooler.
Extended syllabus content: Thermal absorption experiment
If you are studying the Extended syllabus, you will also need to understand an experiment to investigate rates of absorption of thermal radiation by different surfaces. Click 'show more' for this content:
How infrared radiation is absorbed can be investigated using the following apparatus.
Two squares of aluminium are arranged as shown in the diagram.
One is painted dull black, the other is polished and shiny.
To ensure a fair test they are the same area and thickness and are placed the same distance from the Bunsen flame.
Two identical corks are stuck to the back of the plates using equal amounts of candle wax.
The Bunsen is lit.
Quite quickly the cork attached to the black plate falls off.
The cork behind the polished plate takes much longer to fall off.
Conclusion
Both plates received the same quantity of radiation, but the black plate heated up more quickly.
This tells us that a dull, black surface is a better absorber of radiation than a shiny, polished surface.
Key points
- Dark matt surfaces are better at absorbing heat energy than light shiny surfaces.
- Dark matt surfaces are better at radiating heat energy than light shiny surfaces.
- The rate of emission of radiation depends upon the surface temperature and surface area of an object. Hotter objects with larger surface areas emit more radiation.
| Surface | Absorption | Emission |
|---|---|---|
| Dull, matt or rough | Good absorber of heat radiation | Good emitter of heat radiation |
| Shiny | Poor absorber of heat radiation | Poor emitter of heat radiation |
Applications
Saucepans are shiny so that they don鈥檛 emit too much radiation and cool down too quickly.
Cricketers wear white to remain cool in summer as white it is a good reflector (poor absorber) of radiation.
Shiny silver blankets are used to stop heat loss from a person pulled from a cold sea after exercise.
Solar panels are black because black is a good absorber of radiation.
Radiators are often painted with white gloss paint, but they would be better at radiating heat if they were painted with matt black paint.
Teapots and saucepans are shiny so that they don鈥檛 emit too much radiation and cool down too quickly.
Cricketers wear white to remain cool in summer as white it is a good reflector (poor absorber) of radiation.
Shiny silver blankets are used to stop heat loss from a person pulled from a cold sea or evacuated from a swimming pool.
Solar panels should be black because black is a good absorber of radiation.
Radiators are often painted with white gloss paint, but they would be better at radiating heat if they were painted with matt black paint.
They are painted white to make them look nicer.
However, despite their name, radiators actually transfer most of their heat to a room by convection, not radiation.
So, radiators are the wrong colour, and have the wrong name.
Normally, heaters need to be red hot before they are any good at heating by radiation.
Extended syllabus content: Thermal radiation experiment
If you are studying the Extended syllabus, you will also need to understand an experiment to investigate rates of thermal radiation from different surfaces. Click 'show more' for this content:
The transfer of infrared radiation from a hot object to cooler surroundings can be investigated using a piece of apparatus called a Leslie cube.
This is a metal cube with four side prepared in different ways: black, white, shiny, or dull.
It can be filled with hot water or heated on an electrical hot plate so that all four sides are at the same temperature.
Method
- Measure the temperature a fixed distance from each side of a Leslie cube using four identical thermometers.
- Heat the Leslie cube by filling it with boiling water.
- Continue to measure and record the temperatures every 30 seconds for five minutes, then plot a graph of temperature on the y-axis, against time on the x-axis, for each side.
- Compare the four graphs obtained.
Control variables
Distance of each thermometer from the sides of the cube, the type of thermometer used and the time taken for each reading.
Results
The temperature of the thermometer opposite the dull, black side reaches the highest temperature in the same time interval.
From this we can say that dark matt surfaces are better at radiating heat energy than light shiny surfaces.
Extended syllabus content: Complex thermal transfers
If you are studying the Extended syllabus, you will also need to know about times when more than one energy transfer occurs at once. Click 'show more' for this content:
Often more than one energy transfer occurs. The heat you feel from a wood or coal fire has been radiated out from it. Energy is also transferred by convection, through the currents formed when the hot air above it rises.
Radiators cool down the engines of petrol or diesel vehicles like cars. Heat from the engine is transferred by conduction from metal pipes into a coolant liquid, such as water. This heated water is then pumped to the radiator where the energy is conducted into metal cooling fins. These transfer the energy away mainly by radiation, but also convection and conduction.
Quiz
Test your knowledge of temperature changes and heat transfer with these quizzes.
Teaching resources
Are you a physics teacher looking for more resources? Share this short film from Operation Awesome with your students. Presenter Steve Mould and students visit a Fire Safety teaching centre to experiment with infra-red cameras.
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