electricity

physics gcse

Resistance is measured in

Ohms

Charge is measured in

Coulombs, C

Charge is

the value for electricity flowing in a circuit

Current is measured in

Amps

Current is measured with

an ammeter

Current is

the rate of flow of charge/ the flow of negatively charged electrons

For current to flow you need...

-conductors

-voltage provided by a cell or battery

-a complete circuit (no breaks)

Conventional current flows from

positive to negative

Current going into a junction

equals current going out

Potential difference is measured in

Volts

Potential difference is

- the force driving the flow of electrons

- the energy difference per unit charge between two points on a circuit

Potential difference is also called

voltage

Voltage is

the energy transferred per unit of charge passed

A volt is

a joule per coulomb

More voltage...

more current

Less voltage...

less current

Resistance...

slows down current

More resistance...

less current

Less resistance

more current

Factors that affect resistance

- thickness

- length

- temperature (insulators have a higher resistance)

-number of components

More components means

more resistance

Electrical power

the rate of transfer of energy

--\> seen as brightness, heat or speed

equation between voltage, energy and charge

E(energy, J) \= V(voltage, V) x Q(charge, C)

equation between charge, current and time

Q(charge, C) \= I(current, A) x T(time, s)

equation between voltage, current and resistance

V(voltage, V) \= I(current, A) x R(resistance, ohms)

equation between power, current and voltage

P(power, W) \= V(voltage, V) x I(current, A)

equation between energy, power and time

E(enery, J) \= P(power, W) x T(time, s)

equation between energy, voltage, current and time

E(energy, J) \= V(voltage, V) x I(current, A) x T(time, s)

equation between frequency and time

Frequency \= 1/ T(time, s)

Series circuits:

- the current is the same throughout

-the voltage from the cell splits between the components

-total resistance: R1+R2... (you can add up resistances)

In a series circuit, if one component breaks,

the other components do not work

Parallel circuits

- the current splits at a junction

-the voltage is the same for all the components

- the overall resistance decreases

- total resistance: total V \= total I*total R

In a parallel circuit, if one component breaks,

the other branches can still work

Which has more resistance, a conductor or insulator?

insulator

Electrical conductors

- a material that allows electric current to pass through it

- has lots of free electrons that can move

- metals

Electrical insulators

- a material that prevents electric current from passing through it easily

- has no free electrons

- no charges are free to move and carry a current

- plastic, glass, rubber

Lamps and LEDs can be used to

indicate the presence of current in a circuit

Energy comes from

the battery

More voltage

More current

The relationship between the current and the voltage depends on...

the component you have in the circuit

I-V characteristics - Fixed Resistor:

- the resistor is an Ohmic conductor- it obeys Ohms Law

- the IV graph is a straight line through the origin

- linear relationship between the current and the voltage

- current is proportional to the voltage

--\> if the voltage was to double then the current would also double

Fixed Resistor graph

the IV graph is a straight line through the origin

Ohms Law

the current is proportional to the voltage providing the temperature of the resistor does not change

I-V characteristics - Filament Lamp:

- IV graph is curved

- filament lamps do not obey Ohms Law

- the resistance of a filament lamp increases as the potential difference increases because it gets hot

- the atoms in the filament vibrate faster causing more collisions with electrons which slows them down

Filament lamp graph

IV graph is curved

To get negative values of current of voltage...

switch the wires connected to the power pack

I-V characteristics - Diode (or LED):

- the diode or LED will only allow current to flow through it in one direction

- the voltage needs to exceed the threshold voltage before it starts to conduct

- only allows current to flow if it is connected the correct way round

- when connected the wrong way round- INFINITE RESISTANCE

- diodes are very sensitive to large currents, so often a resistor is used to prevent the LED from blowing if too much current flows

diode symbol and what it does

the arrow on the symbol points in the same direction as the current flows which is positive to negative

Diode graph

- one direction

- voltage has to exceed threshold voltage

drawing circuits

- voltmeters in PARALLEL

- ammeters in SERIES

Thermistors

- a temperature dependent resistor

- the higher the temperature the lower the resistance

- it is a non- linear relationship

- thermal energy releases more 'free' electrons available to flow

Uses of thermistors

- thermostat

- kettle

- oven

LDRs

- Light Dependent Resistor

- the brighter the light intensity, the lower the resistance

- it is a non-linear relationship

- LDRs are useful in automatic night-lighting/ burglar detectors

LEDs

indicate the presence of current in a circuit

USES: traffic lights

Batteries provide...

low voltage dc electricity

The mains provides...

230v ac electricity

DC

direct current which flows in one direction

AC

alternating current which continuously changes direction

The plug

- EARTH WIRE - green and yellow

- LIVE WIRE - brown

- FUSE