P1.1 Transfer of energy by heating processes and factors that affect the rate at which that energy is transferred

Give two ways the nature of a surface could be changed so that the surface emits more infrared radiation.

Make the surface darker in colour Make the surface less shiny/ more matt.

What does heat transfer faster through?

Materials with higher U-values than through materials with lower U-values.

Why does the type of material affect the rate of heat energy transferred?

Objects made from good conductors transfer heat away more quickly than insulating materials, e.g. plastic. It also matters whether the materials in contact with it are insulators or conductors. 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.

What do conduction and convection involve the transfer of energy by...

Particles

Describe the process of heat transfer in conduction.

Particles that vibrate faster than others pass on their extra kinetic energy to their neighbours.

What are the three different ways heat is transferred?

Radiation Conduction Convection

What can materials which need to gain lots of energy to warm up also do?

Release loads of energy when they cool down again. They can 'store' a lot of heat.

What is convection more efficient in?

Roundish or squarish containers because they allow the convection currents to work best. Shallow, wide containers or tall, thin ones don't work as well.

What are the three states of matter?

Solid, liquid and gas

Infrared radiation can be emitted by...

Solids, liquids and gases

What is draught-proofing?

Strips of foam and plastic around doors and windows stop draughts of cold air blowing in, i.e. they reduce heat loss due to convection.

What does the rate of heat energy transfer depend on?

Surface area and volume and the type of material

What happens when water vapour comes into contact with cold surfaces?

The air condenses e.g. drink glasses

Specific heat capacity is?

The amount of energy needed to raise the temperature of 1 kg of a substance by 1°C.

What is payback time?

The amount of time it takes for the initial cost to equal the money saved.

Why will the temperature of the gas being lower make the rate of condensation faster?

The average particle energy in the gas is lower - so more particles will slow down enough to clump together and form liquid droplets.

Why will the airflow being less make the rate of condensation faster?

The concentration of the substance in the air will be higher, and so the rate of condensation will be greater.

What happens at the hot end of a metal?

The electrons move faster and collide with other free electrons, transferring energy. These other electrons then pass on their extra energy to other electrons etc.

Why will the density being higher make the rate of condensation faster?

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.

Why will a lower density make the rate of evaporation faster?

The forces between the particles will usually be weaker, so more particles will have enough energy to overcome these forces and escape the liquid.

What happens if the temperature gets cold enough?

The gas particles get close enough together that condensation can take place, the gas becomes a liquid.

How is a vacuum flask designed to limit heat transfer?

The glass bottle is double-walled with a vacuum between the two walls. This stops all conduction and convection through the sides. The walls either side of the vacuum are silvered to keep heat loss by radiation to the minimum. The bottle is supported using insulating foam. This minimises heat conduction to or from the outer glass bottle. The stopper is made by plastic and filled with a cork or foam to reduce any heat conduction through it.

Give one other design feature of the heat sink that helps it to do its job. Explain your answer. (2 marks)

The heat sink is made from metal which is a good conductor, and so will be able to conduct heat away quickly from the processor.

Suggest two ways the shape of the heat sink helps transfer heat away quickly from the processor. Give a reason for each of your answers. (4 marks)

The larger the surface area of an object, the higher the rate of heat transfer will be. The heat sink has fins which give it a large surface area so it will have a high rate of heat transfer. The heat sink is in direct contact with the whole of the processor, which will maximise the heat transfer by conduction between the processor and the heat sink.

What is specific heat capacity?

The measure of how much energy a substance can store.

What is short payback time?

The money you save covers the amount you paid really quickly.

The hotter an object is...

The more radiation it radiates in a given time.

What are the most effective methods of insulation?

The ones that give you the biggest annual saving (they save you the most money each year on your heating bills).

Describe the particles in a gas in terms of their arrangement, kinetic energy and movement.

The particles in a gas have high kinetic energies, move in random directions at high speeds and are not arranged in an pattern - they are free to move.

What happens when a gas cools?

The particles in the gas slow down and lose kinetic energy. The attractive forces between the particles pull them close to together.

What's the same and what's different in the three states of matter?

The particles of a particular substance in each state are the same. Only the arrangement and energy of the particles are different.

In hot conditions, many animals sweat. Explain, in terms of particles energies, how sweating will keep an animal cool. (3 marks)

The particles of sweat that evaporate have high energies. When they evaporate, the average energy of the remaining particles decreases and so the temperature of the sweat decreases, helping to cool the animal.

Conduction of heat energy is...

The process where vibrating particles pass on their extra kinetic energy to neighbouring particles.

Infrared radiation is emitted from...

The surface of an object.

What does the decrease in average particle energy mean?

The temperature of the remaining liquid falls, so it cools.

What happens to the faster particles that have more kinetic energy?

They are more likely to evaporate from the liquid - so when they do, the average speed and kinetic energy of the remaining particles decreases

What happens to particles near the surface of a liquid?

They can escape and become gas particles if: -The particles are travelling in the right direction to escape the liquid. -The particles are travelling fast enough (they have enough kinetic energy) to overcome the attractive forces of the other particles in the liquid.

What do the particles do in evaporation?

They escape from the liquid. Particles can evaporate from a liquid at temperatures that are much lower than the liquid's boiling point.

Why do electric storage heaters store the heat using concrete or bricks?

They have a high specific heat capacity and are designed to store heat over night when electricity is cheapest and release it during the day.

How do thick curtains work?

They reduce heat loss by conduction and radiation

Explain how the mirrored surfaces on the inside of a vacuum flask help reduce heat transfer.

They reflect infrared radiation and so help reduce heat transfer by radiation.

Why do heaters have high heat capacities?

To store lots of energy.

What is condensation?

When a gas turns to liquid

What is evaporation?

When liquid turns to gas

When does convection occur?

When the more energetic particles move from the hotter region to the cooler region - and take their heat energy with them.

How can the cooling effect be useful?

When you exercise you sweat or get hot. As the water from the sweat on your skin evaporates it cools you down.

What happens when you heat a substance?

You give its particles more kinetic energy - they vibrate or move faster. This is what eventually causes solids to melt and liquids to boil.

What is the equation to work out payback time?

payback time = initial cost / annual saving

The lower the U-value...

the better the insulator

The bigger the temperature difference between a body and its surroundings...

the faster energy is transferred by heating.

A 0.5 kg concrete block is heated from 20°C to 100°C. This takes 36,000 J of energy. (a)Calculate the specific heat capacity of the concrete block. (4 marks) Clearly show how you work out your answer. (b)Explain why concrete blocks are often used in heating systems such as storage heaters. (2 marks)

(a) θ = 100-20 = 80 E = m x c x θ c = E / (m x θ) c = 36,000 / (0.5 x 80) = 900 J/kg°C (b)Concrete has a high specific heat capacity and so will be able to store a lot of heat.

(a)Describe how heat is transferred: (i)from the Sun to the solar heating panel. (1 mark) (ii)from the hot water in the pipe to the colder water in the tank. (1 mark) (iii)throughout the water in the tank. (1 mark) (b)Explain why the pipes in the heating panel are painted black. (1 mark)

(a)(i)By heat radiation (ii)By conduction through the metal pipe (iii)By convection currents in the water (b)Because black surfaces are good absorbers of heat radiation.

A student is investigating the factors that affect how quickly a drink will cool. He measures the length of time it takes for a cup of water to cool from 60°C to 30°C. Describe how the time taken for the same temperature change would alter if the following changes were made to the apparatus. Give a reason for each of your answers. (a)The experiment was done in a room where the temperature was 15°C. (2 marks) (b)The plastic cup was replaced with an identically sized metal cup. (2 marks)

(a)The temperature of the water would fall more quickly - the rate of heat transfer would be greater because the difference in temperature between the water and its surrounding is greater. (b)The temperature of the water would fall more quickly - the rate of heat transfer would be greater because the cup is now made from metal, which is a better conductor than plastic.

Where Tom losses heat from his house: 25% through roof 35% through brick walls 15% through open windows and cracks in window frames 10% through glass in windows 15% through concrete floor (a)Calculate how much money Tom wastes every year in heat loss through the roof. (b)Tom decides to insulate the loft and needs to pick from two types of insulation. Insulation A has a U-value of 0.7 W/m²K, and insulation B has a U-value of 0.6 W/m²K. Both types of insulation would cost the same amount to buy and install. (i) Which type of insulation should Tom buy? Explain your answer. (ii) The insulation costs him £350, but reduces the amount he spends on wasted heat to £255 per year. Calculate the payback time for fitting loft insulation in Tom's house.

(a)£300 x 0.25 = £75 (b) (i) Insulation B - it has a lower U-value therefore a better insulator. (ii) 300-255 = £45 saved per year Payback time = cost / saving per year = 350 / 45 = 7.8 years

The rate of evaporation would be faster is...

-temperature is higher -density is lower -surface area is larger -airflow over the liquid is greater

The rate of condensation will be faster if...

-temperature of the gas is lower -airflow is less -temperature of the surface the gas touches is lower -density is higher

Gases have...

1)Almost no forces of attraction between the particles. 2)The particles have more energy than those in solids and liquids. 3)They are free to move, and travel in random directions at high speeds.

What's the immersion heater?

1)Heat energy is transferred from the heater coils to the water by conduction (particle collisions). 2)The particles near the coils get more energy, so they start moving around faster. 3)This means there's more distance between them, i.e. the water expands and becomes less dense. 4)This reaction in density means that the hotter water tends to rise above the denser, cooler water. 5)As the hot water rises it displaces (moves) the colder water out of the way, making it sink towards the heater coils. 6)This cold water is then heated by the coils and rises - and so it goes on. You end up with convection currents going up, round and down, circulating the heat energy through the water.

Solids have...

1)Strong forces of attraction hold the particles close together in a fixed, regular arrangement. 2)The particles don't have much energy so they can only vibrate about their fixed positions.

Why will a greater airflow over the liquid make the rate of evaporation faster?

1)The lower the concentration of an evaporating substance in the air it's evaporating into, the higher the rate of evaporation. 2)A greater airflow means air above the liquid is replaced more quickly, so the concentration in the air will be lower.

Liquids have...

1)Weaker forces of attraction between particles. 2)The particles are close together, but can move past each other, and form irregular arrangements. 3)The have more energy than the particles in a solid. 4)They move in random directions at low speeds.

What's the specific heat capacity of water?

4200 J/kg°C.

Heat sinks are often fitted with fans to cool the air around them. Explain how this will affect the rate of heat transfer from the heat sink to its surroundings. (2 marks)

A fan will increase the difference in temperature between the heat sink and its surroundings, and so will increase the rate of heat transfer.

What is loft insulation?

A thick layer of fibreglass wool laid out across the whole loft floor reduces conduction and radiation into the roof space from the ceiling.

What is an example of a device that has been designed to limit heat transfer?

A vacuum flask

What can any object do, whether or not conduction or convection are also taking place?

Absorb and emit infrared radiation

An object that's cooler than its surroundings...

Absorbs more radiation than it emits (as it warms up).

What causes a rise in temperature at the other side of the solid?

As the conduction process continues through a solid, gradually some of the extra kinetic energy (or heat) is passed all the way through the solid.

Why will a higher temperature make the rate of evaporation faster?

At higher temperatures the average particle energy will be higher so more particles will have enough energy to escape.

Why are metals good conductors?

Because of their delocalised (free) electrons

Why does heat travel so fast through metals?

Because the electrons can move freely. This is a faster way of transferring the heat energy through the metal than slowly passing it between jostling neighbouring atoms.

Why is conduction usually faster in denser solids?

Because the particles are closer together and so will collide more often and pass energy between them.

Why can't convection happen in solids?

Because the particles can't move

Why are they cost-effective?

Because they have a short payback time

What are convection currents all about?

Changes in density

What do the most cost-effective tend to be?

Cheapest

Explain how putting on a coat keeps you warm.

Coats help trap a layer of air around the body, which acts as an insulator and helps stop heat loss by radiation. (The coat itself will also usually be made of an insulating material.)

What is meant by 'condensation'?`

Condensation is when a gas changes state and becomes a liquid.

What is the main form of heat transfer in solids...

Conduction

What is the main form of heat transfer in liquid and gases...

Convection

How does you use a formula triangle for specific heat capacity?

Cover up the thing you're trying to find. The parts of the formula you can still see are what it's equal to.

What surfaces absorb IR falling on them and emit much more IR?

Dark, Matt surfaces

What are the types of insulation?

Draught-proofing Loft insulation Cavity wall insulation Thick curtains Hot water tank jacket

What is the equation for specific heat capacity?

E = m x c x θ

Explain how having large ears could be an advantage to an animal living in a hot climate. (2 marks)

Ears with a large surface area will allow a high rate of heat transfer away from the body which will help the animal to stay cool in hot conditions.

An object that's hotter than its surroundings...

Emits more radiation than it absorbs (as it cools down).

What does heat energy flow away from and to?

Flows away from a hotter object to its cooler surroundings.

What is cavity wall insulation?

Foam squirted into the gap between the bricks reduces convection, conduction and radiation across the gap.

What can convection only happen in?

Gases and liquids

Why does the surface area and volume affect the rate of heat energy transferred?

Heat energy is radiated from the surface of an object. The bigger the surface area, the more infrared waves that can be emitted from (or absorbed by) the surface - so the quicker the transfer of heat. E.g. radiators have large surface areas to maximise the amount of heat they transfer. This is why car and motorbike engines often have 'fins' - they increase the surface area so the heat is radiated away quicker. So the engine cools quicker. 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 area so they can emit heat as quickly as possible. If two objects at the same temperature have the same surface area but different volumes, the object with the smaller volume will cool more quickly - as a higher proportion of the object will be in contact with it's surroundings.

Explain why heated air rises.

Heated air expands, so it becomes less dense than the surrounding cooler air and rises.

What is the radiator example?

Heating a room with a radiator relies on convection currents. Hot, less dense air by the radiator rises and denser, cooler air flows to replace it.

What do U-values show?

How fast heat can transfer through a material

What are humans and animals ways of controlling heat transfer?

In the cold, the hairs on your skin 'stand up' to trap a thick layer of insulating air around the body. This limits the amount of heat loss by convection. Some animals do the same using fur. When you're too warm, your body diverts blood to flow near the surface of your skin so that more heat can be lost by radiation - that's why some people go pink when they get hot. Generally, animals in warm climates have larger ears than those in cold climates to help control heat transfer. Example: Arctic foxes have evolved small ears, with a small surface area to minimise heat loss by radiation and conserve body heat. Desert foxes on the other hand have huge ears with a large surface area to allow them to lose heat by radiation easily and keep cool.

Heat radiation is the transfer of heat energy by...

Infrared radiation

Materials that have larger spaces between their particles conduct heat energy much more slowly - these materials are...

Insulators

What is the steam you see rising from a boiling kettle?

Invisible water vapour condensing to form tiny water droplets as it spreads into cooler air.

What is kinetic energy?

It describes how particles move in solids, liquids and gases. The energy an object (or particle) has because of its movement is called kinetic energy.

Why are some heaters filled with oil?

It has a high specific heat capacity which is around, 2000 J/kg°C. Because this is lower than water's oil heating systems are often not as good as water-based systems. Oil does have a high boiling point though, which usually means oil-filled heaters can safely reach higher temperatures than water-based ones.

Why is water ideal for central heating systems in buildings?

Its a liquid so it can easily be pumped around pipes and has a really high specific heat capacity.

What is a hot water tank jacket?

Lagging such as fibreglass wool reduces conduction and radiation.

What surfaces reflect a lot of IR falling on them?

Light, shiny surfaces

Give three ways of increasing the rate of condensation of a gas onto a cold surface.

Lowering the temperature of the gas Decreasing the airflow Increasing the concentration of the gas Other answers: Decreasing the temperature of the surface the gas is condensing on Increasing the density of the gas

Why will a larger surface area make the rate of evaporation faster?

More particles will be near enough to the surface to escape the surface.