Physics GCSE 2017/2018

Seite 1

P1: conservation and dissipation of energy

P1.1: changes in energy stores

energy is transferred by heating, by waves, by electrical current, or by a force when it moves against an object.

when an object loses energy when it stops moving or when it cools down, most or all of the energy is transferred to the surroundings and is stored as thermal energy or is transferred by sound waves.

P1.2: conservation of energy

energy cannot be created or destroyed, this applies to all energy changes.

a closed system is an isolated system in which no energy transfers take place out of or into the energy store of the system. Total energy of the system is always the same. E.g. a pendulum in a vacuum.

P1.3: energy and work:

work is done on an object when a force makes it move.

energy transferred = work done.

work done is W = F s where F is force and s is distance moved along the line of action of the force.

joules = newtons * metres

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

P1.4: gravitational potential and energy stores

increases in an object as work is done on it to lift it to overcome gravity.

the gravitational potential energy store of anobject is Ep = m × g × h

g = gravitational field strength in newtons per kilogram.

m = mass in kilograms

P1.5: kinetic energy and elastic energy stores

kinetic energy depends on an objects mass and speed.

Ek = m * v² * 1/2

elastic potential energy is energy stored in an elastic object when work is done on it.

Ee = k * e² * 1/2

k = spring constant in newtons per meter

e = extension in meters

P1.6: energy dissipation

waste energy = not useful and transferred by an undesirable path.

wasted energy transferred to the surroundings which become warmer.

as energy dissipates it becomes less useful. E.g. hot water in a boiler.

even useful energy eventually transfers to the surroundings.

P1.7: energy and efficiency

useful energy = useful output / total input (and *100 for a percentage)

no energy transfer can be more than 100% efficient.

machines waste energy. E.g. friction between parts or electrical resistance or air resistance or noise.

machines can be made more efficient. E.g. lubricant.

P1.8: electrical appliances

designed for a particular purpose and to be as efficient as possible.

P1.9: energy and power

power is the rate of transfer of energy.

P = E / t

watts = Joules / seconds

efficiency of application = useful power in / total power out (and *100 for a percentage)

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

Seite 2

P2: Energy transfer by heating

P2.1: Energy transfer by heating

the higher the thermal conductivity of a material, the higher the rate of energy transfer through it.

the thicker a layer of insulating material, the lower the rate of energy transfer through it.

conduction is the flow of heat through objects in contact when there is a temperature difference.

P2.2: Infrared radiation

All objects emit and absorb infrared radiation.

the hotter an object is, the more infrared radiation it emits in a given time.

blackbody radiation is radiation emitted by a body that absorbs all the radiation incident on it.

an object that has a constant temperature emits radiation across a continuous range of wavelenghts.

P2.3: More about infrared radiation

Green house effect.

The temperature of an object increases if it absorbs more radiation than it emits.

P2.4: Specific heat capacity

The specific heat capacity of a substance is the amount of energy that is needed to change the temperature of 1kg of the substace by 1°C.

△E = m c △θ

use a joulemeter to measure energy change and a thermometer to measure temerature change.

P2.5: Heating and insulating buildings

rate of energy transfer from house can be reduced by using: loft insulation, cavity wall insulation, double-glazed windows, aluminium foil behind radiators.

cavity wall insulation is used to fill the cavity between two brick layers of an external house wall and work by trapping pockets of air preventing conduction.

U-value is a number that indicated how easily heat passes through a material. The best insulators have the lowest U-value.

Payback time - the length of time that an appliance will take to save you as much money as it costs to install.

Seite 3

P3: Energy resources

P3.1: energy demands

biofuels like methane and ethanol are renewable.

Uranium release much more energy per kilogram compared to fossil fuels.

to make electricity you need coils of copper wire, a magnetic field(magnets) and movement.

renewable - replaced at the same rate it is used.

P3.2: energy from wind and water

hydroelectric generators turned by water flowing downhill.

a tidal power station traps each high tide and uses it to turn generators while releasing it back to the sea.

P3.3: power from the sun and the earth

solar cells are flat solid cells and use the sun's energy to generate electricity directly. Converts less than 10% of what it absorbs.

solar heating panels use the sun's energy to heat water directly

geothermal comes from the energy transferred by radioactive substances deep inside the earth.

water pumped into hot rocks underground produces steam to drive turbines at the earths surface to generate electricity.

P3.4: energy and the environment

renewable resources can cover large areas and can disturb natural habitats.

resources can be evaluated in terms of reliability, environmental effects, pollution, efficiency and waste.

P3.5: Big energy issues

gas fired stations and pumped storage stations (type of hydroelectric) can meet variation in demand as low start up time.

nuclear power, and carbon capture with it, is very expensive to build, run and decommission, very low start up time.

all resources likely to be needed for future energy supplies.

hydroelectric - reservoirs could run dry.

wind and waves weak on calm days so unreliable.

tidal varies monthly and yearly.

solar - not always sunny.

supply and demand.

use nuclear or coal for base load and gas and pumped storage for variation in winter and renewable when demand is high and conditions are suitable.

Seite 4

P4: Electric circuits

P4.1: Electrical charges and fields

some insulating materials become charged when rubbed together.

Polythene rods gain electrons from dry cloth while perspex rods lose electrons.

the force between two charged objects is a non-contact force.

charged objects create an electrical field around themselves.

lines of force are drawn to represent an electric field.

when objects are oppositely charged, electrons in the air molecules between the objects experience a force towards the positive object.

power = energy / time

power,P - Watts,W

P=E/t

P4.2: Current and charge:

a battery consists of two or more cells.

the size of an electric current is the rate of flow of charge.

I=Q/t current = charge / time

A=C/s amperes = couloumbs / seconds

current is the rate of flow of electrons.

potential difference is the amount of energy transferred per unit charge.

P4.3: Potential difference and resistance:

potential difference,V - volts,V

potential difference across a component,V = energy transferred,E / charge Q V=E/Q

resistance,R - ohms,Ω

R=V/I

Ohm's law states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.

Reversing the potential difference across a resistor reverses the current through it.

resistance is a measure of a components opposition to ht e flow of current.

resistance is when moving electrons collide with the ions in a metal, making it difficult for the current to flow.

E = VQ I=Q/t V=E/Q Q=It E=VIt P=E/t P=VIt/t P=VI

W=VA

p4.4: Component characteristics:

A filament lamp's resistance increases if the filament's temperature increases.

Diode forward resistance low; reverse resistance high.

The diode has very high resistance at low voltage values however it's resistance becomes low when the voltage increases past a certain point.

A thermistor's resistance decreases if its temperature increases.

An LDR's resistance decreases if the light intensity on it increases.

P4.5: Series circuits:

The current is the same in each component.

The total potential difference is shared between the components.

For cells in series, acting i the same direction, the total potential difference is the sum of their individual potential differences.

Total resistance is calculated by adding the resistance of all the components and resistors.

adding more resistors in series increases the total resistance as the current through ht e resistors is decreased and the total potential difference across the m is unchanged.

P4.6: Parallel circuits:

Total current is the sum of the current through the separate components.

Potential difference across each component is the same as the power supply.

current is split at the junctions and rejoins with the circuit.

The bigger the resistance of a component, the smaller the current that will pass through that component.

Adding more resistors in parallel decreases the total resistance because the total current through the resistors is increased and the total potential difference across them is unchanged.