P1.1.3 Energy Transfer by Heating

Question 1

Define conduction of heat energy.

Answer 1

Conduction of heat energy is the process where vibrating particles pass on their extra kinetic energy to neighbouring particles.

Question 2

Explain the process of conduction in heating an object.

Answer 2

Heat flows through an object as vibrating particles pass on their extra kinetic energy to neighbouring particles. This process continues throughout the solid, so that the end that isn’t being directly heated gains some kinetic energy (which causes a gradual rise in temperature at the other side of the solid). This also leads to an increase in the heat radiating from its surface.

Question 3

In terms of arrangement and movement of particles, what properties allow denser solids to be good conductors?

Answer 3

Usually conduction is faster in denser solids, because the particles are closer together and so will collide more often and pass energy between them.

Question 4

In terms of arrangement and movement of particles, what properties allow less dense solids to be good insulators?

Answer 4

Materials that have large spaces between the particles conduct heat energy much more slowly.

Question 5

Explain why metals are good conductors.

Answer 5

Metals conduct well because they have free electrons. At the hot end the electrons move faster and collide with other free electrons, transferring energy and these other electrons then pass on their extra energy to other electrons. Because the electrons are free, this allows a faster way of transferring the energy through the metal rather than slowly passing it between vibrating particles.

Question 6

Explain which metal shape conduction is more efficient in.

Answer 6

A short, fat rod rather than through a long, thin rod where electrons have to transfer energy further away.

Question 7

If metal and wood had been left in the sun, the metal would feel hotter than the wood, but it wouldn’t be hotter. If it had been left in the cold, it would feel colder but it wouldn’t be colder. Why?

Answer 7

The metal isn’t actually hotter because it just conducts the heat into your hand much quicker than the wood, and so you hand heats up quicker. In cold weather it would feel colder because they take the heat away from your hand quicker (Remember that if an object is hotter than it’s surroundings it would emit more radiation than it absorbs and if it’s colder it would absorb more radiation than it emit - this is similar).

Question 8

Which materials can convection happen in and why can’t it happen in others?

Answer 8

Fluids (gases and liquids) have particles that can move around, which allows them to transfer hear by convection. Convection doesn’t happen in solids because the particles cannot move.

Question 9

Explain the process of convection.

Answer 9

1. Heat energy is transferred to the water near the heat coils by conduction.
2. The particles near the coils gain more energy as they heat up, allowing them to move around faster.
3. These means the heated water expands and becomes less dense as there’s more distance between the moving particles.
4. This reduction is density means that the hotter water tends to rise above the denser, cooler water.
5. As the hot water rises it displaces the colder water, making it sink towards the heater coils.
6. This cold water is then heated by the coils and rises.
7. A convection current is then created as the fast-moving particles collide with slow moving-particles and transfer heat, meaning the changes in density are often.

Question 10

What containers is convection more efficient in?

Answer 10

It’s more efficient in round or square containers which allow the convection currents to work best, rather than shallow, wide containers or tall, thin ones.

Question 11

Describe the process of condensation.

Answer 11

1. When a gas cools, the particles in the gas slow down and lose kinetic energy. The attractive forces between the particles pull them closer together.
2. If the temperature gets cold enough and the gas particles get close enough together that condensation can take place, the gas becomes a liquid.

Question 12

Describe the process of evaporation.

Answer 12

Evaporation is when liquids turn into gases because particles escape from a liquid. The particles in a liquid have different energies. Some will have enough energy to escape from the liquid and become a gas.

Question 13

In regards to evaporation, particles of a liquid can escape if…

Answer 13

• The particles are near the surface.
• The particles are travelling in the right direction to escape the liquid.
• The particles are travelling fast enough, with enough kinetic energy to overcome the attractive forces of the other particles in the liquid.

Question 14

Explain which factors increase the rate of evaporation.

Answer 14

1. A higher temperature - the average particle energy will be higher and so more particles will have enough energy to escape.
2. Density is lower - the forces between the particles will usually be weaker, so more particles will have enough energy to overcome theses forces and escape the liquid.
3. Surface area is larger - more particles will be near enough to the surface to escape the liquid.
4. Airflow over the liquid is greater - the lower the concentration of an evaporating substance in the air it’s evaporating into, the higher the rate of evaporation. A greater airflow means air above the liquid is replaced more quickly, so the concentration gradient is kept constant.

Question 15

Explain the cooling effect evaporation has.

Answer 15

• The fastest particles are most likely to evaporate from the liquid, and so when they do the average speed and kinetic energy of the remaining particles decreases.
• This decrease in average particle energy means the temperature of the remaining liquid falls, cooling the liquid.

Question 16

Give an example when the cooling effect from evaporation is useful.

Answer 16

When you’re hot, or you exercise, you sweat. As the water from the sweat on you skin evaporates, it cools you down.

Question 17

Explain which factors increase the rate of condensation.

Answer 17

1. Temperature of the gas is lower - the average particle energy in the gas is lower so more particles will slow down enough to clump together and form liquid droplets.
2. Temperature of the surface the gas touches in lower.
3. Density is higher - the forces between the particles will be stronger. Fewer particles will have enough energy to overcome these forces and will instead clump together and form a liquid.
4. Airflow is less - the concentration of the substance in the air will be higher, and so the rate of condensation will be greater.

Question 18

The _ _ _ _ _ _ the temperature difference between an _ _ _ _ _ _ and its _ _ _ _ _ _ _ _ _ _ _ _ , the _ _ _ _ _ _ the rate at which energy is transferred by heating.

Answer 18

The bigger the temperature difference between an object and its surroundings, the faster the rate at which energy is transferred by heating.

Question 19

What factors affect the rate at which an object transfers energy by heating?

Answer 19

• surface area and volume.
• the material from which the object is made.
• the nature of the surface with which the object is in contact.

Question 20

Heat energy is radiated from the surface of an object, and therefore the _ _ _ _ _ _ the surface area, the _ _ _ _ infrared waves that can be emitted from or absorbed by the surface. Give 3 examples where this applies.

Answer 20

1. Radiators have large surface area to maximise the amount of heat they transfer.
2. Cooling fins on engines increase the surface area so heat is radiated away quicker.
3. Heat sinks are devices designed to transfer heat away from objects they’re in contact with, e.g. computer components. They have fins and a large surface are so they can emit heat as quickly as possible.

Question 21

If two objects at the same temperature have the same surface area but different volumes, which object will cool quicker?

Answer 21

The object with the smaller volume will cool more quickly as a higher proportion of the object will be in contact with its surroundings i.e. there is a larger surface are to volume ratio.

Question 22

Give two ways that the type of material affect the rate of heat energy transfer.

Answer 22

1. The material of the object effects the rate - objects made from good conductors transfer heat away more quickly than insulating materials.
2. The materials that the object is in contact with also affect the rate - if an object is in contact with a conductor, the heat will be conducted away much faster than if it is in contact with a good insulator.

Question 23

How is a vacuum designed to reduce heat energy transfer?

Answer 23

1. The glass bottle is double-walled with a vacuum between the two walls. This stops all conduction and convection through the sides.
2. The walls either side of the vacuum are silvered to keep heat loss by radiation to a minimum, as silver surfaces are good reflectors of radiation.
3. The bottle is supported using insulating foam which minimises heat conduction to or from the outer bottle.
4. The cap is made of plastic and filled with cork or foam, as good insulators reduce heat energy transfer by conduction.

Question 24

Give three ways that humans control heat transfer in the cold.

Answer 24

• The blood vessels supplying the skin capillaries constrict, causing less blood to flow nearer the surface of the skin, so less heat can be lost by radiation.
• The body shivers - this contraction of muscles means the muscles respire to releasing energy, and this energy warms up the body.
• The hairs on your skin ‘stand up’ to trap a thicker layer of insulating air around the body.

Question 25

Give three ways that humans control heat transfer when hot.

Answer 25

• The blood vessels supplying the skin capillaries dilate causing more blood to flow nearer the surface of the skin, so more heat can be lost by radiation.
• As the water from the sweat on your skin evaporates it cools you down because the particles that evaporate are usually the fastest, reducing the average particle energy and lowering the temperature of the remaining liquid.
• The hairs on your skin lay flat.

Question 26

Give three ways that animals limit heat loss.

Answer 26

1. Animals living in cold conditions have a compact shape to keep keep their surface area to a minimum in comparison with their volume (Small ears, with a small surface area minimises heat loss by radiation).
2. Thick, hairy coats to keep the body insulated and some animals have hairs that stand up to trap a layer of insulating air.
3. They have a think layer of blubber for insulation.

Question 27

Give three ways that animals increase heat loss.

Answer 27

1. Large surface area to volume ration allows animals to lose more heat. (Large ears with a large surface area all them to increase heat loss by radiation).
2. They produce very little sweat.
3. Desert animals have very think layers of body fat and a thin coat to help them lose body heat.