Physics: Unit One

P1.1 - Changes in energy stores

Chemical energy stores include fuels, food, or the chemimcals found in batteries. The energy is transferred during chemical reactions.

Kinetic energy stores describe the energy an object has because it is moving.

Gravitational potential energy stores are used to describe the energy stored in an object because of its position, such as an object above the ground.

Elastic potential energy stores describe the energy stored in a springy object when you stretch or squash it.

Thermal energy stores describe the energy a substance has because of its temperature.

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P1.1 - Changes in energy stores (2)

Energy can be stored in a variety of different energy stores.

Energy is transferred by heating, waves, electrical current, or by a force when it moves an object.

When an object falls or gains speed, its store of gravitational potential energy decreases and its kinetic energy store increases.

When a falling object hits the ground without bouncing back, its kinetic energy store decreases. Some or all of its energy is transferred to the surroundings - the thermal energy store of the surroundings increases, and energy is also transferred by sound waves.

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P1.2 - Conservation of energy

Energy cannot be created or destroyed.

Conservation of energy applies to all energy changes.

A system is an object or a group of objects.

A closed system is an isolated system in which no energy transfers take place out of or into the energy stores of the system.

Energy stores can be transferred between energy stores within a closed system. The total energy of the system is always the same, before and after, any such transfers.

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P1.3 - Energy and work

Work is done on an object when a force makes the object move.

energy transferred = work done

work done, W (joules) =

force applied, F (newtons) x distance moved along the line of action of the force , s (metres)

Work done to overcome friction is transferred as energy to the thermal energy stores of the objects that rub together and to the surroundings.

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P1.4 - Gravitational potential energy stores

Gravitational potential energy Ep is the transfer of energy from the chemical energy store in the muscles to the gravitational energy store of the object.

The force you need to lift an object at constant velocity is equal and opposite to the gravitational force on the object. So the upward force you need to apply to it is equal to the object's weight.

The gravitational potential store of an object increases when it moves up and decreases when it moves down.

It increases as it moves up, because work is done on it to overcome the gravitational force.

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P1.4 - Gravitational potential energy stores (2)

change in object's gravitational potential energy store (J)= weight (N) x change of height (m)

The gravitational field strength at the surface of the Moon is less than on the Earth (about a sixth).

change of gravitational potential energy store, ^Ep (J)= mass, m (kg) x gravitational field strength, g (N/kg) x change of height, ^h (m)

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P1.5 - Kinetic energy and elastic energy stores

Energy in the kinetic energy store of a moving object depends on its mass and its speed.

kinetic energy, Ek (J) =                                     mass, m (kg) x speed^, v^ (m/s^)

Elastic potential energy is the energy stored in an elastic object when work is done on the object.

elastic potential energy, Ee =                               1/2 x spring contact, k (N/m)                               x extention^, e^ (m^)

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P1.6 - Energy dissipation

Useful energy is energy transferred to where it is wanted in the way that it is wanted.

Wasted energy is the energy that is not usefully transferred.

The waste energy is dissipated (spreads out) and as a result may be stored in less useful ways.

Wasted energy is eventually transferred to the surroundings, which become warmer.

Useful energy is eventually transferred to the surroundings too eg. in tyres.

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P1.7 - Energy and efficiency

Weight is measured in Newtons. When on the Earth's surface 1kg = 10N.

Energy is measured in Joules. 1J = 1N lifted 1m.

efficiency = useful output energy transferred(J)

total input energy supplied to the device (J)

No energy transfer can be more than 100% efficient.

Machines waste energy because of friction between their moving parts, air resistance, electrical resistance, noise.

Machines can be made more efficient by reducing the energy the energy they waste. Eg, lubricating is used to reduce friction between moving parts.

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P1.8 - Electical appliances

Light bulb -                                           light emitted from the glowing filament (useful).           energy transfer from the filament, which is now heating the surroundings (waste).

Electric heater -                                           energy heating the surroundings (useful).                   light emitted from the glowing element (waste).

Electric toaster -                                         energy heating bread (useful).                             energy heating the toaster case and the air around it (waste).

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P1.8 - Electical appliances (2)

Electric kettle -                                         energy heating water (useful).                             energy heating kettle itself (waste).

Hairdryer -                                               kinetic energy of air driven by fan (useful).           sound of fan motor (waste).                             energy heating air flowing past heater filament (useful). energy heating hairdryer itself (waste).

Electric motor -                                       kinetic energy of objects driven by motor (useful).       gravitational potential energy, objects lifted by motor(u) energy heating motor, energy transferred by sound waves (w).

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P1.8 - Electical appliances (3)

Electricity and gas and/or oil supply most of the energy you use in your home.

Electrical appliances can transfer energy in the form of useful energy at the flick of a switch.

Uses of everyday electrical appliamces include heating, lighting, making objects move (using an electrical motor), and producing sound and visual images.

An electrical appliance is designed for a particular purpose and should waste as little energy possible.

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P1.9 - Energy and power

The more powerful an appliance is, the faster the rate at which it transfers energy.

Power is the rate of transfer of energy.

power, P (W) = energy transferred to appliance, E (J)

time taken for energy to be transferred, t (s)

efficiency = useful power out

total power in       (x100%)

power wasted = total power in - useful power out

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