P1 Conservation and dissipation of energy

Energy stores

• Chemical energy stores e.g. chemicals found in batteries, food, fuels
• Kinetic energy stores - energy because of movement
• Gravitational potential energy stores - energy because of position, e.g. above the ground
• Elastic potential energy stores - energy stored in a squashing or squeezing an object
• Thermal energy stores - energy because of temperature

Energy transfer

Energy can be transferred by heating, by waves, by an electric current or by a force when it moves an object. 

Falling object

The object falls, gains speed - gravitational potential energy decreases and kinetic energy increases. When the falling object hits the ground and stops - its kinetic energy decreases its energy is transferred to surroundings - the thermal energy and energy transferred by sound waves.

What is the conservation of energy?

The total amount of energy present stays the same before and after any changes. 

The pendulum

The pendulum shows the principal of conservation of energy in action. Gravitational potential energy is converted to kinetic energy and back, over and over again, as the pendulum swings.

• As it swings downwards the gravitational potential energy decreases and kinetic energy increases. 
• As it swings back upwards the kinetic energy decreases and gravitational potential energy increases.

• At all points during the swing, the total (GPE + KE) is constant.

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

Energy can be transferred between energy stores within a closed system. Total = always same.

Work done

When an object is moved by a force, work is done on the object by the force. Force transfers energy to object.

Work done = Energy transferred

The work done on an object depends on the size of the force and the distance moved.

Work done = Force (Newtons, N) x Distance (metres, m)

Work done to overcome friction

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

Rubbing hands makes them warm. Muscles do work to overcome friction between the hands. Work done by the hands is transferred as thermal energy that warms the hands.

Changes in GPE stores

• GPE store of an object increases when it moves up, and decreases when it moves down.
• GPE store of an object increases when the object is lifted up because work is done on it to overcome the gravitational force on the object. 
• Energy required to lift object = gravitational force acting on object

change in object's GPE (joules, J) = weight (newtons, N) x change of height (metres, m)

GPE stores and mass

Objects are easy to lift on the moon because the gravitational field strength on the moon is less than Earth's. Less gravity acting on object. 

change of GPE energy store (joules, J) =

mass (kg) x gravitational field strength (newtons per kilogram, N/kg) x change of height (metres, m)

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

Kinetic energy equation

Kinetic energy = 1/2 x mass (kg) x speed2 (metres per second, m/s2)

Using elastic potential energy 

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

Elastic potential energy (joules, J) = 1/2 x spring constant (newtons per metre, N/m) x extension2 (metres, m2)

2 means squared 

Energy for a purpose

• Useful energy is energy in the place we want it and in the form we need it. It is energy transferred in the intended way. 
• Wasted energy is the energy that is not usefully transferred - dissipated (spreads out)

E.g. plane uses chemical energy in the fuel. Some of this energy increases the kinetic energy and GPE energy but some is wasted as it heats the plane and by sound waves. 

Friction

Friction between two objects causes energy to be wasted.

Bike rider = friction acts between the brake pads and wheels. Energy is wasted as it transfers from the kinetic energy of the bike to the thermal energy stores of the brake pads and wheels which are heated by friction. 

As energy spreads out, it becomes less useful. 

Difficult topic - learn the examples of energy dissipation due to friction.

Some energy is useful and some is not. The more efficient a device is, the more energy transferred is useful. 

Efficiency of a device (%) =

useful output energy/ total input energy supplied to device x 100

No device can be more than 100% efficient - you can never get more energy from a machine than what you put into it. 

Improving efficiency

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

Improving efficiency

• Friction between the moving parts causes heating 

Lubricate the moving parts to reduce friction.

• Resistance of a wire causes wire to get hot when a current passes through it.

Use wires with as little electrical resistance as possible.

• Air resistance causes a force on a moving object that opposes its motion. Energy transferred from the object to the surroundings by this force is wasted. 

Streamline the shapes of moving shapes to reduce air resistance.

• Sound created by machinery causes wasted energy transfer.

Cut out noise (tighten loose parts, to reduce vibration)

Everyday electrical appliances

• Electricity, gas and oil supply most of the energy used in the house. 
• Electrical appliances are useful because they transfer energy in the form of useful energy at the flick of a switch.

Appliances

• Light bulb - useful: light emitted from the glowing filament. wasted: energy transfer from filament heating surroundings.
• Electric heater - useful: energy heating the surroundings. wasted: light emitted from the glowing element. 
• Electric toaster - useful: energy heating bread. wasted: energy heats surroundings.
• Electric kettle - useful: energy heating water. wasted: energy heats kettle itself

Clockwork radio - uses no electricity

Clockwork spring winds, increases EPE of spring. 

Spring unwinds, EPE transferred to kinetic energy which turns electric generator in radio. 

Power is the rate of transfer of energy. 

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

1 watt is equal to the rate of transferring 1 joule of energy in 1 second. (1 W = 1 J/s)

Power (watts, W) =

energy transferred (joules, J) / time taken for energy transfer (seconds, s)

Efficiency of an appliance (%) = useful power out / total power in x 100

Power wasted by an appliance = total power input - useful power output