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physics gcse summary

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  • Created by: Dementia
  • Created on: 13-11-11 22:38

Electric Current

Current: is a rate of flow of electrons round a circuit and the unit is amps and it's measured by an ammeter

Voltage: is the driving force that pushes the current round and is measured in volts by a voltammeter

Resistance: is anything in the circuit that slow down the flow and measured in ohms

Electrons flow the opposite way of the conventional current

AC: keeps changin directions

DC: flows in one direction, the voltage doesn't vary so the current has a constant value too

The mains electricity is AC and DC is used in batteries, solar cells and DC generators

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Generating Electricity

Electromagnetic Induction: Rotating a magnet in a coil of wire induces a voltage. As you turn the magnet, the magnetic field through the coil changes, this induces a voltage allowing the current to flow in the wire.

3 factors that affect the size of the induced voltage:

  • strength of the magnet
  • number of coils of wire
  • speed of the movement

Generators create current by either rotating the magnet or coil of wire. A dynamo is a generator which is often used in bikes to power the lights -magnet is rotated by the wheels rotation.

Ammeters are always connected in a series circuit and voltammeters are connected in a parallel circuit. The supply of voltage doesn't change, using the variable resistor, you adjust to pick various values for the current.  A fixed resistance produces a straight line graph whereas a filament lamp would create a positive s shape. 

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Varying resistance

LDR have lower resistance in bright light, which is useful for electronic sensors such as automatic night lights and burglar detectors. 

Thermistors have varying resistance depending on the temperature. Most of them have high resistance in the cold - useful for car engines + central-heating thermostats

A modern camera can automatically adjust how long the shutter is open depending on the light. When the shutter is open it causes a capacitor to charge and when it's full, it activates a switch, closing the shutter.The resistance is higher when there's less light.

Data logging systems uses sensors to automatically record data about how a physical quantity changes over a period of time.

Advantages: Accurate, no human error, machines can go where humans can't, warns you if data is beyond a certain value, readings could be taken at a shorter or longer period of time and it's easier to process the data as a graph, table or a chart. 

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Batteries and their uses

Dry-Cell Batteries: handy when you have no power supply to recharge the batteries and they are cheaper. But they have to be thrown away when they run out, which is wasting resources and also risking toxic chemicals getting into landfill sites.

Rechargeable batteries: can be reused so don’t need to make many, it uses less energy, little impact on natural resources and less toxic waste in landfill site. More expensive to start with and is usually cheap overall. You need a recharger in order get power supply to recharge it, some have a ‘memory effect’ so if you charge it before it was discharged it will remember its short charge time and will not charge fully. All rechargeable batteries deteriorate over time. Chemical are more toxic than dry-cell batteries.

Battery Capacity is how much current it can supply for how long which is measured in Amp-Hours (Ah).

You can measure the voltage and the current change over time and the battery gives more or less constant output voltage till the battery is about to run out.   

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Superconductivity gave us Maglev trains:

Superconductivity was discovered by scientist studying what happened to materials at a really low temperature and found out that the electrical resistance of some materials dropped to zero. These are called superconducters.

The Lack of resistance meant that they could carry high currents without overheating or wasting energy.

They also produced magnetic fields.

The idea of magnetic levitation was around and if these super-cooled superconductors could carry current without burning out, it could be made into super-powerful electromagnets, strong enough to levitate a train.

Therefore there wouldn’t be any friction which means that it can go really fast.

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Computers are getting smaller and faster: The processing power of computers depend on how many electric circuits you can fit inside them. The first electric circuits used valves to switch signal around, this was replaced by smaller, faster transistors. In the future nanotechnology can be used to build smaller circuits. Carbon nanotubes can be used to make diodes a few nanometers thick however; we don’t know how to mass-produce them. Quantum computers can also be used using a property called entanglement. This can produce smaller and powerful computers.

Electricity and telephones have changed the world: Electricity powers everything from heaters and lights to washing machines and mp3 players. It also drives our industries, hospitals and banks, starts up cars, trains, and planes. Before electricity and phones communication was through letter or courier (before 1875). Nowadays, cables, microwave receivers and satellites let communication signals be sent anywhere in the world in seconds.

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