AQA Physics 1a

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Heat - Transferred in 3 Ways

Heat energy can be transferred by radiation, conduction or convection.

Heat radiation is the transfer of heat energy by infrared (IR) radiation.

Conduction and convection involve the transfer of energy by particles.

Conduction is the main form of heat transfer in liquids and gases.

Infrared radiation can be emitted by solids, liquids and gases.

Any object can both absorb and emit infrared radiation, whether or not conduction or convection are also takin place.

The bigger the temperature difference between a body and its surroundings, the faster energy is transferred by heating.

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Infrared Radiation

Emission of electromagnetic waves.

All objects are continually emitting and absorbing infrared radiation. Infrared radiation is emitted from the surface of an object.

An object that's hotter than its surroundings emits more radiation than it absorbs (as it cools down). And an object that's cooler than its surroundings absorbs more radiation than its emits (as it warms up).

The hotter an object is, the more radiation it radiates in a given time.

You can feel this infrared radiation if you stand near something hot like a fire or if you put your hand just above the bonnet of a recently parked car.

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Surface Colour and Texture

Dark, matt surfaces absorb infrared radiation falling on them much better than light, shiny surfaces, such as gloss white or silver. They also emit much more infrared radiation (at any given temperature).

Light, shiny surfaces reflect a lot of the infrared radiation falling on them. E.g. vacuum flasks have silver inner surfaces to keep heat in or out, depending on whether it's storing hot or cold liquid.

Solar hot water panels contain water pipes under a black surface (or black painted pipes under glass). Radiation from the sun is absorbed by the black surface to heat the water in the pipes.

This water can be used for washing or pumped to radiators to heat the building.

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Kinetic Theory

The three states of matter are solid, liquid and gas. The particles of a particular substance in each state are the same - only the arrangement and energy of the particles are different.

SOLIDS - strong forces of attraction hold the particles together in a fixed, regular arrangement. The particles don't have much energy so they can only vibrate about their fixed position.

LIQUIDS - there are weaker forces of attraction between the particles. The particles are close together, but can move past each other, and form irregular arrangements. They have more enery than the particles in a solid - they move in random directions at low speeds.

GASES - there are almost no forces of attraction between the particles. The particles have more energy than those in liquids and solids - they are free to move, and travel in random directions and at high speeds.

When you heat a substance, you give its particles more kinetic energy (KE) - they vibrate or move faster. This is what eventually causes solids to melt and liquids to boil.

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Conduction

Conduction of heat - occurs mainly in solids.

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

This process continues through the solid and gradually some of the extra kinetic energy (or heat) is passed all the way through the solid, causing a rise in temperature at the other side of the solid. And hence an increase in the heat radiating from its surface.

Usually conduction is faster in denser solids, because the particles are closer together and so will collide moe often and pass energy between them. Materials that have larger spaces between their particles conduct heat energy much more slowly - these materials are insulators.

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Metals

Metals are good conductors because of their free electrons.

Metals "conduct" so well because the electrons are free to move inside the metal.

At the hot end the electrons move faster and collide with other free electrons, transferring energy. These other electrons pass on their extra energy to other electrons, etc.

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

This is why heat energy travels so fast through metals.

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Convection

Convection of heat - liquids and gases only.

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

This is how immersion heaters in kettles and hot water tanks and convector heaters work. Convection simply can't happen in solids because the particles can't move.

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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 reduction 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 colder 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.

Convection currents are all about changes in density.

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Things to Note:

  • Convection is most efficient in roundish or squarish containers, because they allow the convection currents to work best. Shallow, wide containers or tall, thin ones just don't work quite so well.
  • Because the hot water rises (because of the lower density) you only get convection currents in the water above the heater. The water below it stays cold because theres almost no conduction.
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Radiator

1) Heating a room with a radiator relies on convection currents too.

2) Hot, less dense air by the radiator rises and denser, cooler air flows to replace it.

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Condensation

Condensation is when gas turns into a liquid.

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.

3) Wter vapour in the air condenses when it comes into contact with cold surfaces e.g. drinks glasses.

4) The steam you see rising from a boiling kettle is actually invisible water vapor condensing to form tiny water droplets as it spreads into cooler air.

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Evaporation

1) Evaporation is when particles escape from a liquid.

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

3) Particles near the surface of a liquid 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 over come the attractive forces of the other particles in the liquid.

4) The fastest particles (with the most kinetic energy) are most likely to evaporate from the liquid - so when they do, the average speed and kinetic energy of the remaining particles decreases.

5) This decrease in average particle energy means the temperature of the remaining liquid falls - the liquid cools.

6) This cooling effect can be really useful. For example, you sweat when you exercise or get hot. As the water from the sweat on your skin evaporartes, it cools you down.

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Rate of Evaporation - faster if the...

  • Temperature is higher - the average particle energy will be higher, so more particles will have enough energy to escape.
  • Desity is lower - the forces between the particles will usually be weaker, so more particles will have enough energy to overcome these forces and escape the liquid.
  • Surface area is larger - more particles will be near enough to the surface to escape the liquid.
  • 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 if evaporation. A greater airflow means air above the liquid is replaced more quickly, so the concentration in the air will be lower.
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Rate of Condensation - faster if the...

  • 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.
  • Temperature of the surface the gas touches is lower.
  • Density is higher - the forces of attraction 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.
  • Airflow is less - the concentration of the substance in the air will be higher, and so the rate of condensation will be greater.
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Rate of Heat Energy

  • 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 motorbikes often have 'fins' - they increase the surface area so 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 away as quickly as possible.
  • If two objects at the same temperature have the same surface area but different volumes, te object with the smaller volume will cool more quickly - as a higher proportion of the object will be in contact with it's surroundings.
  • Other factors like the type of material, affect the rate too. 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 awat much faster than if it is in contact with a good insulator.
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Limiting Heat Transfer

Some devices are designed to limit heat transfer.

Vacuum Flask:

1) The glass bottle is double-walled with a vacuumbetween 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.

3) The bottle is supported using insulating foam. This minimises heat conduction to or from the outer glass bottle.

The stopper os made of plastic and filled with cork or foam to reduce any heat conduction through it.

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Humans & Animals

1) In the cold, the hairs on your skin 'stand up' to trap a thicker layer of insulating air around the body. This limits the amount of heat loss by convection. Some animals do the same using fur.

2) When you're too warm, your body diverts more 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 too hot.

3) Generally, animals in warm climates have larger ears than those in cold climates to help control heat transfer.

For example:

Artic foxes have evolved to have 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.

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Effectiveness & Cost-Effectiveness

1) The most effective methods of insulation are ones that give you the biggest annual saving (They save you the most money each year on your heating bills).

2) Eventually, the money you've saved on heating bills will equal the initial cost of putting in the insulation (the amount it cost to buy). The time it takes is called the payback time.

PAYBACK TIME = initial cost / annual saving

3) The most cost-effective methods tend to be the cheapest.

4) They are cost-effective because they tend to have a short pay back time - this means the money you save covers the amount you paid really quickly.

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Types of Heat Transfer Involved

  • Cavity Wall Insulation - foam squirted into the gap between the bricks reduces convection and radiation across the gap. Pockets of air in the foam reduce heat transfer by conduction.
  • Loft Insulation - a thick layer of fibreglass wool laid out across the loft floor and ceiling reduced heat loss by conduction and convection.
  • Draught-Proofing - stips of foam and plastic around doors and windows stop draughts of cold air blowing in, i.e. they reduce heat loss by convection
  • Hot Water Tank Jacket - lagging such as fibreglass wool reduces coduction and radiation.
  • Thick Curtains - big bits of cloth over the window to reduce heat loss by conduction and radiation.
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U-Values

U-Values show how fast heat can transfer through a material.

1) Heat transfers through materials with higher u-values than through materials with low u-values.

2) The better the insulator the lower the u-value.

E.g. The U-value of a typical duvet is about 0.75Wm2K, where as the u-value of loft insulation is around 0.15Wm2K.

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Specific Heat Capacity

1) It takes more heat energy to increase the temperature of some material than others.

E.g. you need 4200J to warm 1Kg of water by 1 degree C, but only 139J to warm 1Kg of mercury by 1 degree c.

2) Materials which need to gain lots of energy to warm up also release loads of energy when they cool down again. They can 'store' a lot of heat.

3) The measure of how much energy a substance can store is called it specific heat capacity.

4) Specific heat capacity is the amount of energy needed to raise the temperature of 1Kg of a substance by 1 degree C.

Water has a specific heat capacity of 4200J/Kg degrees C.

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SHC Formula

E = m x c x θ

energy transferred (J) = mass (kg) x specific heat capacity (J/kg degrees C) x temperature change (degrees C)

(Make sure masses are in kg.)

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Heaters

Heaters have high capacities to store lots of energy.

1) The materials used in heaters usually have high specific heat capacities so that they can store large amounts of heat energy.

2) Water has a really high specific heat capacity. It's also a liquid,so it can easily be pumped around in pipes - ideal for central heating systems in buildings.

3) Electric storage heaters - are designed to store heat energy at night (when electric is cheaper), and then release it during the day. They store the heat using concrete or bricks, which have a high specific heat capacity (around 880 J/kg degrees C).

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

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Nine Types of Energy

  • Electrical Energy - whenever a current flows.
  • Light Energy - from the Sun, light bulbs, etc.
  • Sound Energy - from loud speakers or anything noisy.
  • Kinetic Energy, or Movement Energy - anything that's moving has it.
  • Nuclear Energy - released only from nuclear reactions.
  • Termal Energy, or Heat Energy - flows from hot objects to colder ones.
  • Gravitational Potential Energy - possed by anything that can fall.
  • Elastic Potential Energy - stretch spings, elastic, rubber bands, etc.
  • Chemical Energy - possed by foods, fuels, batteries, etc.

Gravitation Potential, Elastic Potential and Chemical Energy are forms of stored energy. The energy is not obviously doing anything, it's kind of waiting to happen, i.e. waiting to be turned into one of the other forms.

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Conservation of Energy Principle

Energy can be transferred usefuly from one form to another, store or dissipated - but it can never be created or distroyed.

(dissipated - spread out and lost)

Energy is only useful when it can be converted from one form to another.

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Energy Transfers

1) Useful devices are only useful because they can transform energy from one form to another.

2) In doing so, some of the useful input energy is always lost or wasted, often as heat.

3) The less energy that is 'wasted', the more efficient the device is said to be.

4) The energy flow diagram is pretty much the same for all devices.

Energy Input -----> Useful Device -----> Useful Energy Output + Wasted Energy (heat & sound)

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Calculating Efficiency

A machine is a device which turns one type of energy into another. The efficiency of any device is defined as:

Efficiency = Useful Energy Out / Total Energy In

If you don't know the energy inputs and outputs of a machine, you can still calculate the machine's efficiency as long as you know the power input and output:

Efficiency = Useful Power Out / Total Power In

You can give efficiency as a decimal or you can multiply your answer by 100 to get a percentage.

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Useful Energy Input

Useful energy input isn't usually equal to total energy output.

No device is 100% efficient and the wasted energy is usually spread out as heat.

Electric heaters are the exception to this. They're 100% efficient because all the electricity is converted to "useful" heat. Ultimately, all energy ends up as heat energy.

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Wasted Heat

1) Useful energy is concentrated energy. The entire energy output by a machine, both useful and wasted, ends up as heat.

2) This heat is transferred to cooler surroundings, which then become warmer. As the heat is transferred to cooler surroundings, the energy becomes less concentrated - it dissipates.

3) The total amount of energy stays the same. The energy is still there, but as it becomes increasingly spread out, it can't be easily used or collected back in again.

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Light Bulb Example

Thinking about cost-effectiveness and efficiency when choosing appliances.

1) A low-energy bulb is about 4 times as efficient as an ordinary light bulb.

2) Energy-efficient light bulbs are more expensive to buy but they last much longer.

3) If an energy-saving light bulb cost £3 and saved £12 of energy a year, its payback time would be 3 months.

4) Energy-saving light bulbs are normally more cost effective than ordinary bulbs.

5) LED light bulbs are even more efficient than low-energy bulbs, and can last even longer.

6) But they are more expensive to buy and don't give out as much light as the other two types of bulb.

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Replacing Old Appliances

Thinking about cost-effectiveness and efficiency when choosing appliances.

1) New, efficient appliances are cheaper to run than older, less efficient appliances. But new appliances can be expensive to buy.

2) You've got to work out if it's cost-effective to buy a new appliance.

3) To work out how cost-effective a new appliance will be you need to work out its payback time.

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'Waste Energy'

Sometimes waste energy can actually be useful.

1) Heat exchanges reduce the amount of heat energy that is 'lost'.

2) They do this by pumping a cool fluid through the escaping heat.

3) The temperature of the fluid rises as it gains heat energy.

4) The heat energy in the fluid can then be converted into a form of energy that's useful again - either in the original device, or for other useful functions.

For example, some of the heat from a car's engine can be transferred to the air that's used to warm the passenger compartment.

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Sankey Diagrams

The idea of Sankey diagrams is to make it easy to see the at a glace how much of the total energy in is being usefully employed compared with how much is being wasted.

The thicker the arrow, the more energy it represents - so you see a big thick arrow going in, then several smaller arrows going off it to show the different energy transformations taking place.

You can have either a little sketch or a properly detailed diagram where the width of each arrow is proportionall to the number of joules it represents.

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Kilowatt-hours

Kilowatt-hours (kWh) are "units" of electricity.

  • 1) Electrical appliances transfer electrical energy into other forms - e.g. sound and heat energy in a radio.
  • 2) The amount of energy that is transferred by an appliance depends on its power (how fast the appliance can transfer it) and the amount of time that the appliance is switch on.

Energy = Power x Time            E = P x T

  • 3) Energy is usually measure in joules (J) - 1 J is the amount of energy transferred by a 1 W appliance in 1 s.
  • 4) Power is usually measured in watts (W) or kilowatts (kW). A 5kW appliance transfers 5000J in 1 s.
  • 5) When you're dealing with large amounts of electrical energy (e.g. the energy used by a home in one week), its easier to think of the power and time in kilowatts and hours - rather than in watts and seconds.
  • 6) So the standard unitsof electrical energy are kilowatt-hours (kWh) not joules.

A kilowatt-hour is the amount of electrical energy used by a 1 kW appliance left on for 1 hour.

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Calculating the Cost of Electricity

No. of Units (kWh) Used = Power in (kW) x Time (in hours)

Units = kW x hours

Cost = No. of Units x Price Per Unit

Cost = Units x Price

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Electricity Meter

How to read an electricity meter:

  • The units are in kWh - (but make sure to check).

You may be given two meter readings and askeed to work out the total energy that's been used over a particular time period. Just subtract the meter reading at the start of the time (the smaller one) from the reading at the end to work this out.

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Choice of Electrical Equipment

1) There are often a few different appliances that do the same job.

In the exam you may be asked to weigh up the pros and cons of different appliances and decide which one is most suitable for a particular situation.

2) You might need to work out whether one appliance uses less energy or is more cost-effective than another.

3) Yoy might need to think about the practical advantages and disadvantages of using different appliances. E.g. 'Can an appliance be used in areas with limited electricity supplies?'

4) You might need get asked to compare two appliances that you haven't seen before. Just take your time and think about the advantages and disadvantages - you should be able to make a snesible judgement.

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E.g. Clockwork Radios & Battery Radios

1) Battery radios and clockwork radios are both handy in areas where there is no mains electricity supply.

2) Clockwork radios work by storing elastic potential energy in a spring when someone winds them up. The elastic potential energy is slowly released and used to power up the radio.

3) Batteries can be expensive, but powering a clockwork radio is free.

4) Battery power is also only useful if you can get hold of some new batteries when the old ones run out. You don't get that problem with clockwork radios - but it can get annoying having to wind them up every few hours to recharge them.

5) Clockwork radios are also betterfor the environment - a lot of energy and harmful chemicals go into making batteries, and they're often tricky to dispose of safely.

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Standards of Living

1) Most people in developed countries have access to mains electricity. However, many people living in the worlds poorest countries don't - this has a big effect on their standard of living.

2) In the UK, our houses are full of devices that transform electrical energy into other useful types of energy. For example, not only is electric lighting useful and covenient, but it can also help improve safety at night.

3) Refigerators keep food fresh for longer by slowing down the growth of bacteria. Refrigerators are also used to keep vaccines cold. Without refigeration it's difficult to distribute important vaccines - this can have devastating effects on a country's population.

4) Electricity also pays an important role in improving public health in other ways. Hospitals in developed countries rely heavily on electricity, e.g. for X-ray machines. Without access to these modern machines, the diagnosis and treatment of patients would be poorer and could reduce life expectancy.

5) Communications are also affected by a lack of electricity. No electricity means no internet or phones - making it hard for people to keep in touch, or for people to send or receive new and information.

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