0625 · 2.3Transfer of thermal energy

Thermal energy transfer basics

0:00A saucepan heats on a hob, water circulates inside it, and your hand feels warmth without touching the metal. These effects involve three different methods of thermal energy transfer. They are conduction, convection and radiation. By the end of this video, you will understand how each works, how to investigate it, and why several methods often act together. First, remember that temperature and thermal energy are not the same thing.

0:24Temperature tells us how hot something is, and relates to the average kinetic energy of its particles. Thermal energy depends on more than temperature, including how much material there is. When objects have different temperatures, thermal energy transfers overall from the hotter object to the colder object. The word overall matters because objects can exchange energy in both directions. The hotter object loses more energy than it receives from the colder object.

0:49An insulator slows this transfer rather than stopping it completely. We will use these ideas to explain everything from a pan handle to the temperature of the Earth.

Conduction in solids

0:58Imagine leaving a metal spoon in hot soup, with its handle sticking out. Eventually, the handle becomes warm even though it is not touching the soup. Thermal energy has travelled along the spoon by conduction. In conduction, energy transfers through a material without the material moving as a whole from the hot region to the cold region. Now we are adding the Supplement explanation required for Extended students. In a solid, atoms or molecules vibrate about fixed positions in a lattice.

1:24Heating one end makes the particles there vibrate more vigorously. Through interactions with neighbouring particles, energy passes along the solid towards cooler regions. The particles do not travel all the way along the spoon. This lattice vibration mechanism occurs in all solids, but metals have another important way to transfer energy. They contain free electrons, also called delocalised electrons, which can move through the metal. Electrons in the hotter region gain kinetic energy and move through the material.

1:53Their interactions transfer energy to particles elsewhere, helping metals conduct thermal energy rapidly. Copper and aluminium are good thermal conductors, so they transfer thermal energy quickly under comparable conditions. Wood and many plastics are bad thermal conductors, also called thermal insulators. However, materials do not fit into just two extreme categories. Many solids, including glass, conduct better than some insulators but less well than good metallic conductors. We can compare conduction using rods of different materials with equal lengths and cross-sectional areas.

2:27Attach identical pins using equal amounts of wax at the same distance from one end. Place those ends equally deeply into the same hot-water bath. Start with the rods at the same temperature, and record how long each pin takes to fall. Energy conducts along each rod until the wax becomes warm enough to melt and release its pin. Under these controlled conditions, an earlier fall indicates faster conduction along that rod.

2:50A pin that remains attached only shows that insufficient energy reached its wax during the observation time. It does not prove that the material transfers no thermal energy at all. This experiment also shows why fair comparisons matter. A shorter or thicker rod could transfer energy differently even if its material were unchanged. Keep the heating conditions, distances and wax amounts the same. Hot water and heated equipment require teacher supervision, and you should never test conductivity by touching a heated rod.

3:19For Extended students, we also need to explain why gases and most liquids are poor conductors. Gas particles are widely separated, so energy transfer through particle collisions is relatively inefficient. Most liquids lack the fixed lattice of solids and the free-electron mechanism of metals. Particle interactions still transfer energy, but usually much less effectively than in good metallic conductors.

Convection currents

3:43Water can be a poor conductor and still heat throughout a saucepan fairly quickly. That happens because the water itself moves and carries energy with it. This method is called convection. It is an important method of thermal energy transfer in liquids and gases. These substances can flow, unlike the particles fixed within a solid lattice. Follow the sequence when water is heated from below. The water near the heater becomes warmer and usually expands.

4:08Its mass stays the same, but it occupies a larger volume, so its density decreases. The warmer, less dense water rises through the surrounding cooler, denser water, carrying thermal energy upwards with it. Cooler, denser water moves down to replace the rising water. That water is then heated, and the circulation continues. This circulating movement is called a convection current. The important explanation is not simply that warm water rises.

4:34Heating causes expansion, expansion reduces density, and the density difference produces the circulation. A teacher can demonstrate this using a transparent beaker of water and a small amount of coloured dye near the bottom. Heat the water gently beneath one side, without stirring it. The coloured water rises near the heated region, travels across the top, and moves down elsewhere. The dye reveals the movement of the water rather than causing that movement.

5:00Heating near the top gives a very different result. The warmer, less dense water is already above the cooler, denser water, so strong circulation is not established. Energy can still conduct downwards, but water conducts poorly. This explains why the position of the heat source matters, not just how much energy it supplies. Convection in air can be demonstrated with a smoke box containing two chimneys. A candle beneath one chimney heats the air, which expands, becomes less dense and rises out.

5:28Cooler air enters through the other chimney to replace it. Smoke introduced above that cooler chimney makes the airflow visible. The demonstration should be performed under teacher supervision. A room heater produces a similar circulation. Air warmed near the heater rises, while cooler air moves in to replace it. The current transfers thermal energy around the room rather than leaving it only beside the heater. Do not say that heat always rises, because conduction and radiation can transfer energy in any direction.

Infrared radiation and surfaces

5:56Radiation explains how you feel warmth from a fire without touching it or standing in its rising hot air. Thermal radiation is infrared radiation, and all objects emit it. It is a form of electromagnetic radiation rather than the movement of warm particles. Unlike conduction and convection, thermal radiation does not require a medium. A medium means material through which a transfer takes place, such as air, water or metal.

6:21Radiation can travel through a vacuum, where there are no material particles. This is why energy from the Sun can reach Earth through space. The Sun emits several kinds of electromagnetic radiation, while everyday warm objects mainly emit infrared radiation. An object does not have to glow visibly to emit infrared radiation. Your body, a table and the walls all emit it because of their temperatures. Objects also absorb radiation arriving from their surroundings.

6:47Their temperature depends on the overall balance between energy received and energy transferred away, not simply on whether they emit radiation. Surface colour and texture affect absorption, reflection and emission. Dull black surfaces are good absorbers and good emitters of infrared radiation. In the usual school comparisons, black surfaces absorb and emit better than white surfaces with comparable finishes. Shiny surfaces are poor absorbers and emitters, and good reflectors of infrared radiation.

7:15Keep absorption and reflection separate in your explanation. Absorbed radiation transfers energy to the object, whereas reflected radiation is sent away from its surface. A shiny surface reflects much of the infrared falling on it, so less is absorbed. Being a good emitter means transferring energy away efficiently by radiation at a given temperature and surface area.

Radiation investigations and energy balance

7:37The following radiation investigations are Supplement content for Extended students. To compare absorbers, use identical metal plates, one dull black and one shiny. Position them equally far from the same infrared source, with the same exposed area and orientation. Start them at the same temperature and measure their temperature rises over the same time. The dull black plate should show a greater temperature rise because it absorbs infrared more effectively.

8:02Keep the material, mass and thickness the same, so surface finish is the important difference. Reduce draughts and other unequal energy transfers that could affect the measurements. Record temperatures rather than deciding which surface feels warmer by touch. To compare emitters, use a Leslie cube containing hot water, with faces of different surface finishes. Measure the infrared from each face using a detector at the same distance and angle. The faces have equal areas and are maintained at approximately the same temperature.

8:31A larger detector reading from the dull black face shows that it is the better emitter. Controlling temperature is essential because hotter surfaces emit infrared at a greater rate. Surface area matters too. A larger area emits more energy each second when temperature and finish are unchanged. Therefore, a comparison of surface finishes must keep both temperature and area constant. Otherwise, a larger reading might have more than one possible explanation.

8:59This next idea is also Supplement content. For an object to remain at a constant temperature, it must transfer energy away at the same rate that it receives energy. If it receives energy faster than it transfers energy away, its temperature rises, provided it is not changing state. If it loses energy faster than it receives energy, its temperature falls. Let us work through a numerical example using energy transfer rates.

9:22An object receives two hundred joules each second and transfers away one hundred and fifty joules each second. Its net gain each second is 200 − 150, giving fifty joules. Over twenty seconds, 50 × 20 gives a gain of one thousand joules. Now consider an object receiving eighty joules each second while transferring away one hundred and twenty joules each second. Its net loss each second is 120 − 80, giving forty joules.

9:50Over thirty seconds, 40 × 30 gives a loss of one thousand two hundred joules. Its temperature falls unless a change of state or another energy process affects the result. Earth’s temperature also depends on an energy balance. Some incoming solar radiation is reflected by clouds, the atmosphere and the surface, while the rest is absorbed. The warmed surface emits infrared radiation. If the Earth system absorbs energy faster than it transfers energy to space, it warms.

10:18The opposite imbalance produces cooling. Reflectivity affects how much incoming energy is absorbed, so more reflection generally means less warming when other factors remain unchanged. Greenhouse gases absorb some infrared emitted by the surface and emit infrared in different directions. Some is directed back towards the surface, reducing the net energy transferred away at a given surface temperature. The temperature then adjusts until incoming and outgoing rates balance again.

Combined transfer methods and recap

10:45Now we can explain the saucepan more completely. Energy reaches its base from the hob, and the metal conducts it through the base and sides. Water next to the hot metal warms, and convection distributes energy through the water. A wooden or plastic handle reduces conduction towards your hand, although it can still become hot. A room radiator also uses more than one method, despite its name. It emits infrared radiation, but it also warms nearby air and creates convection currents.

11:13Metal conducts energy through the radiator itself. Understanding the separate stages is more useful than choosing one method for the whole object. The following combined applications are Supplement content. In a wood or coal fire, hot gases rise and transfer energy by convection. The flames and glowing fuel emit radiation that warms nearby people and objects. Conduction transfers energy through pieces of fuel and along any metal poker in contact with the fire.

11:39The direction of transfer helps identify the mechanism. Strong warming beside the fire is not mainly caused by convection, because the hottest gases rise above it. Infrared radiation can travel sideways from the fire to your body. A poker becoming hot along its length instead shows conduction through the metal, without the poker moving towards you. A car radiator removes energy from hot coolant arriving from the engine. Energy transfers from the coolant to the metal tubes and conducts through the tubes and attached fins.

12:08Air moving between the fins carries energy away by convection. The fins provide a large surface area for transfer to the air, and the radiator also emits infrared radiation. A fan or the car’s movement maintains airflow through the radiator. Moving warmed air away and replacing it with cooler air helps maintain energy transfer. As the coolant circulates, it carries energy from the engine to the radiator. Despite the name radiator, convection to the passing air is especially important in this application.

12:38Consider an exam-style question asking you to explain why air above a room heater rises. Build a chain from heating to motion rather than naming convection alone. A suitable answer is that the air is heated and expands. Its density decreases, so it rises through the surrounding cooler, denser air. Cooler air moves in to replace it, forming a convection current. Another question could ask you to describe an experiment comparing infrared absorption by two surfaces.

13:05State the apparatus and what you measure, then identify the variables that must remain constant. Explain that identical plates at equal distances receive radiation for equal times. The plate with the greater temperature rise is the better absorber under those controlled conditions. Watch for three misleading shortcuts in your answers. Cold does not flow into an object, because thermal energy transfers overall towards the colder region. Solid particles do not travel along a conductor.

13:32Their vibrations transfer energy, and free electrons also contribute in metals. Finally, radiation needs no medium, whereas convection requires a moving liquid or gas. Let us bring the ideas together. Conduction transfers energy through particle interactions and, in metals, moving free electrons. Convection transfers energy through the bulk movement of liquids and gases caused by density differences. Thermal radiation is infrared, and it needs no medium. How quickly an object warms or cools depends on its surface.

14:03It also depends on how much energy it absorbs compared with how much it emits.