1.4 Energy

Work (in terms of energy)

A measure of the energy transferred from one store to another

Work (in terms of forces)

When the force causes a displacement of the object

The equation linking work, force and distance (in the direction of the force)

W = Fd

Unit for energy and work

Joules (J)

Unit for force

Newtons (N)

Unit for distance

Metres (m)

What one joule is equal to

1J = 1Nm
e.g lifting a (1N) apple 1m

Alternative unit for work done by a force

Newton-metre (Nm)

Effect of doing work against friction

rise in the temperature of the object

Power

rate at which energy is transferred or work is done

The equation linking power, energy transferred and time

P = E/t

Unit for power

Watts (W)

Unit for time

Seconds (s)

One watt is equal to

1W = 1 J/s

The equation linking power, work done and time

P = W/t

Kinetic energy

The energy of a moving object

The equation linking kinetic energy, mass and speed

E_k = ½ mv²

Unit for mass

Kilograms (kg)

Unit for speed or velocity

Metres per second (m/s)

Gravitational potential energy

The energy of an object raised against gravity

The equation linking gravitational potential energy, mass, gravitational field strength and height

E_p = mgh

Unit for gravitational field strength

Newtons per kilogram (N/kg)

Unit for height

Metres (m)

Efficiency

how good a device is at transferring energy input to useful energy output

Total input energy

The total amount of energy supplied to a device

Useful energy output

The amount of energy that is transferred to useful energy stores

Wasted energy output

The amount of energy that is wasted or dissipated to the surroundings e.g heat

The equation linking efficiency and energy

Efficiency = useful output energy transfer ÷ total input energy transfer

The equation linking efficiency and power

Efficiency = useful power output ÷ total power input

Percentage efficiency

multiplying the efficiency by 100

What is meant by an efficiency greater than 1 or 100%

More energy is being transferred than is being supplied

Why can't you have an efficiency greater than 1 or 100%

would break the law of conservation of energy as energy is being created

Reasons devices waste energy

1. Friction between moving parts

2. Heat due to electrical resistance

3. Unwanted sound or light

Ways to improve efficiency

1. Lubricate mechanical to reduce friction between moving parts

2. Insulate heating to reduce dissipation of thermal energy

Energy resource

A useful supply or store of energy

Non-renewable

sources used at a higher rate than can be replaced so will eventually run out (finite)

Examples of non-renewable resources

Fossil fuels (coal, crude oil, natural gas)

nuclear fuels (uranium, plutonium)

Finite

Something that has a limited number of uses before it runs out

Renewable

Energy sources that are (or can be) replenished as they are being used so will not run out

Examples of renewable resources

Bio-fuels, solar, wind, geothermal, wave, tidal, hydroelectric

Replenishing renewable resources

Human action, natural processes

Fossil fuels

Fuels formed from the remains of living organisms (coal, crude oil, natural gas)

Nuclear fuels

Radioactive materials used in nuclear reactors (uranium, plutonium)

Bio-fuels

Fuels made from plant and animal waste (wood; bio-diesel)

Resources used for transport

Fossil fuels, bio-fuels

Resources used for heating

Fossil fuels, bio-fuels, geothermal, solar

Resources used to generate electricity

Fossil fuels, nuclear fuels, bio-fuels, solar, wind, geothermal, wave, tidal, hydroelectric

Reliable (energy resource)

an energy resource that can supply enough energy to meet demand at predictable times

Examples of reliable resources

Fossil fuels, nuclear fuels, bio-fuels, tidal, hydroelectric and geothermal

Environmental impact

the damage to the environment caused by using an energy resource to produce electricity

Examples of environmental impact

Pollution, harmful waste products and loss of habitats

Pollution

Damage to the land, air or water caused by a toxic chemical or an object

Atmospheric pollution

Carbon dioxide from burning fossil/bio fuels

sulfur dioxide from burning coal

Carbon neutral

Burning bio-fuels releases same amount of carbon dioxide as crops absorbed for photosynthesis

Closed system

system with no net change to the total energy

Law of conservation of energy

Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed

Useful energy transfer

When energy is transferred by a device to energy stores we want

Wasted energy transfer

When energy is transferred by a device to energy stores we don't want

Dissipate

When energy spreads out and is transferred to energy stores into less useful forms

Dissipated energy

Energy which is wasted and is usually transferred to the surroundings, causing them to heat up

Lubricate

When a substance is used to reduce wasted energy transfers by reducing the friction between moving parts

Insulate

When a substance is used to reduce wasted energy transfers by reducing the amount of heat that is transferred to the surroundings

Thermal conductor

A material that allows charge or heat to pass through it easily

Thermal insulator

A material that does not allow charge or heat to pass through it easily

Thermal conductivity

The rate of energy transfer by conduction through a material

Conduction

transferred through solid

vibrating particles collide and transfer energy to one another

Convection

transferred through a moving liquid or gas

differences in densities

Radiation

transferred from a hot object

object emitting infrared (heat) radiation

System

An object or group of objects

Kinetic energy

The energy of a moving object

Heat (thermal) energy

energy of the particles in an object due to its temperature

Light (radiant) energy

carried by a light wave

Gravitational potential energy

when an object is raised against gravity

Chemical energy

stored in food, fuels and batteries

Sound energy

released by vibrating objects

Elastic potential energy

stored when an object is stretched or squashed

Electrical energy

stored from moving charges or static electric charges

Nuclear energy

stored in the nuclei of atoms

Magnetic energy

stored when repelling poles are pushed together or attracting are pulled apart

Doing work

The scientific way of saying that energy has been transferred

Energy

The capacity to do work