P2 - Electricity

Define electrical current

the rate of flow of electrical charge

Define component

Something that transfers electrical energy to another form

How is an ammeter connected?

In series

What does an ammeter measure?

Current (amps)

What does a voltmeter measure?

Potential difference (v)

How is a voltmeter connected?

In parallel to the component

What is the unit for charge?

Coulombs

Potential difference=

current x resistance

Current rule in series

Same current flows through all components

PD rule in series

PD is shared between components

PD rule in parallel

The supply PD is equal to the PDs across each component in each loop

Current rule in parallel

Total current supplied is the sum of the currents in each separate branch of the circuit

Define resistance

a property of components in a circuit that opposes the current

What are the variables affecting resistance?

Material of wire

Temperature

Area of wire cross section/diameter

Length of wire

Resistance rule in series

As the number of resistors increases, the total resistance increases too

Resistance rule in parallel

As the number of resistors increase, the total resistance of the circuit decreases because there are more possible pathways for the electrons to get through

Graph for filament bulb PD against current

Graph for ohmic/fixed resistor PD against current

Graph for diode PD against current

In a diode, what is the current rule

Current can only flow in one direction

In an ohmic resistor, what can be said about the relationship between current and potential difference?

Directly proportional

What happens to bulbs as we add more in series?

They get dimmer

What happens to bulbs as we add more in parallel?

They stay the same brightness

Which direction does current flow in a circuit?

From the positive terminal to the negative terminal

A resistor whose resistance decreases when temperature increases is a:

thermistor

A resistor which cannot vary the amount of resistance it provides is a:

fixed resistor

In which type of resistor does the resistance vary with light intensity?

light dependent resistor

The resistance of a thermistor decreases when the temperature _______

increases

The current flow in a thermistor is greatest when the temperature is

brighter

Diode

Light emitting diode

Fuse

Fixed resistor

Variable resistor

Light dependent resistor

Thermistor

Letter used for charge

Q

Letter used for current

I

Charge (Q) =

current (I) x time (t)

Energy transferred (e) = power x

time

Energy transferred (e) = charge (Q) x

potential difference (V)

(for p2) power =

potential difference (V) x current (I)

In series, potential difference is...

shared across all components

In series, current is...

the same everywhere

In series, the total resistance is...

the sum of the resistance of each component

In series, components with a greater resistance will have a...

higher voltage

In parallel, the potential difference is...

the same everywhere

In parallel, current is...

shared between loops

Loops that have a greater resistance will take a...

lower share of the current

As more loops added in parallel, the total resistance will...

decrease

Power = resistance x

current^2

What are the stages of energy distribution?

Power station -> step-up transformer -> pylon cables -> step-down transformer -> consumer

What do power stations do to cope with surges?

Operate at below maximum output, have lots of spare capacity

Why must current be kept low?

High currents through wires cause high temperatures due to resistance, meaning energy is lost as heat

What do step up transformers do?

Increase the voltage to around 400 000 V

What do step down transformers do?

Decrease the voltage to around 230 V

Is UK mains AC or DC?

AC

What is the frequency of the UK mains supply?

50 Hz (50 cycles per second)

What is the voltage of the UK mains supply?

230 V

Is the current in a cell/battery AC or DC?

DC

Which type of current periodically reverses its direction?

AC

What is the pd of the live wire?

(+-) 230V AC

What is the pd of the neutral wire?

0V

What is the pd of the earth wire?

0V (unless there is a fault)

What colour is the earth wire?

green and yellow

What colour is the live wire?

brown

What colour is the neutral wire?

blue

What does the earth wire do?

Provides an alternate pathway for current to flow away from the casing of the appliance if the live wire is touching it

What is a surge?

a sudden increase in current

What are disadvantages of a fuse that don't exist in a circuit breaker?

A fuse cannot be repaired once broken and needs to be replaced but a circuit breaker is just tripped

What four ways help prevent electric shocks?

Earthing,

Double insulation,

Fuse,

Circuit Breaker

Define static electricity

A build up of charge on insulating materials

What causes a spark?

If two objects have been rubbed together and one has built up lots of negative charge so it has a potential difference between it and an earthed object, causing electrons to jump across the gap

What is an earthed object?

Has 0V

Why does static electricity not build up on conducting materials?

The electrons can just flow back so no charge is built up

When two materials are rubbed together, which particles are transferred?

electrons

What are the rules for drawing field lines

Positive go outward, negative go inward, at perpendicular angles to the surface

Why can sparks travel in air which is normally an insulator?

Air is ionised to positive ions by very charged particles so it can conduct electricity