Energy 2 (Convection and Conduction)

Question 1

Which is a more effective process: convention or conduction?

Answer 1

Convection as gases and liquids jus slosh around.

Question 2

Explain how convection works

Answer 2

Convection occurs when the more energetic particles MOVE from the HoTTER REGION to the COOLER REGION and TAKE THEIR HEAT ENERGY WITH THEM.

Question 3

Set out how convection occurs in an immersion heater

Answer 3

1. HEAT ENERGY is TRANSFERRED from the heater coils to the water by conduction (PARTICLE COLLISIONS).
2. PARTICLES near the coils get MORE ENERGY so they start MOVING around FASTER
3. As a result water expands and becomes less dense.
4. This reduction in density means the HOTTER WATER tends to his above the DENSER, COOLER WATER.
5. As the HOT WATER rises it DISPLACES (moves) the COLDER water out of the way, making it SIN towards the heater coils

6 This cold water is then HEATED BY THE COILS and rises and so it goes on.

Question 4

In an immersion heater is there convection below the heater coils - explain the reason for your answer.

Answer 4

There is no convection because the heat element is at the top so the water that is heated will rise but it then becomes part of the convection cycle and will not return to BELOW the heater coil - hence the water below the heater coil will remain cold.

Question 5

Convection Currents are all about CHANGES in ?

Answer 5

DENSITY - don’t forget that

Question 6

Explain how a radiator creates convection

Answer 6

It’s not hard!

The radiator heats the air, which then becomes less dense and rises. This displaces the cold air and the cold air which is more dense falls to the ground to fill the gap left by the heated air.

This falling air is then heated by the radiator and rises again etc. etc.

Question 7

What is condensation?

Answer 7

This is when gas turns to liquid.

Question 8

Explain how condensation works

Answer 8

1. When a gas cools, the particles in the gas slow down and lose kinetic energy. This then means the attractive forces between the particles pull closer together.
2. If it gets cold enough and the gas particles get close enough together that condensation can take place, the gas becomes a liquid.
3. Water vapour in the air condenses when it comes into contact with cold surfaces.

Question 9

What is evaporation?

Answer 9

Evaporation is when Liquid turns to Gas. The particles escape from a liquid.

Question 10

Does evaporation need to be at a liquid’s boiling point?

Answer 10

No, particles can evaporate from a liquid at temperatures that are much lower than the liquid’s boiling point.

Question 11

What 3 things are required for particles to escape the liquid (evaporation).

Answer 11

1. The particles are travelling in the RIGHT DIRECTION.
2. The particles are travelling FAST ENOUGH (they need enough kinetic energy)to overcome the ATTRACTIVE FORCES of the OTHER PARTICLES.

Question 12

Which PARTICLES are most likely to evaporate from the liquid?

Answer 12

The fastest particles (with most kinetic energy) are MOST LIKELY to evaporate from the liquid

Question 13

What happens when particles leave the liquid in relation to evaporation

Answer 13

The average speed and kinetic energy of the remaining particles DECREASES.

The decrease in average particle energy means the TEMPERATURE of the remaining liquid falls - the liquid COOLS.

Question 14

Why is the cooling effect useful?

Answer 14

When you exercise or get hot the water from the sweat on your skin evaporates thereby cooling you down.

Question 15

What speeds up the rate of evaporation?

Answer 15

1. TEMPERATURE IS HIGHER - average particles
2. DENSITY is LOWER - forces between the particles
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 into the higher the rate of EVAPORATION.

Question 16

What 4 things will speed up the rate of CONDENSATION?

Answer 16

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 is lower.
3. DENSITY is higher - the forces between the particles will have enough energy to overcome these forces and will instead clump together
4. AIRFLOW IS LESS - the concentration of the substance in the air will be higher and so the rate of condensation will be GREATER. Is

Question 17

How does the size of the surface area affect heat transfer?

Answer 17

The bigger the surface area the more infrared waves can be emitted so the QUICKER the TRANSFER OF HEAT.

Question 18

What function do FINS have on engines?

Answer 18

Fins increase the surface area so heat is radiated away quicker

Question 19

What are heat sinks?

Answer 19

Heat sinks are devices designed to transfer heat away from objects they’re in contact with.

Question 20

If two objects at the same temperature have the same surface area but different volumes which one will cool more quickly?

Answer 20

The object with the smaller volume will cool quicker because a higher proportion of the object will be in contact with its surrounding.

Question 21

Does material have an impact upon the rate of heat transfer?

Answer 21

Yes.

Objects made from good conductors transfer heat away more quickly than insulating materials such as plastic.

Question 22

Does the material that is contact with the object affect rate of heat transfer?

Answer 22

Yes, if an object is in contact with a conductor the heat will be CONDUCTED away faster than a good INSULATOR.

Question 23

Explain four ways that show how a vacuum flask limits heat transfer.

Answer 23

1. Glass bottle is double-walled with a VACUUM between each wall (stops convection and conduction).
2. Walls either side of the vacuum are SILVERED to keep hear loss by radiation to a minimum
3. The bottle is supported using INSULATING FOAM. (minimises heat conduction to and from the outer glass bottle)
4. The STOPPER is made of PLASTIC and filled with CORK or FOAM to reduce any HEAT CONDUCTION.

Question 24

How do animals control heat transfer?

Answer 24

1. HAIRS STAND UP - when cold humans hair stands up to trap a THICKER layer of insulating air.
2. BLOOD FLOWS NEAR TO SURFACE OF SKIN - when too warm blood moves closer to skin to lose more heat (why people go pink after exercise or when very hot)
3. LARGER EARS - in hotter climates animals tend to have ledger ears so as to lose more heat due to larger surface area.