# Energy

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## Energy Stores and Transfers

SystemA single, or group of objects that have the ability to do work.

Closed System: A system that experiences no net change in its total energy when energy transfers occur within it.

Dissipation: When energy is scattered in all directions and lost to the surroundings.

Conservation of Energy: Energy cannot be created or destroyed, only transferred to one store to another. 1) Assumed non is dissipated 2) Some dissipated e.g. Phone dissipates thermal energy.

Energy Stores: Nuclear, Elastic, Thermal, Graviatational potential, Electrostatic, Magnetic, Chemical, Kinetic. E.g. Energy is transferred mechanically from the elastic potential store of the bow to the kinetic store of the arrow.

Energy Transfers: Mechanical, Radiation (Infrared, Light, Sound), Heating (Convection, Conduction), Electrical.

Example: Boiling water in a kettle. Water is the system. Energy is transferred to the water (from the kettle's heating element) by heating, to the water's thermal energy store (temperature rise).

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## Equations

Mechanical Work - The amount of energy transferred by force.
Work (J) = Force (N) x Distance (Along the line of force) (m)

Electrical Work - When charge flow in a circuit is the amount of energy transferred.

Energy Transferred (Work) (J) = Charge Flow (Q) x Potential Difference (V)

Kinetic - In moving objects.

Kinetic Energy (J) = 0.5 x Mass (Kg) x Velocity2 (m/s)

Elastic Potential Energy - In a stretched, compressed or twisted object.

Elastic Potential Energy (J) = 0.5 x Spring Constant (N/m) x Extension2 (m)

Gravitational Potential Energy - In objects raised above a planet's surface.

GPE (J) = Mass x Gravitational Field Strength (N/Kg) x Height (m)

1 Kilojoule (KJ) = 1,000J (103J) / 1 Megajoule = 1,000,000 J (106J)

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## Specific Heat Capacity

The thermal (Internal) energy store in a system changes if its temperature changes.
Specific Heat
Capacity
(c) – The amount of energy required to raise the temperature of 1Kg of a substance by 1oC.

Change in Thermal Energy (J) = Mass (J) x Specific Heat Capacity (J/KgoC) x Temperature Change (oC)

Example:  When the heater was left on for 5 mins, the heater supplied 10,800J of thermal energy to the aluminium block. The temperature of the 2Kg block of aluminium rose 6oC.

Specific Heat Capacity = 10,800 / 2 x 6 = 900J/KgoC

Investigation: (Need: Solid - 2 holes (e.g. Copper), Heater and Thermometer)

1) Measure mass of block and insulate. Insert thermometer and heater 2) Measure initial temperature, set power pack to 10V and switch on. Start stop watch 3) Take readings of temperature and current every minute for 10 minutes 4) Plot temperature (y) energy transferred (x) graph using P = I V and E = P t to find the energy transferred - Creates a linear graph

5) Calculate Specific Heat Capacity from graph by using the formula: 1 / gradient x mass

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## Power

Power – The rate at which energy is transferred. The rate at which work is done. Power is measured in Joules/second. 1 J/s = 1 Watt

Power (W) = Energy Transferred (J) / Time (s)

Power = Work Done / Time

Example: A motor transfers 4.8KJ of energy in 2 minutes. Find its power.

Energy transferred = 4.8KJ = 4,800J

Take Taken = 2 minutes = 2 x 60s = 120s

P = E / t = 4800 / 120 = 40W or 40J/s

Example:How long does it take for a 55W motor to do 110J of work?

Rearrange: t = W / P = 110 / 550 = 0.2s

Note: A powerful machine is one which can transfer lots of energy in a short space of time.

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## Conduction and Convection

Conduction - The process by which vibrating particles transfer energy to neighbouring particles.

Thermal Conductivity - A measure of how quickly energy is transferred through a material by conduction. Materials with a high thermal conductivity transfer energy between their particles quicker, so the faster energy can transferred by conduction. Occurs mainly in solids, as particles collisions in liquid and gases are less frequent due to being freer.

Thermal Conductors - Materials with high thermal conductivity.

Thermal Insulators - Materials with low thermal conductivity.

Convection - Where energetic particles move away from hotter to cooler regions. Happens in gases or liquids. E.g. The heated air becomes hotter and less dense, as the particles move quicker. It cools, becomes less dense and sinks. This creates a convection current cycle.

Questions:

What is conduction and describe how energy is transferred by conduction. What is convection? In which state of matter does convection not happen?

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## Reducing Unwanted Energy Transfers

You can reduce the amount of energy lost using thermal insulators and Lubricants.

Dissipation Through Walls Depends On:

1) Temperature difference inside and outside 2) Area of walls (larger = more loss) 3) Thermal conductivity of the walls 4) Thickness of walls.

Shiny Foil: (Radiation) Prevents radiation from reaching walls by reflecting it back into the room.

Curtains: (Convection/Radiation) Prevents draughts. As opaque, radiated heat cannot pass.

Cavity Wall Insulation: Trapped air reduces conduction. Foam can prevent convection in cavity.

Double Glazing: Extra pane and trapped air stops conduction. Removing air stops convection.

Loft Insulation: (roof) (Conduction/Convection) Foam contains trapped air - an insulating layer.

Draught Excluders: (Convection) Hairy/spongy strips that close gaps reducing heat loss.

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## Efficiency

Efficiency - The amount of useful energy you get from an energy transfer, compared to the energy input.

Efficiency = Useful Output Energy Transfer / Total Input Energy Transfer (x100 for percentage)

Efficiency = Useful Power Output / Total Power Input (x100 for percentage)

Example: The wind turbine produces 120MW of electrical energy for every 500MV of kinetic energy provided by the wind. Calculate the efficiency of the wind turbine.

120 / 500 = 0.24 efficient or 24% efficient.

Wasted Energy - No device is 100% efficient and the wasted energy is usually transferred to useless thermal energy stores.

Improving Efficiency - You can improve the efficiency of energy transfers by insulating objects, lubricating them or making them more streamlined.

Questions: A machine with an efficiency of 68% transfers 816J usefully. Find the total energy.

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