# Physics P1a

These cards are to help me with my year 9 Edexcel Physics exam. June 19th 2009

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• Created by: Tess
• Created on: 18-06-09 20:34

## Producing electricity

There aretwo types of electric current- direct current (DC) and alternating current (AC). An electric current can be produced by moving a magnet inside a coil of wire. The size of this induced current can be increased by moving the magnet faster, by using a stronger magnet, or by increasing the number of turns on the coil and increasing its area. A dynamo is a type of electricity generator.

Direct current flows only in one direction

Alternating current constantly changes direction. Mains electriciy is an AC supply. The UK mains supply is about 230V. It has a frequency of 50Hz (50 Hertz), which means that it changes direction and bakc again 50 times a second.

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## Inducing current

An electric current is produced when a magnet is moved into a coil of wire in a circuit. We say that the electric current has been induced, and the process is called induction. The direction of the induced current is reversed when the magnet is moved out of the coil again. It can also be reversed if the other pole of the magnet is moved into the coil.

Increasing the induced current:

move the magnet faster

use a stronger magnet

increase the number of turns on the coil

increase the area of the coil

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## Dynamos

It is not practical to genrate large amounts of electricity by passing a magnet in and out of a coil of wire. Instead, generators induce a current by spinning a coil of wire insiede a magnetic field, or by spinning a magnet inside a coil of wire. As this happens, a potential difference- voltage- is produced between the ends of the coil, which casues a current to flow.

One simple example of a generator is the bicycle dynamo. The dynamo has a wheel that touches the back tyre. As the bicycle moves, the wheel turns a magnet inside a coil. This induces enough electricity to run the biycles's lights.

The faster the bicycle moves, the greater the induced current and the brighter the lights.

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## Voltage, current and resistors

Current is measured in amps (A), using an ammeter in series.

Voltageis measured in volts (V), using a voltmeter in parallel. There is a relationship between voltage, current resistance

voltage= current x resistance.

The resistance in a circuit can be increased by adding by adding more components, such as resistors and lamps.

The filament lamp does not follow Ohms's Law.

The resistance of a thermistor changes with temperature, and the resistance of an LDR changes with different light levels. The components are useful for controlling electrical devices.

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## Circuit diagrams

Two things are importanat for a circuit to work:

~There must be a complete

~There must be no short circuits

To check for a complete circuit, follow a wire coming out of the battery with your finger. you should be able to go out of the battery, through the lamp and back to the battery.

To check for a short circuit, see if you can find a way past the lamp without going through any other component. Of you can, then there is a shortcircuiot and the lamp will not light.

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## CURRENT AND VOLTAGE @

You need to know how to measure the current that flows through a resistor,a dn how to measure the voltage across the resistor.

A current flows when an electric chargemoves around a circuit. No current can flow if the circuit is broken, for example, when a switch is open.

Current is measured in ampreres

Amperes is often abbreviated to amps or A

Current flowing through a component in a circuit is measured using an ammeter.

The ammeter must be connected in series with the component!!!

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Voltage

A voltage across an electrical component- such as a lamp- is needed to make a current flow through it. Cells or batteries often provide voltage needed.

Voltage is measured in volts, often abbreviated to V

Voltage across a component in a circuit is measured using a voltmeter

The voltmeter must be connected in Parallel!!!! with the component.

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## Resistance and OHMS law

Resistance

An electric current flows when charges particle called electrons move through a conductor. The moving electrons can collide with the atoms of the conductor. This makes it more difficult for the current to flow and causes resistance. Electrons collide withthe atoms mroe often in a long wire than they do in a short wire. A thin wire has fewer electrons to carry the current than a thick wire. This means that the resistance in a wire increases as:

The length of the wire increases

The thickness of the wire decreases

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OHMS LAW

resistance is measured in ohms. The symbol for an ohm looks like like a hat! The greater the number of ohms, the greater the resistance.

The equation below shows the relationship between voltage, current and resistance:

potential difference(volt, V) = current(ampere, A) x resistance (ohms)

The current flowing through a resistor at a constant temperature is directly proportional to the voltage across the resistor. SO if you double the voltage, the current also doubles. This is called Ohms's Law.

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## Changing resistance

Series circuits

When compontents are connected to in series, their total reisstance is the sum of their individual reistances. For example, if a 2ohm resistor, a 1ohm resistor and a 2ohm resistor are connected side by side, their total resistance is 2+1+3= 2ohm.

Of you increase the number of lamps in a series circuit, the total reistance will increase and less current will flow.

Variable resistors

The resistance in a circuit can be also be changed using variable resistors. For example, these components maybe used in dimmer switches, or to control the volume of a CD player.

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## Filament lamp

The filament lamp is a common type of light bulb. It contains a thin coil of wire called the filament. This heats up when an electric current passes through it and produces light as a result.

Ohm's Law revisited

Remeber that the current flowing through a resistor at a constant temperature is directly proportional to the voltage across the resistor.

The filament Lamp

The filament lamp does not follow Ohm's Law. the resistance of a filament lamp increases as the temoerature of its filament increases. As a result, the current flowing through a filmanet lamp is not directly proportional to the voltage across it.

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## Thermistor and LDR's

The thermistor

Thermistors are used as temperature sensors, for examplem in fire alarms. Their resistance decreases as the temperature increases:

~ At low temps, the resistance of an thermistor is high, and little current can flow through them.

~At high temps, the resistance of a thermistor is low, and more current can flow through them.

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The LDR

LDRs (light-dependant resistors) are used to detect light levels, for example, in automatic security lights. Their resistance decreases as the light intensity increases:

~in the dark adn at low light levelsm the resistance of an LDR is high, and little current can flow through it.

~In bright light, he resistance of an LDR is low and more current can flow throught it. LDRs are also useful for controlling how long the shutter should remain open of a digital camera. Changes in the reistance are measured and if the light level is low, the shutter stays open for longer than if the light level is high

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## Efficiency

Efficiency = Useful Output

Total input

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## Batteries

Batteries can supply electrical energy.The capacity of a battery is measured in amp-hours- symbol is Ah. Rechargable and non-rechargable batteris have different advantages and disadvantages.

Battery capacity

The ability of a battery to supply electrical energy is called caoaciuty. the capacty of a battery is measured in amp-hours. This unti ha the symbol Ah.

The equation for working out a battery's capacity:

Capacity= current supplied x length of time supplied

SO, a battery that can supply amps for 4 hours has a capacity of 2x4=8 Ah.

HOW LONG CAN THEY LAST?

re-arrange the eqautuon like this

length of time supplied = capacity/ current supplied

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## types of batteries

Non-rechargable

ordinary 'dry-cell' batteries are non-rechargable. As the reactants inside them become used up in chemical reactions, the output from these batteris gradually falls. Once all the reactants have been used up, these batteries go 'flat' and cannot supply electrical energy anymore.

Re-chargable batteries

Rechargable batteries usually maintain a constant output until just before they go flat. They contain reactants that can be made again just by passing electricty throuhg thr proudcts. Once the battery has gone flat, it can be connected to a recharge. this uses electrical energy to reverse the chemical reaction that hppened in the battery while it was in use.

Rechargable batteries are more expensive to buy but cheaper in the long run.

Non- rechargable batteris are cheap to buy but more expensive in the long run becasue they can only be used once.

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## generating electricity

Fossil fuels

teh fosslil fuels are coal, oil adn natural gas. They are fuels becasue they release heat energy when they are burned. They are fossil fuels becasue they were formed from the remains of living organisms millions of years ago. About three-quarters of the electicity generated in the UK comes from power stations fuelled by fossil fuels.

Disadvantages of using fossil fuels are that they are nonp-mrenewable resources. Their supply is limited and they will eventually reun out. Fossil fiels do not renew themselves, while fuels such as wood can be renewed endlessly.

Fossil fuels release carbon dioxide whent ehy burn, which adds to the greenhouse effect and increases global warming. Of the three fossil fuels, for a given amount of energy release, coal produces the most carbon dioxide adn natural gas produces the least. Coal adn oil release sulfur dioxide when they burn, whicbh casues breathing problems for living creatures and contributes to acid rain.

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## Different types of generating energy

Wave energy uses kinetic energy to drive electrical generators.

Hydroelctric power using gravitational potential energy which is transferred as kinetic energy as the the water drive electrical generators.

Solar energy

Solar celles are devices that convert light energy directly into electrical energy. You may have seen small solar cells in calculators. Larger arrays of solar cells are used to power road signs in remote areas and even large arrays are used to power satellites in orbit around earth.

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Solar Panels

SOlar panels do not generateelectricity, but rather they heat up water, They are often located on roofs of buildings where they an recieve heat energy from the sun.

~Solar energy is a renewable energy resource and there are no fuel costs.. No harmful polluting gases are proudced.

~solar cells are expensive and inefficient, so the cost of their electricity is high

~solar panels may only produce very hot water in very sunny climates, and in cooler areas may need to be supplemented with a convectional boiler.

~Solar panel do not work at night as there is not sunlight

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