Energy

what is a system

an object or group of objects

what are the main energy stores?

• Thermal (heat)

• Gravitational potential

• Kinetic

• Chemical

• Magnetic

• Electrostatic

• Nuclear

Energy in each store isn’t fixed, can be transferred from one to another

How can energy be transferred

• Heating

• Electrically

• Radiation

• Mechanically

what is an open system

where the matter of the system can exchange with all matter outside the system.

Can lose/gain energy as it interacts with the outside world.

What is a closed system

where matter and energy can’t leave the system and interact with matter outside the system.

Even though energy can be transferred within the system, the overall change is always 0.

what is another way of saying energy transferred

Work done

Work done equation

work done = force x distance

W = fs

What are the two main types of work

• mechanical

• Electrical

Law of conservation of energy

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

Kinetic energy (KE) equation

E_k = ½mv²

gravitational field strength on Earth

9.8 N/kg

Gravitational field strength on the moon

1.6N/kg

what force of attraction does an object experience if it comes into a gravitational field

weight

Weight equation

W = mg

What is mass

amount of matter in an object

Always stays the same

What type of energy is used to overcome gravity

gravitational potential energy

Increases as object’s height increases

gravitational potential energy (GPE) equation

E_p = mgh

What happens during a transfer between KE and GPE?

• as an object rises, KE is converted to GPE

• As an object falls, GPE is converted to KE

• KE gained = GPE lost

• GPE lost = KE gained

The maximum speed a cyclist can travel on a level road is 14 m/s.

How does cycling uphill affect the maximum speed of this cyclist?

Explain your answer.

maximum speed is lower

because maximum power output of cyclist is constant

But GPE of cyclist is increased

In a moving object, how is a small amount of energy lost?

• due to friction between moving object and surface

• Due to air resistance acting on the object

These losses are not taken into account in calculations, unless stated otherwise

Internal energy

• total energy stored by the particles making up a substance/system

• Made up of potential energy stores and kinetic energy stores

• Whenever a substance is heated up, it transfers energy to the KE store of all the particles. Increases internal energy. We measure this as an increase in temperature (temperature is a measure of the avrg internal energy of a substance)

specific heat capacity

The amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius

Specific heat capacity equation

change in thermal energy = mass ×specific heat capacity × temperature change

∆E = m c ∆θ⁢ 

Wasted energy

energy that goes to forms other than the one intended. Generally in the form of heat

where is energy transferred to in a heated object

thermal energy store

What are the 3 ways a substance can be heated

• conduction (solids)

• Convection (liquids and gases)

• Radiation (empty space)

Conduction

When an object is heated, particles gain kinetic energy and vibrate faster. This causes them to collide with neighbouring particles more often and with more energy. Energy is transferred by collisions with the other particles.

why does conduction only occur in solids

because particles are close together

Thermal conductivity

How well objects transfer energy by conduction

Thermal conductivity of different materials

• metals have high conductivity

• Plastics have low thermal conductivity (used as insulators)

• Fluids have low thermal conductivity

Convection

particles in fluids gain kinetic energy and move around faster when heated. By process of random diffusion, more energetic particles more spread out, so fluid in warmer region expands as it is heated and is less dense. Particles close to heat source gain energy and spread out. They rise above colder particles, which sink and take their place, Hot particles cool and sink, cold particles heat up and rise. Cycle continues, known convection currents.

Radiation

• thermal energy transferred through a vacuum

• Infrared waves carry energy

• All objects constantly absorb and emit radiation

• Hotter object = more radiation

methods of insulation in homes

• foam seals around doors and windows to reduce heat loss by convection

• Houses have thicker walls made of materials with low thermal conductivity

• Cavity walls (two layers of brick with air gap). Air gap reduces conduction (air is a poor conductor) but allows convection. Gap filled with insulating foam

• Single glazing - 1 pane of glass, heat easily lost by conduction

• Double glazing - 2 layers of glass with air gap to reduce convection

Friction

• the resistance that one object encounters when moving over a solid/through a fluid

• Reduces efficiency of energy transfer, causes substance to heat up

Power

rate of energy transfer/work done

Power equations

P = E/t

P = W/t

efficiency

Proportion of energy supplied that is transferred to useful energy output

efficiency equation

useful energy output or useful power output/total energy input or total power input

Increasing efficiency of an intended energy transfer

• Reducing friction

• Reducing electrical resistance

• Reducing air resistance

• Reducing noise

how can friction be reduced?

lubrication e.g. oil

How can electrical resistance be reduced

• sound results in unwanted energy transfers by heating to the surroundings as sound waves cause the particles in the air and nearby objects to vibrate

• Can be reduced by