series and parallel circuits

how will current flow

from negative end of cell to the positive and the electrons carry energy which they will pass to the components

convention current

before many scientists thought electrons travel from the positive end and although incorrect we sometimes draw the arrow from positive to negative

current in series circuit

never used up in a circuit. it will be the same all the way round

current in a parallel circuit

current will split and travel through different branches -the current in the branches adds up to the total current leaving the cell

what is pd

a pdf of 1 volt tells us that 1 joule of energy is transferred for each coulomb of charge thats is moving through the circuit

pd in series circuit

will be shared between the circuit components

pd in parallel circuits

for components connected in parallel, the pd across each component is the same

resistors in series

will add together -the current has to pass through each resistor in turn. it cannot bypass any resistor

resistance in parallel

the total resistance in parallel is less than the resistance of the smallest individual resistor -with 2 resistors in parallel, we have two pathways for the current to take. More total current will through the circuit. If the current is increased but the pd has not changed, then the total resistance must have decreased

resistance of LDR in dark and bright conditions

In dark conditions, the LDR has a high resistance but in light will have a lower resistance

an use of LDR

used to turn mobile phones off when pressed against ear to conserve battery

-garden lamps where the light turns on in the dark

resistance of a thermistor

the resistance of a thermistor decreasesif the temp increases

how are thermistors useful

as thermostats which are devices which turn appliances on or off depending on the temperature

incubator for premature babies -if temp falls, this is sensed by a thermistor which causes a lamp to turn on and an alarm to sound

energy transfer for blender and fan

electrical energy - kinetic energy of electrical motors

energy transfer of iron and kettle

electrical energy- thermal energy

energy transfer of hair dryer and washing machine

electrical energy - kinetic energy of electrical motors +thermal energy w

hich appliances generally have a higher power rating

generally appliances used to generate thermal energy tends to have a higher power rating

how to calculate the energy transferred

power x time(s)

power (watts)

voltage x current

(current)*2 x R

Direct current (DC)

current flows in one direction

alternating current

mains electricity in the uk is ac and the current is constantly changing direction

benefit of AC

very easy to use a transformer to increase or decrease the pd

in the UK, alternating current has frequency of and pd

50 hertz (Hz)

230 volts

how to calculate frequency from oscilloscope trace

1/ time to take to complete one cycle

live wire

-brown

-carries alternating potential difference from the supply (230 V)

-connected to a fuse in the plug

neutral wire

blue

-completes the circuit

-pd of 0V compared to the live wire

earth wire

green/yellow striped wire

-this is a safety wire to stop appliances from becoming live

why is live wire still dangerous even if switch is off

because the part of the live wire before the switch still has a p.d of 230V meaning if someone touched the live wire, a current would flow through the person to earth because earth has pd of 0V

why are appliances with metal cases dangerous and how does earth wire prevent this

if live wire becomes loose and touches metal case, the case can become live and risk of giving someone electric shock if they touch it but the metal case is attached to the Earth wire which is connected to ground with a metal rod -if case becomes live, a huge current flows to Earth. The fuse melts and shuts off the current which prevents electric shock

what is the national grid

The National Grid is a system of cables and transformers linking power stations to consumers.Electrical power is transferred from power stations to consumers using the National Grid.

step-up transformers and step-down

Step-up transformers are used to increase the potential difference from the power station because then less energy is lost in the power cables when the pd is very higb to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use.

explain why the National Grid system is an efficient way to transfer energy.

-energy is always lost in the power cables due to the resistance of the wires but reduce energy loss by building power stations near to homes

static charge

When certain insulating materials are rubbed against each other and due to the friction they become electrically charged. Negatively charged electrons are rubbed off one material and on to the other. The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.

spark

can occur when a person with a large build up of charge comes into contact with an uncharged person as there is a large potential difference between them. Sparking often occurs between a charged insulator and an earthed conductor.- objects being earthed prevents any charge from building up and leading to a spark

electric field

A charged object creates an electric field around itself. The electric field is strongest close to the charged object. The further away from the charged object, the weaker the field.

A second charged object placed in the field experiences a force. The force gets stronger as the distance between the objects decreases.

-field lines need to be perpendicular to surface and arrows need to point from positive to negative