Electricity

Current

• Rate of flow of charge

• Measured in Amps

Charge flow, current and time are linked by the equation:

Charge = current x time

Q = It

A current has the same value at any point in a single closed loop

potential difference

• force driving the flow of electrons

• Provided by cell/battery

• Voltage, measured in volts

What does the current flowing through a component depend on?

both the resistance of the component and the potential difference across the component. The greater the resistance of the component the smaller the current for a given potential difference (pd) across the component.

Resistance

• something that resists the flow of electrons

• Measured in Ohms

• Resistance increases with temperature

Current, potential difference or resistance can be calculated using the equation:

Potential difference = current x resistance

V = IR

Convential current

• showing current as flowing from positive to negative

• Because electrons are negative, they flow from negative to positive but no one knew about this when electricity was discovered

How are the current through an ohmic conductor and potential difference across it related?

• directly proportional

• Resistance remains constant as current changes

• The resistance of components such as filament lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component.

Resistance of a filament lamp

• The resistance of a filament lamp increases as the temperature of the filament increases

• This is because as current passes through the filament, the wire heats up, which increases resistance

• Less current can flow per unit of p.d at higher voltages as temperature and resistance are higher

• Curve gets less steep as current increases

Resistance of a diode

• The current through a diode flows in one direction only.

• This is because the diode has a very high resistance in the reverse direction, so no current can flow in that direction.

Resistance of an LDR (Light Dependent Resistor)

• resistance decreases as light intensity increases

• Used in circuits that switch lights on when it gets dark, burglar alarms, etc.

Resistance of a thermistor

• resistance of a thermistor decreases as the temperature increases

• Used in thermostats, car engines, etc.

What are the two different ways of joining electrical components

• in series

• In parallel

Series

• single loop, components connected one after the other

• there is the same current through each component

• the total potential difference of the power supply is shared between the components. Sum of p.ds of individual components

• the total resistance of two components is the sum of the resistance of each component

Parallel

• more than 1 loop

• Each loop usually contains a single component

• the potential difference across each component is the same

• the total current through the whole circuit is the sum of the currents through the separate components

• the total resistance of two resistors is less than the resistance of the smallest individual resistor

why adding resistors in series increases the total resistance whilst adding resistors in parallel decreases the total resistance

• in series, current has to pass through each resistor in turn, so resistances add up. Total resistance is therefore increased.

• In parallel, the total resistance is less than the resistance of the smallest individual resistor. This is because with multiple resistors in parallel there are more pathways for the current to take. More total current will flow through the circuit. If current has increased but p.d has not changed, then total resistance decreases

explain the design and use of dc series circuits for measurement and testing purposes

DC series circuits connect components end-to-end in a single path, making them ideal for testing because current is constant throughout, resistance adds up, and voltage divides, allowing simple measurement of total/individual resistance (using ammeter in series, voltmeter in parallel) and voltage drops (V=IR) to verify laws like Ohm's Law and study component behaviour in basic setups like voltage dividers.

National grid

• a system of cables and transformers linking power stations to consumers

• Electrical power is transferred from power stations to consumers using the National Grid

How do power stations generate electricity

• generate loads of heat

• Convert thermal energy into electrical energy

What does the amount of electricity generated depend on

• demand

• Electricity demand peaks in the afternoon and evening

• To cope with surges in demand, power stations need spare capacity, so run well below their maximum power output

what does the national grid need to transmit huge amounts of power

• high voltage

• High current

What happens to voltage before it is sent across the country

• increased with step up transformers

• They increase the voltage to around 400,000 volts

• This decreases the current

why is the voltage increased?

• whenever a high current flows through a wire, lots of heat is generated due to resistance. Lots of thermal energy transferred to surroundings

• Very high p.d means very low currents

• Which means less thermal energy transferred to surroundings

• Which increases the efficiency of power transmission

what happens to voltage before power reaches homes

• decreased with step down transformers

• they decrease the voltage to around 230V

• This increases the current

why is p.d decreased before reaching homes

High voltages are dangerous and could damage household appliances