# Electricity and Circuits (EdExcel)

## Electric Charge

❂Objects can have positive or negative charges

❂ Like charges repel, opposite charges attract

❂ Electric charge is measured in coulombs

❂ Electric charges create electric fields in the areas surrounding them

❂ Field lines always go away from positive charges and towards negative charges

❂ Electrons are negatively charged particles

❂ Negative charges are the result of gaining electrons

❂ Positive charges are the result of losing electrons

❂ Conductors are materials that allow charge to pass through

❂ Insulators do not allow charge to pass through

## Induction

❂ When a charged material is brought near an uncharged material, the charged material will induce a charge in the uncharged material

❂ If a negatively charged rod is brought near an object, it will attract positive charges towards the rod and the negative charges will gather on the opposite side

❂ Positively charged rod will attract negative charges

Current

❂ The flow of charge is known as current

❂ When two oppositely charged conductors are connected by a conductor, the charges will flow, producing a current

❂ The greater the flow of charge, the greater the current

❂ Current is the charge passing a point in a circuit every second

❂ Current = Charge/Time

❂ The unit of current is Amperes (A)

❂ Current can be measured using an ammeter

❂ Ammeters are always connected in series to the part of measurement

❂ The flow of free electrons in a metal causes current

❂ Electrons flow from negative to positive terminals

❂ Conventional current flows from positive to negative terminals

## Electromotive Force

❂ The electromotive force is the work done in driving a charge around a complete circuit

❂ It is the potential difference across the battery

❂ It is measured in Volts (V)

❂ It is the energy per coulomb

## Potential Difference

❂ Potential difference is the work done in driving a charge through a component

❂ It is measured in volts (V)}

❂ It is the energy per coulomb

❂ Potential difference can be measured using a voltmeter

❂ The voltmeter should be connect in parallel to the component of which the p.d is to be measured

## Resistance

❂ Resistance is the opposition to current

❂ The higher the resistance, the lesser the current

❂ Potential Difference = Current x Resistance

❂ V = IR

❂ Resistance is measured in ohms

❂ To measure the resistance of a component, the current is measured by attaching an ammeter in series and potential difference is measured by attaching a voltmeter in parallel

❂ The readings are used to calculate the resistance by R=V/I

❂ Resistance is caused when the electrons flowing in a metal collide with the metal’s ions

❂ The longer the wire, the greater the resistance

❂ The greater the cross sectional area of the wire, the lesser the resistance

❂ Resistance is directly proportional to the length, and inversely proportional to the area

## Ohm’s law

❂ Ohm’s law states that resistance is directly proportional to current when physical conditions such as the temperature are constant

❂ If the temperature varies, such as in a filament lamp, it will not obey ohm’s law

❂ Resistors that obey Ohm’s law are known as ohmic resistors

❂ Resistors that do not obey Ohm’s law are non-ohmic

❂ The IV graph of an ohmic graph is a straight line

❂ The graph of non-ohmic conductors is curved towards the voltage

❂ In non-ohmic conductors such as filament lamp, the temperature increases with time

❂ The temperature causes the resistance to increase

❂ A higher resistance means a lower current

## Electrical Energy

❂ Energy is transferred from the power source to the components when current flows

❂ Energy transferred = Current x Voltage x time

## Electrical Power

❂ Power is the rate of energy transfer

❂ Power = Current x Voltage

❂ P=IV

❂ The unit of power is Watt (W)

❂ Watt is the the same as Joules/second

❂ Energy = Power x Time

❂ E=Pt

## Series Circuits

❂ Components are connected next to each other in series

❂ The current is same at all points in a series circuit

❂ The total EMF for batteries connected in series is the sum of their individual EMFs

❂ In a series circuit, the sum of the p.d of individual components is equal to the total emf

❂ If one component stops working, the whole circuit doesn’t work

❂ the total resistance in a series circuit is the sum of all individual resistances

## Parallel Circuits

❂ The components are attached on separate branches

❂ Components can be individually controlled

❂ if one component stops working the rest will continue to function

❂ The current splits up in parallel circuits

❂ The sum of currents in each branch is equal to the emf

❂ The current does not split equally always

❂ The potential difference is same in every branch

❂ Resistors connected in parallel have decreased resistance

❂ If two identical resistors are attached in parallel, the resistance is will be half of one component

❂ The reciprocal of the total resistance is equal to the sum of the reciprocals of the individual resistors

❂ 1/Rt = 1/R1 + 1/R2 …+ 1/Rn

## Potential Dividers

❂ When two resistors are connected in series, voltage is shared between them

❂ The larger resistance resistor will get greater potential difference

❂ Increasing the resistance of one resistor will increase the voltage across it and reduce the voltage across the other resistor

❂ A potentiometer consists of a coil of wire with a sliding contact

❂ Moving the sliding contact changes the resistance as it changes the length of the coil the current has to pass through

❂ Adjusting the slider can be used to control the voltage across components in series

## Relay

❂ A relay consists of an electromagnet and a magnetic switch

❂ The switch is within the magnetic field range of the electromagnet

❂ When there is current in the electromagnet coil, it attracts the magnetic switch

❂ Attracting the switch closes it, allowing the current to pass through the circuit of the switch

❂ Turning the current off in the electromagnet turns the switch back open

## Thermistor

❂ Thermistors are resistors that have variable resistance depending upon heat

❂ As the thermistor gets hotter, its resistance decreases

❂ As the thermistor gets colder, its resistance increases

❂ The temperature and resistance are inversely proportional

## LDRs

❂ Light dependant resistors change resistance according to the light on it

❂ When more light shines on the LDR, the resistance decreases

❂ When the light is lesser, the resistance increases

## Diode

❂ Diode is a component that allows current to pass in one direction only

❂ If the current direction is opposite to the diode, there will be no current

❂ It converts a.c current to d.c current

❂ There will be no current half the time with a.c current

## Hazards of Electricity

❂ Electricity can be lethal, a few volts of shock can be hazardous

❂ Touching an exposed wire due to damaged insulation can cause an electric shock

❂ Passing too much current through a wire can cause overheating and lead to fires

❂ Moisture can come into contact with live wires in damp conditions, conducing electricity and causing a short circuit or even posing risk of electrocution

## Fuses

❂ Fuses are a safety measure to cut off electricity to an appliance if the current is too large

❂ Fuses consist of a glass cylinder with a thin metal wire

❂ If the current is too large, the wire will heat up and melt

❂ The circuit will be broken and the current wont be able to pass through

❂ Fuses have different ratings which are decided by the power of the appliance

❂ The power of the component is found using P=IV

❂ The rating of the fuse is the next size up higher than the power rating

❂ The power should fall in the middle of the fuse’s range

❂ The fuse is attached on the live wire so that the current can not damage the device or cause shock

## Earthing

❂ Electrical appliances can have metal cases which can cause hazards

❂ If a live wire comes into contact with the metal case, touching it would cause a dangerous shock

❂ The earth wire provides a low resistance path to the earth

❂ If the current becomes too large, it will cause a surge in the earth wire, blowing the fuse

❂ the fuse cuts off the supply of electricity, making the appliance safe