# Physics - Energy Transfers (AQA Syllabus A)

The transfer of heat energy is one of the topics studied in GCSE Science. This is one of eight quizzes on that subject and it looks specifically at things which affect the rate of heat energy transfers through convection, conduction, evaporation and condensation.

Thermal energy can be transferred from one place to another by doing work or by heating processes. The rate at which heat transfers depends on several things. The heating processes are conduction, convection, evaporation and condensation and they all depend on how the particles of a particular substance are arranged. Lets have a look at these different heating processes in turn, starting with conduction:

Particles of solids are fixed in place but they can vibrate. They transfer heat energy into kinetic energy, so the more heat that is present, the more they vibrate. Metals are good heat conductors because the particles are closely packed together and the vibrations can be transferred efficiently. They also contain free electrons which drift through the metal - the hotter it is, the quicker they move. So heating one end of a metal bar makes the free electrons move faster at that end and transfer heat quickly throughout the metal bar. In all other substances there are no free electrons to carry the heat through the structure and so conduction is a much slower process. Conduction of heat cannot take place in liquids or gases because the particles are not fixed in place.

Secondly, convection. The particles of a liquid are not fixed in place, so when they are heated they move around faster. They therefore occupy more space and that means the heated part of the liquid is less dense than the cooler parts. In liquids, less dense areas will move upwards so the warmer part of the liquid rises away from the heat source. Cooler liquid moves in to take its place and is heated in its turn, so a convection current is set up that transfers the heat throughout the liquid.

Finally, evaporation and condensation. The fastest moving particles of a liquid are able to escape from the surface, this is called evaporation. The temperature of a liquid is a measure of the average speed of its particles. As these faster particles leave, it means that the average speed of the ones left behind is lower, so evaporation causes cooling. The faster the rate of evaporation, the greater the cooling. Condensation is the opposite of evaporation.

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1. At the back of a refigerator there is a condenser which is designed to change the hot refrigerant gas into a liquid. The tubes of the condenser are attached to hundreds of small metal 'fins'. Why?
Cooling fins make the process more efficient. The rate of thermal energy transfer to the surroundings is faster than it would be if they weren't there
2. During the cold weather, birds can often appear bigger than they do when the weather is warm. Which of the following is the most likely explanation?
Lifting the feathers slightly further than in summer traps more air next to the bird's body. Since trapped air is a good insulator, this keeps them warmer
3. Which of the following statements is false?
Plastics are good insulators for exactly to opposite reason. It is metals that contain free electrons which help to make them good conductors
4. The rate at which an object transfers thermal energy depends on what?
A hot object with a small surface area, in contact with an insulating surface will transfer its heat energy very slowly
5. During the winter, moisture often condenses on the inside of windows of old houses. This rarely happens during the summer. Which of the following is likely to be the main reason for this?
Moisture in the air condenses on colder surfaces
6. Based on the information that "the chemical called ether evaporates much faster than water", which of the following statements is true?
The faster something evaporates, the cooler it makes the surface on which it is placed
7. In which direction does thermal energy (heat) flow?
Heat energy always travels from a higher temperature to a lower temperature. There are no exceptions
8. Why are cooling fins thin and flat?
Since their job is to transfer thermal energy into the air, a large surface area compared to their volume makes them more efficient
9. A student investigated the rate of heat transfer from four identical containers of water. The water was heated to different temperatures (shown below) and she noted the temperature drop during the first two minutes of cooling. Which answer puts them in the correct order, from lowest rate to highest rate?

Container A: 35oC
Container B: 55oC
Container C: 45oC
Container D: 80oC
Heat is lost more slowly where the difference in temperature and the surroundings is lower. Hot objects therefore cool down quickly at first and then more slowly as they get closer to the temperature of the surroundings
10. A student investigated the rate of heat transfer from four identical containers of water. The water was heated to different temperatures (shown below). She then placed containers B and C in a freezer at -5oC and containers A and D in a room with a temperature of 20oC, and noted the temperature drop during the first two minutes of cooling. Which of the following statements is correct?

Container A: 35oC
Container B: 55oC
Container C: 45oC
Container D: 80oC
This question is checking to see if you understand about temperature gradients.The difference in temperature and the surroundings is called the temperature gradient - the greater the difference, the greater the gradient and the faster the cooling