# Physics: P1A

Electricity

HideShow resource information

## Development and Introduction to Currents

1860 - The light bulb was developed
1875 - The telephone was invented
1967 - The first satellite TV network
1971- First email using @
1983 - Hand held mobile phones available

Alternating Current (AC) Constantly changing direction. This is usually found in main sockets at home
Direct Current (DC) Stays in the same direction. This is usually found in batteries.
Lights plugged into AC dont flicker because the current changes direction so quickly
Conventional Current - Flows from positive to negative.
Electrical Current is a flow of netgatively charged particles c alled Electrons which go from - to +.
When current is measured in amps, it is actually measuring the number of electrons flowing per second.
An ammeter is placed in series of the circuit.

Direct Current - Batteries are made up of individual cells connected together. The msot common cell is the dry cell - Non rechargable. A chemical reaction inside the cell causes the current. Rechargable batteries are used in mobile phones. Solar cell is a cel that produces a DC current without a chemical reaction.

1 of 13

## Producing Electrical Currents

Around every wire there is a magnetic field. if it is moved through the field a voltage is made. Voltage is the amunt of electrical push that makes the current flow. Moving the magnet or wire creates a voltage. This is called electromagnetic induction or the dynamo effect.
If yu move the magnet or the wire in the opposite direction then voltage will also change direction. If you turn the magnet around then the current will also change direction. The current consists of negative charges moving from the negative to the positive. The voltage is measured in volts and the voltmeter is parrallel to the circuit. The current is measured in amps and is in series to th circuit

Resistance describes how good a material is at conducting electricity, It is measured in ohms. Adding more components to a circuit will increase its resistance so it is harder for th current to flow and so it will be less. Air doesnt conduct electricity but in a thunderstornm the voltage is so high it causes the air to conduct.

2 of 13

## Magnets and Resistance Lamps and Computers

To increase voltage and current
Move the magnet closer to the wire
Add more coils to the wire
Use a stronger magnet
Move the magnet faster

The relationship between a current and voltage ina  resistor is different to a filmanet.
The resistors line is straight whereas the filaments line is curved. The voltage increases, the resistance increases.

Advantage of using computers: Possible to collect lots of information and sets of readings quickly which couldnt be done by hand. Easier to measure and analyse data. Thousands of readings can be safely recorded, looked at and analysed at a later time.

3 of 13

## Capacity, Time, Cells and Recharging and Changing

The capacity of a cell is given in amp-hurs. It measures how much energy a cell can store.
Battery rating in amp hours/Current used in amps = Time in hours

Rechargable batteries cannot produce as much chemical pollution as dry cells and provied cheaper electricity in the long run.
Disadvantage: Originally cost more, Need a recharging unit and the storage of power is usually less.

A dry cell batterie's power slowly falls over time as you use them. The output from a rechargable battery remains constant but drops quicly at the end.

In a dimmer switch, th main component is a variable resistor. This is the same in stereos, car windscreen wipers and a fan. When changed, you alo change the resistance.

eg. A camera has a LDR which changes the photo to makie sure it is not too light or too dark. As the light intensity changes, its resistance changes. This change is easured to control the amount of time the shutter is kept open for.

4 of 13

## LDRs and Thermistors

Thernistor - A type of resistor that responds tto changes in the temperature. As the temp increases, the Resistance decreases. Used in pools and fridges.

LDR - A type of resistor that responds to changes in lgiht intensity. As the light increases, the resistance decreases. Used in cameras and alarms.

5 of 13

97% of electricity used in the UK comes from non renewable fuels. Supplies of theese fuels can never be
replaced (Coal, Oil, Natural Gases). Renewable resources do not have a limited lifespan. There are no fuel costs, only running and matinance costs - Wind Solar Wave and Tidal power. They do not produce pollution - CO2.

The national grid is a network of pylons and cables that distributes electricity to our homes In the UK, most electricity c omes from nuclear or fossil fuel stations. In rural areas, solar power can be used
Disadvantages: Building costs are very expensive. Construction can have a bad effect on the enviroment, Safety issues.
Alterntives: A small scale renewable power generator conencted directly to local homes.

Wind Power - Very little matinance
Energy is free
Wind strength is unrelaible
Lots of turbines needed
Buildig the turbines can be expensive.

6 of 13

## Superconductors

Trains are levitating, they hover a few centimetres over the track. It is supported by very strong magnets which reduces friction. Superconductors can be used to make strong electromagnets. Magnetic levitating trains work by having superconducting electromagnets on both the train and the rail. They are charged constantly so one always attracts and one repels. Graph of a superconductor:

7 of 13

## Electric Motors

The direction of the wire can be changed by:
Reversing the direction of the current through the wire (Swapping connection)
Reversing the direction of the permanat magnet field (Turning it around)

Every electric motor contains magnets and wires and hey all pass currents through the wires to the magnets and make them move. The first motor was built in 1821. They are everywhere, cars, phones, music platers. All you need is some wire and a current

The motor effect - A wire is near a permanant magnet and it will move.
The force on a single wire can be small but the force can be made larger an make the wire move by increasing the size of the current, causing a stronger magnetic field causing it to move

8 of 13

## Electric Motors

As the coil spins, the motor reverses the current in the coil every half turn which keeps it in the same direction:
To make it faster
Increase turns on coil
Increase strength on magnets

How a motor works:
1. Inside a magnetic field a coil of coper wire is wrapped around a core that spins on its axis.
2. The wire is free to spin without getting tangled up.
3. The current always flows in the same direction but the current is continuously reversin in the wires
4. A current is passed through the coil to make it magnet
5. It interacts with the magnet and spins.

9 of 13

## Fuses

Brown wire is the live wire that carries the current into the appliance.
Blue wire is the netural wire that carries the current away.
The yellow and green striped wireis the Earth wire that prevents electrocution.

RRCB detects the difference between the curents in the live wire and netural wire. They are safety devices that measure the current flow. They are quicker to respond than fuses and easy to reset.

The fuse is a thin piece of wire with a low melting point inside glass or ceramic casing. If the wire gets to hot it melts and breaks th circuit. This protects the appliance and stops it from overheating and catching fire. All appliances with a metal oiutercasing must be connected to earth with an earth wire. When  a fault occurs the whole circuit becomes live. The metal casing is attached to the earth wire. The current increases bease the earth wire offers less resistance.

10 of 13

## Power

Power is abut tme and work done. Time is the rate. Electrical motors convert electrical energy into kinetic energy. The rate of conversion is called hte power of the motor. Power is measured in Watts

Power = Current x Voltage
Efficiency = Useful output/Total output x 100%

Efficiency tells us how much of the nergy we have is useful:
Some energy is always transferred into an unwanted form, it is not possible to achieve 100% efficiency

Efficiency Stickers: A - G ranging from most efficient to least. efficient appliances cost more but work out cheaper in the long run.

11 of 13

## Power, Payback time and Solar Cells

To calculate the number of units of electricty that something uses you need to know: The power of the rating (kw) , The time it is on for (h)
Energy = Power / Time
Total Cost = Power x Time x Cost of 1kwh
Payback time = Cost of measure/Amount of money saved yearly
Payback time is the time it will take to save enugh money to cover the ost of an energy saving measure.

Solar cells - Used to heat water and turn solar energy into electrical.
Unreliable. Installation is very exensive.

Heat Loss
Roof 25%- Loft insulation
Doors 15%- Draft excluders
Windows 10%- Double Glazing
Walls 35%- Cavity Wall Insulation
Floors 15%- Sealed flooorboards

12 of 13

## Technology, Electricity and Medicine

Treatments using Electricity
Transcutaneaus Electrical Nerve Syndrome- Uses electrical signals to help reduce pain often used during childbirth.
Conduction Testing- Electrical signals are sent along nerves to test how well they are working
Electronic muscle simulation- Electrical signals stimulate and move the muscles of paralysed people in physiotherapy
Pace maker- Implanted into the chest to provide a regular pulse of electricity used in modern medicine.

Diagnosing and treating heart problems - Defbrilliators treat the heart when its not beating properly. Causing the heart an electric shock.
Electrocardiograms help doctors diagnose heart conditions. Electrodes are placed across the body and measure small electridcal c urrents generated during heat activity.

13 of 13