Heat Transfer

Question 1

What are the properties of a solid?

Answer 1

Strong forces of attraction hold the particles close together in a fixed rebuild arrangement. Little energy so only vibrate

Question 2

What are the properties of a liquid?

Answer 2

Weaker forces between particles
Close together but can move
They have more energy so can move in random directions slowly

Question 3

What are the properties of a gas?

Answer 3

Almost no forces of attraction

They have the most energy and are free to move in random directions at high speeds

Question 4

What is conduction?

Answer 4

Conduction is the process of vibrating particles passing on their extra kinetic energy to neighbouring particles

Question 5

Why are metals good conductors?

Answer 5

Because of their free electron that can move and pass on energy by colliding with other free electrons
This is a much faster way of transferring the energy causing heat to travel faster in metals

Question 6

What happens if an object is hotter than its surroundings?

Answer 6

It emits more radiation than it absorbs

Question 7

What is convection?

Answer 7

When more energetic particles move from the hotter region to the cooler region taking their heat with them

Question 8

What is a convection current?

Answer 8

When particles are heated so becomes less dense and rise and displace the cooler particles which sink

Question 9

Where can convection take place?

Answer 9

Liquids and gases

Question 10

What is condensation?

Answer 10

The transformation of a gas into a liquid

Question 11

Why does condensation happen?

Answer 11

When a gas cools the particles slow down and lose energy and attractive forces between particles pull them closer together so if the temperature gets cold enough then it will condense

Question 12

What is evaporation?

Answer 12

When liquid turns into a gas

Question 13

Why does evaporation take place?

Answer 13

When particles on top of a liquid are hot enough (have enough energy) to escape the liquid and become a gas

Question 14

When can evaporation happen?

Answer 14

The particles are travelling in the right direction to escape a liquid
The particles have enough kinetic energy to overcome the forces in liquid particles

Question 15

Which particles are more likely to evaporate from the liquid?

Answer 15

The particles with the most kinetic energy and when they escape the average speed and kinetic energy of remaining particles decreases so the temperature falls

Question 16

What can make the rate of condensation faster?

Answer 16

Temperature of the gas is lower
Temperature of the surface is lower
Density is higher
Airflow is less

Question 17

What makes the rate of evaporation higher?

Answer 17

Higher temperature
Lower density
Surface area is larger
Airflow over liquid is greater

Question 18

Why is conduction usually faster in denser solids?

Answer 18

Because the particles are closer together they will colour more often and pass energy between them

Question 19

What happens when you heat a substance?

Answer 19

You give it’s particles more kinetic energy they vibrate and most faster which is what causes solids to melt and liquids to boil

Question 20

Describe the immersion heater example for convection?

Answer 20

Heat energy is transferred from the heater coils to the water by Conduction
Particles near coils get more energy so move around faster
The water expands and becomes less dense
Reduction in density means that the hotter water tends to rise above the denser cooler water
As the hot water rises it displaces the cooler water making it sink towards the heater coils
Convection current is formed circulating heat energy through the water

Question 21

In what three different ways is heat transferred?

Answer 21

Radiation
Conduction
Convection

Question 22

What is heat radiation?

Answer 22

The transfer of heat energy by infrared radiation

Question 23

What can any object do?

Answer 23

Absorb and emit infrared radiation

Question 24

Where is infrared radiation emitted from?

Answer 24

The surface of an object

Question 25

What does an object cooler than its surroundings do?

Answer 25

Absorbs more radiation than it emits

The cooler an object is the less radiation it radiates in a given time

Question 26

What type of surface absorb and emit radiation more successfully?

Answer 26

Dark Matt surfaces

Not light shiny surfaces as they reflect a lot of the radiation

Question 27

How do solar panels work with radiation?

Answer 27

Solar hot water panels contain water pipes under a black surface
Radiation from the sun is absorbed by the black surface to heat the water in the pipes

Question 28

How does surface area affect the rate of heat transfer?

Answer 28

The bigger the surface area the more infrared waves than can be emitted from the surface so the quicker the transfer of heat

Question 29

How does type of material affect the rate of heat transfer?

Answer 29

Objects made from good conductors transfer heat away more quickly than insulating materials

Question 30

Why do car engines have fins?

Answer 30

Increase the surface area so heat is radiated away quicker and the engine cools quicker

Question 31

How is a vacuum flask designed to reduce heat transfer?

Answer 31

The glass bottle is double walled with a vacuum between the walls to prevent conduction and convection
The walls are silver to minimise heat loss by radiation
The stopper is made of plastics and filled with cork to reduce conduction through it

Question 32

How do humans and animals control heat transfer?

Answer 32

In cold hair stands up on end to trap layer of insulating air around the body reducing convection

When too hot body transfers more blood flow to surface of skin so heat is lost by radiation

Question 33

When can the cooling effect or evaporation be useful?

Answer 33

When you get hot and sweat

Question 34

At what temperatures can liquide evaporate?

Answer 34

At temperatures much lower than the liquids boiling point