# Physics Unit 1 Notes

For those sitting Unit 1 Physics exams, whether it be a module or a resit...hope these help! :)

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• Created by: Emma Jay
• Created on: 28-05-12 10:59

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Topic 1 how is heat transferred?
Heat Flow
Objects get hot if heat energy (thermal energy) passes into them. We know they get hot because
the temperature rises. It is important that we know the difference between temperature and
heat:
Temperature - the measure of how hot something is, measured in degrees Celsius;
Heat - a flow of energy from a hot object to a cold object. Heat is measured in joules (J).
Note: Temperature is not heat
A big heat flow does not mean a high temperature. If we heat up a beaker of ice, there is a big
heat flow, but the temperature does not rise above 0 oC until all the ice has melted.
Heat always flows from hot to cold. Heat does not flow from a cold object to a hot object. For
example the heat from a hotplate passes into a cold pan to heat up the water in the pan.
The greater the temperature difference between the hot object and the cold object, the
greater is the flow of heat.
How Heat Flows
There are three methods for heat to flow:
Conduction;
Convection;
Radiation.
Conduction
If you put a metal bar into a candle (or bunsen) flame, it gets hot
quickly. Soon you can't hold it. If you put a glass bar into a candle, it
won't get too hot to hold. But if you touched the end that was in the
flame, you would find that it was really hot!
The process in which heat passes through a solid substance is called conduction. Metals are good
conductors of heat. Non-metals are generally bad conductors of heat. Liquids and gases are bad
conductors of heat as well. A bad conductor of heat is called an insulator. Your duvet traps air
which is a good insulator.
Conduction works like this:
You will know that atoms and molecules in a solid are in a fixed lattice like this. They are bound
with bonds (like springs). The molecules vibrate about a central point.

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If we apply heat, the molecules vibrate with bigger vibrations and set their neighbours vibrating
with bigger vibrations. These pass on the vibrations to their neighbours in turn. The bigger the
vibrations, the hotter the material. If the vibrations are passed on easily, the material is a good
conductor. (In reality the situation is more complicated than this, but this is all you need to know
at this level.)
Convection
Convection occurs only in liquids and gases. We call liquids and gases fluids.…read more

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Energy can neither be created nor destroyed.
It is only transferred from one form to another.
What this means is that you cannot get something for nothing. Nor can you lose it. Every joule of
energy has to be accounted for. This is what this topic is about.…read more

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In this system the vehicle uses a generator to charge up a battery. When the vehicle accelerates,
the battery gives power to a motor.
Low Grade Heat
There are ways of recovering waste heat. Many devices have heat exchangers which use waste
heat going out to warm up air or water going in. In the end all energy ends up as low grade heat.
The further along the energy process, the lower the grade of heat.…read more

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The energy efficiency is always a fraction less than 1, which is multiplied by 100 to give a
percentage. (If you get 2/3 correct in you exam, you get 67 %.)
Examples of efficiency:
Device Efficiency (%)
Steam engine 10
Car 35
Power station 45
Electric motor 70
Transformer 95
You never get devices that are 100 % efficient. If your answer gives 100 % efficiency or more, you
have done it wrong! Watch out for this bear trap.…read more

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To do a useful job for us, electrical energy has to be converted into other forms of energy. For
example:
A drill:
Electrical Energy Movement (Kinetic) Energy
A radio:
Electrical Energy Sound Energy
Appliance Useful Energy Other Energy
Drill Kinetic Noise/heat
Iron Heat Heat (surroundings)
Hoover Kinetic (moving Air) Heat/Noise
TV Set Sound and Light Heat
The power of an appliance
Electrical energy is measured in joules (J).…read more

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TV set 200 0.20
Kettle 2200 2.2
Shower 7500 7.5
How much does it cost to use an appliance?
When we work out the cost of using an appliance, we pay for the energy that has been used. We
could pay for the number of joules used, but the joule is only a small unit. So we need a bigger
unit. This is called the kilowatt-hour (kWh) or unit.…read more

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How much does Lorraine's shower cost?
Lorraine has used 2.5 kWh
Cost = 2.5 kWh × 8 p = 20 pence
How do we get electricity to our homes?
Electrical energy cannot be stored as electricity. In a battery it is converted to chemical energy.
We are constantly using electricity, so it has to be constantly generated.
Electricity is generated in large quantities in power stations.…read more

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Why High Voltage ?
The reason for voltage being so high is that the current is lower. Therefore the wires can be
thinner, and don't get so hot. Therefore less energy is wasted. Even so there is a fair amount of
energy lost in heating up the wires which heats up the countryside.
Topic 4 How should we generate the electricity we need?
Fossil Fuels
Fossil Fuels are the remains of plants and animals that lived many millions of years ago.…read more

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To make the power station more efficient, the steam is cooled in a condenser (6). The waste
heat is carried to the cooling tower (7), where it returns as cool water (8). The clouds you see
around cooling towers are not smoke; they are water vapour.…read more

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