Energy and electricity
Nothing would happen without energy. Energy is needed to:
• keep our bodies working
• make machines work
• heat homes, schools and offices.
Energies in action
• heat energy
• light energy
• sound energy
• electrical energy
• kinetic (movement) energy.
Some energy has to be stored so that it is ready for use when we need it.
• Chemical energy is stored in food, fuels and cells. • Gravitational potential energy is stored in high up things. • Strain energy is stored in stretched or squashed things. • Nuclear energy is stored inside atoms.
How is energy measured?
Energy is measured in joules (J) or kilojoules (kJ). A kilojoule is 1000 joules.
Energy needs to be changed to be useful.
in the cells in the wires in the bulb
An energy flow diagram.
Many energy changes take place in everyday life. Often wasted energy is produced in the forms of heat or sound.
A car engine produces
kinetic energy, which is
useful. It also produces
heat and sound
Energy cannot be made or destroyed, but can only be changed from one form to another. This is the law of conservation of energy.
A circuit must have a cell or power supply to provide a voltage. The voltage pushes
the electrons around the circuit and gives them energy. This electrical energy is transferred to other components in the circuit, which convert it to other forms of energy. For instance, a light bulb transfers electrical energy to heat and light energy. The voltage of a cell can be measured using a voltmeter. The units for voltage are
volts (V).The voltage across a component is a way of measuring how much energy the component is transferring. The voltage across all the components in a series circuit adds up to the voltage across the cell.
Electricity and cells
Electricity is supplied to homes and factories as mains electricity. This travels along
cables connected to the National Grid.
Fossil fuels are transported to power stations where they are burnt to release heat energy. This heats water, turning it to steam. The steam drives turbines which turn
generators. The electricity generated flows along cables into the National Grid.
Nuclear fuel is made from a radioactive metal called uranium. The energy in nuclear fuel did not come from the Sun.
Electricity can be generated from renewable resources such as wind and moving water. These will become more important as fossil fuels run out.
Sometimes we need a source of portable electricity when we are not close to the mains. This can be supplied by cells (sometimes called batteries). These store
chemical energy which can be changed to electrical energy. Cells go flat when they run out of chemical energy. Some cells can be recharged.
Energy cannot be made or destroyed, but it can be changed to different forms. Not all energy is turned into a form that we want. Often it is turned into heat that we do not need. This is wasted energy. A car engine produces kinetic energy, which is useful. It also produces heat and sound which are wasted forms of energy.
The percentage of useful energy produced by something is known as its efficiency.
The human body is 25% efficient.
Gravity and space
Mass and weight
The mass of something is the amount of substance or ‘matter’ it contains. It is measured in kilograms (kg). Weight is the force of gravity pulling on a mass. It is a
force, so it is measured in newtons (N).
Gravity is the force of attraction between two masses. The force of gravity is stronger if:
• the objects have large masses
• the objects are close together.
On Earth, the gravity pulls on every kilogram of mass with a force of 10 N.
Gravity is not as strong on the Moon, because the Moon has a much smaller mass than the Earth. If you went to the Moon your mass would not change, but your weight would be less than on Earth because the Moon’s gravity is weaker.
If a rocket travels away from the Earth, the force of gravity gets less and less as it gets further from Earth. If it is heading for the Moon, it will eventually reach a place where the Earth’s gravity is cancelled out by the Moon’s gravity. After that, the Moon’s gravity will be pulling it towards the Moon.
The Sun’s gravity keeps all the planets moving in elliptical orbits around it. If there
was no gravity from the Sun, the planets would all fly off into space. The Earth’s gravity keeps the Moon in orbit around the Earth.
A satellite is anything that orbits around a planet. The Moon is the only natural
satellite of the Earth.
Artificial satellites can be put into orbit around the Earth. They can be used for communications (transmitting telephone calls or television programmes), for navigation, or to take pictures of the Earth or the planets and stars.
Satellites can also be put into orbit around other planets. They can take pictures and take measurements, and send all the information back to Earth.
Changing ideas about the Solar System
People have known that the Earth is spherical for thousands of years, but they have only believed that the Sun is at the centre of the Solar System for about 500 years.
Early ideas had the Earth in the centre of the Solar System, with the Sun, the planets and the stars moving in circular orbits around the Earth. These ideas were used to make predictions about where the planets would be in the sky, but the predictions were not very accurate.
Copernicus suggested that the Sun was in the centre of the Solar System, but his model still had the planets moving in circular orbits. The predictions made using this model were a bit more accurate, but there were still errors.
Kepler suggested that the planets actually move in elliptical orbits around the Sun. His model could be used to make very accurate predictions. After Newton had worked out how Kepler’s model could be explained using his ideas about gravity, most scientists accepted that this was the correct way of thinking about the Solar System.
Speed tells us how fast something is going.
We can work out the mean (average) speed of something by using this formula:
mean speed = distance travelled ? time taken.
Speed can be measured in:
• metres per second (m/s)
• kilometres per hour (km/h)
• miles per hour (mph).
We can show how things move on a distance–time graph. This graph shows Kieron
walking to school.
Balanced forces are forces which are the same size but work in opposite directions. Unbalanced forces make things change speed, change shape or change direction.
If forces are balanced:
• a stationary object stays stationary
• a moving object continues to move at the same speed.
If forces are unbalanced:
• a stationary object will start to move
• a moving object will change its speed or direction.
The motorbike is
accelerating because the
forward force is greater
than the backward force.
The motorbike is going at
a steady speed. The forces
A car or motorbike uses fuel to move at a steady speed because it needs a force from the engine to balance the forces of air resistance and friction.
The amount of air resistance on something can be reduced by giving it a smooth, streamlined shape. The air resistance increases as the speed increases, so cars use up more fuel per mile when they are travelling fast. Air resistance is caused by air particles hitting the moving object. The particles transfer energy to the object, which
is why objects moving through air can get hot.
The forces on a skydiver change during a jump. Her weight is the same all the time, but her air resistance changes during the jump. We can use a speed–time graph to
show what happens.
• At A she has just jumped out of the
plane so she has only just started to
move downwards. Her air resistance
is very small.
• At B her air resistance is bigger, but
not as big as her weight so she is still
• At C the forces on her are balanced so
she falls at a steady speed.
• At D she has opened her parachute.
The air resistance force is suddenly a
lot bigger than her weight, so she
• At E the forces are balanced again,
and she will continue to fall at a
steady speed until she reaches the
Pressure and moments
Pressure on solids
The thumb is putting a The thumb is putting a A B
force onto the head of force on the board. The
the pin. The force is area of the thumb is
transferred to the point much larger than the
of the pin. This is a area of the pin point,
very small area, so so there is only a small
there is a very large pressure on the board.
pressure on the board, The thumb does not go
and the pin goes in. into the board.
Examples of a small area giving a large pressure:
Sharp knife. Ice skates.
Examples of a large area giving a small pressure:
Snow shoes. Camel on sand.
We can work out the pressure on something by using this formula:
pressure = force ? area
Pressure can be measured in: 2• newtons per square metre (N/m) 2• newtons per square centimetre (N/cm)
• pascals (Pa).
21 Pa = 1 N/m
Pressure in liquids and gases Both gases and liquids are fluids. Fluids can flow. Pressure in fluids acts in all
directions. The particles in fluids are moving all the time and hitting the walls of
containers or other things they come into contact with. The force of the collisions
causes pressure which acts in all directions.
The swimmer is floating because pressure in the water provides a force called upthrust,
which balances the force of gravity. As you
go deeper into the sea, pressure increases because there is more water above you
pressing down. Dams are made with thicker walls at the bottom to withstand the pressure.
Uses of pressure in liquids and gases
Gases can be compressed. The pressure in
a compressed gas is higher because there are more molecules moving around and
hitting the walls of the container.
Pneumatic tyres contain compressed air
and this keeps the tyre inflated and helps to soften a bumpy ride.
Liquids cannot be compressed. Liquids are used in hydraulic systems which can be used to increase the size of a force. Hydraulics are used in car braking systems.
Pressure = force ? area
The pressure on the water is 25 N . 2 5 cm
2This is 5 N/cm.
2The area at the end of the other syringe is 12 cm.
Force = pressure ; area
22The output force is 5 N/cm ; 12 cm = 60 N.
Forces can be used to turn objects around
pivots. A pivot is also known as a fulcrum.
Levers work by magnifying the force that is put in or the distance it moves.
The hammer is acting as a force multiplier.
A turning force is called a moment. Moments are measured in newton centimetres (N cm) or newton metres (N m).
Small moment. Big moment.
The longer the distance the greater the moment. It is easier to turn the long spanner
than the short one.
When an object is balanced, the anticlockwise moment = the clockwise moment.
In the example above:
the anticlockwise moment = 300 N ; 2 m
= 600 Nm
the clockwise moment = 400N ; 1.5 m
= 600 Nm
The clockwise and anticlockwise moments are the same, so the seesaw is balanced or in equilibrium.
Cranes use the principle of moments. The moment from the load is balanced by the moment from the concrete blocks to stop the crane toppling over.