# [V] Physics - P5

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• Created by: Olivia
• Created on: 09-04-14 13:50

## Static electricity

When your hair is attracted to a charged comb or charged balloons, they move away from each other, so there must be a force acting. In both these examples, the force is an ELECTROSTATIC FORCE caused by positive and negative charges.

When you rub two balloons together, electrons will rub off one and move onto the other. One balloon will be negatively charged and the other positively charged. They will be attracted and move together. The rule for the way charges is: LIKE CHARGES ATTRACT, UNLIKE CHARGES REPEL. Atoms are made up of charged particules. They have a positive nucleus with negative electrons absorbing it. Atoms have no overall change. We say they are NEUTRAL.

Each electron is held in the atom by an electrostatic force between the positive nucleus and the negative electron. The further the electron is from the nucleus, the weaker this force. Sometimes some of the outer electrons can be removed by rubbing. If you rub a balloon on your jumper, electrons will be rubbed onto the balloon, making it negatively charged. Each of the two rubbed objects will become oppositely charged.

Lightning is an example of electrostatics. Charge is built up by friction in the clouds. When it becomes large enough to break down the insulation of air, the charge flows between the cloud and the Earth. The release of energy appears as a flash of light.

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## Moving charges

Wires are covered with plastic for safety. Copper conducts electricity but plastic does not. The plastic covering prevents electricity in a wire from flowing through us or through parts of the structures of our homes. It prevents electrocution and reduces the risk of fires.

Whether a material is an insulator or a conductor depends on its structure at atomic level. The outer electrons of atoms of materials that are good conductors of electricity - usually metals - are loosely held and can break free easily. They are then free to move in the metal. These electrons are known as FREE electrons.

Metals are electrical CONDUCTORS. Plastic is an electrical INSULATOR, as it has few charges that are free to move. A circuit could be used to test if a material is a conductor or an insulator. Crocodile clips are attatched to the sample, and also to a battery and a lamp. When the bulb is lit there is an ELECTRIC CURRENT in the circuit. The testing circuit needs a power supply to provide the energy, conducting wires to provide a pathway and a bulb to indicate when the current is flowing.

The circuit could include a switch. When the switch is open, it creates a gap in the circuit that stops the current flowing. When the switch is closed, the current flows.

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## Increasing the current

Good conductors have free electrons. When a circuit is complete the power supply provides the energy, or push, which makes all the free electrons in the components and wires flow in one direction. Electrons carry a negative electric charge. This flow of charge is the ELECTRIC CURRENT. In an electric curret there are free electrons everywhere, so as soon as the circuit is completed, there is a movement of all electrons throughout the circuit. As soon as a switch is closed and the circuit completed, the light goes on (or a motor starts spinning).

The positively charged particles in the metal also have a force on them, but they are massive and not free to move. It is the movement of electrons that is important to electric circuits.

How fast the electrons move depends on the amount of push they get from the power supply (cell or battery). As the electrons go through the power supply they gain energy. The more energy they gain, the faster they travel and the larger the current.

Current is the RATE OF FLOW OF CHARGE, or the charge flowing per second. Current is measured in amps. Charge is not used up in an electric circuit. When electrons flow through a bulb, their energy is transferred to the surroundings as heat and light. They need a push from the battery to keep moving and keep transferring energy to the bulb to keep it lit. The electrons flow in a continous loop around the circuit. In an electric circuit, CHARGE IS CONSERVED AND ENERGY IS TRANSFERRED.

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## Measuring current and voltage

Batteries do work by pushing the electrons around the circuit. The electrons gain energy.

The higher the voltage, the more energy is transferred and the more work is being done.

An AMMETER is connected in a series with the components in the circuit so that it can measure the current THROUGH the circuit, in amps. In an electric circuit the current gains energy from the power supply, such as a battery and transfers (gives out) energy when it passes through the components such as lamps, motors and so on. The larger the voltage of the power supply (such as a battery), the larger the current and so the more energy is transferred to the components.

A VOLTMETER measures how much energy is transferred in a particular part of the circuit, for example in an electrical device such as a motor. We call this the VOLTAGE across the motor, and the unit we use is the VOLT. Voltmeters are connected across the device (in parallel).

There are different types of ammeters and voltmeters. The meter with the pointer is an analogue meter. The pointer can take any position on the dial according to the size of the current of voltage being measured. A digital meter gives the size of the current of voltage as a number. Digital meters are easier to read but both types can be very accurate.

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## Energy in a circuit

In an electric circuit, the charges (free electrons) are energy carriers. There are charges throughout the circuit and these charges can accept energy from a battery and transfer energy to a component. The battery does work, giving the charges a "push" and they gain energy, just as a swing gains energy when it is pushed. This means that charges leaving the battery have more energy than charges entering the battery on the other side.

At the bulb, the reverse happens. The charges do work. They transfer their energy to heat and light, which is radiated from the circuit to the surroundings. The charges will have more energy on one side of the bulb than on the other side of the bulbs. The exhausted charges are then pushed back towards the battery.

Voltmeters measure this difference in energy between the terminals of a battery or a bulb. The number of charges flowing in a circuit always remains the same: there is nowhere else for them to go.

At the top of a slide, a child has more potential energy than at the bottom. In much the same way, the charges on either side of the battery have a different amount of energy; electrical potential energy. This is because the battery gives the charges energy as they flow through it. This difference in energy per charge is the potential difference. This is another term for voltage. It is measured in volts.

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## Electrical resistance

Similarly, the charges either side of a lamp in a circuit will have different amounts of electrical potential energy. There is a potential difference between the terminals of the lamp, measured in volts by a voltmeter.

The potential difference between two points in a circuit is one volt if one joule of electrical energy is transferred to another form of energy when one unit of charge passes between the points.

In electric circuits, the current can be controlled by changing the number of cells in the circuit. If there are more cells, then more energy will be supplied. (Cells = batteries). This means that there will be a faster flow of charge and therefore a bigger current. In other words, the larger the voltage of the power supply, the larger the current. The current is also affected by the amount of RESISTANCE in the circuit. All components resist the current flowing through them. If there is more resistance in a circuit then the charges move slower and the current will be lower.

A VARIABLE RESISTOR is a device that allows you to vary the amount of resistance in a circuit by moving a slide or rotating a knob, so that more of less resistance wire is connected to the circuit. It is shown on a diagram as a rectangle on a line, with a positive diagonal arrow through the centre of the rectange. Resistance is a measure of how much a conductor opposed the current. Its unit is the OHM.

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## Ohm's law

Good conductors, like copper, have low resistance. The resistance of copper connecting wires is so small that it can be ignored. There is an important relationship between CURRENT, VOLTAGE and RESISTANCE.

- When a current flows through a conductor the size of the current is affected by the voltage. The greater the voltage applied, the more current flows.

- The size of the current is also affected by the resistance of the conductor. The greater the resistance, the less current flows.

Resistance, voltage and current are related by the formula:
Resistance = voltage / current

If you plot a graph of current against voltage for a fixed resistor, it will give a straight line. The straight line through the origin shows that current through the resistor is directly proportional to the voltage across it. This means that the resistance is constant. You can use the gradient of the graph to calculate the resistance:

Resistance = 1 / gradient of the graph

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## Series circuits

A higher resistance gives a lower gradient.

OHM'S LAW describes this current-voltage relationship. It states that the current, through a metallic conductor is directly proportional to the voltage across its ends if the temperature and other conditions are constant.

This only applies to some components, such as resistors. For other components the value of the resistance changes as the current increases.

In the "smart dust" there are electric circuits. Despite these being very small, they have parts where components are connected in SERIES and other places were components are connected in PARALLEL. Components connected in series are in a line.

In a circuit, two cells and two lamps are connected in a series with a switch. If the circuit is broken, both the lamps will go out. The current is the same everywhere in a series circuit: all the ammeters read the same. Remember that current is the flow of charge through the circuit. There is only one path through the circuit, so all the charge has to follow it. In a series circuit, the supply voltage, or potential difference, is shared between the components.

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## Series circuits

Battery PD = PD across lamp + PD across resistor.

In general, for two components in a series circuit, supply PD = V1 + V2

For each component, the resistance is given by R = V / I which can be arranged as V = IR

Since the current, I, is the same throughout all the components, if the resistance of the lamp is twice the resistance of the resistor, then the PD across the lamp will be twice the PD across the resistor.

If the resistance in the whole circuit is increased the current will be smaller Resistances in series add up.

R (total) = R1 + R2 + R3

The resistance in the whole circuit increases because the battery has to move charge through more resistors and so the extra work will mean that the charges will flow slower. If another identical battery is added to the circuit, the overall voltage of the battery will double and this will increase the rate of flow of charge round the circuit. The current will also double.

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## Parallel circuits

Components connected in parallel are each connected separately to the power supply. In a parallel circuit, the current from the battery is shared between each branch. The charge travelling through the circuit it has a choice of pathways. The current to and from the battery is the sum of the current throughout the branches.

The total current from the battery is greater than if any of these resistors were alone in the circuit. This is because the parallel circuit provides more pathways for the charges to move in and so overall increases the rate of flow of charge to and from the battery. Seen from the battery, connecting more resistors in parallel decreases the overall resistance in the circuit.

I stands for current.

1 (total) = I1 + I2 + I3

Branches in parallel circuits behave like individual circuits: each branch gets the full PD provided by the battery. The current through each branch is the same, as if each branch was a series circuit connected seperately to the battery.

More work is done moving charge through a large resistance.

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## Circuits and energy

In an electric circuit work is done transferring energy to and from the charges in the circuit. In a series circuit, work is done by the battery to provide energy to the charge flowing in the circuit. The lamp then uses some of the energy to do work to provide heat and light, and work is done in the resistor and heat is produced. The amount of work done providing energy to the charge by the battery is equal to the energy transferred out of the circuit by the components. In general, in a series circuit this results in the expression:

Supply PD = V1 + V2 + V3 +

Mork work is done moving charge through a large resistamce than a small resistance. This leads to the largest PD being across the largest resistance in a series circuit.

Space travel involves extremes of temperature that many devices may not cope with. A thermistor, however, is an electrical component made of materials that can cope with a large range of temperatures. Thermistors can monitor temperatures and control cooling settings.

The messenger probe entered orbit around Mercury in March 2011. It needs to withstand very high temperatures since Mercury is close to the Sun.

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## The thermistor and LDR

The THERMISTOR is a semi-conductor device. Its resistance changes with temperature. In the most common thermistors, the resistance decreases as the temperature increases. Thermistors are used as temperature sensors, for example, in digital thermometers. They can be used with a thermostat to control temperature, for example to switch on a heater when the temperature of an incubator falls below a certain value. The symbol for a thermistor is similar to that of a resistor. It is a rectange on a horizontal line, with a positive line going through it and in the bottom left there is a horizontal line.

The LIGHT DEPENDANT RESISTOR (or LDR) is another semi-conductor device. Its resistance varies with the amount of light falling on it. In bright light its resistance is low but in darkness it has a high resistance. An LDR can be used to switch on street lighting when night falls. The symbol for an LDR is a horizontal line with a rectangle on it and two arrows pointing to the top of the LDR.

The temperature of a thermistor thermometer can be calibrated with temperature by heating it in a beaker of water and noting the temperature at regular intervals. The resistance of an LDR is highest in the dark. The resistance falls as the surroundings get brighter. The graphs for these will be a curve in a negative correlation.

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## Explaining varying resistance

A conventional light bulb filament gets very hot when a current flows through it. The filament is a thin metal wire. Is its resistance affected by temperature?

When the current (I) through a lamp is measured with increasing potential difference (V) across it, a graph of I against V will be a positive curve.

All metals behave like the light bulb filament when they get hot. As the metal gets hot its resistance increases. This is because the positive ions in the metal structure have more energy and so jiggle about more. The free electrons collide more often with them and so their overall speed is slower. There is a smaller rate of flow of charge (carried by the negative electrons) through the metal. This is opposite to the thermistor and LDR, where the extra heat or light energy provides more free electrons. In these semi-conductor devices there is a greater rate of flow of charge because there is more change available. Their resistance decreases.

The basis of a device that would open your curtains without you getting out of bed might be an LDR in series with a motor. When it gets light the resistance of the LDR will decrease. This will reduce the PD across it. Since the total PD across the components must equal the PD of the supply this will increase the PD across the motor. The overall resistance in the circuit will fall and the current will increase. This may be sufficient to get the motor to work and open/close curtains.

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## Producing electricity

A FIELD is a space in which a particular force acts. If you put a magnetic material in the space around a magnet it will experience a force. We use the idea of field lines to help visulise this MAGNETIC FIELD.

Magnetic field lines may be imaginary, but they are a useful model. Remember that they go from the North pole to the South pole and they never cross. Where they are closer together, the magnetic field strength is greater.

Magnets may be "permanent" magnets or ELECTROMAGNETS. An electromagnet only acts as a magnet while a current is flowing, so is called a "temporary" magnet. Electromagnets can be very strong.

In the 1830s Michael Faraday discovered that if he moved a magnet near a piece of wire then a voltage was INDUCED between the ends of the wire. If the ends of the wire were joined to complete the circuit a current flowed. The wire can be moved instead - by moving the wire up and down between the poles of a very strong magnet a current will register on a sensitive meter.

Moving the wire up induces a current; moving it down induces a current in the other direction.

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## Induced voltage and current

- the direction of the current changed if the motion of the wire was reversed.

- if the wire was kept still, then no current flowed at all.

Faraday also used coils of wire instead of a single wire. He found that the more turns of wire he used, the bigger the induced voltage and therefore the bigger the current on the ammeter. Using a stronger magnet induced a bigger current too.

Enough current can be induced to light a small bulb by moving a bar magnet in and out of a coil a few hundred turns. The magnet needs to be moved rapidly. The slower it goes, the smaller the current produced.

When the magnet is pulled out of the coil the current flows in the opposite direction to when the magnet is pushed in. If the magnet is reversed, so is the current flow. A voltage is induced whenever there is RELATIVE movement between the magnet and the coil. Either the magnet or the coil could move, or both. The coil could spin in the magnetic field, or the magnet could spin in front of one end of the coil. Faster relative motion induces a bigger current and having an iron core in the coil also makes the current bigger.

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## Electromagnetic induction

Faraday introduced the idea of visualising a magnetic field using magnetic field lines. He used it to explain the induced voltage, which he called ELECTROMAGNETIC INDUCTION. Field lines need to "cut" through the electrical conductor to induce a voltage, and the greater the rate at which the field lines cut, the bigger the voltage induced.

Imagine the field lines around a bar magnet. They will "cut" through the turns of the coil if the magnet (or coil) is moving and will cut faster if the movement is faster or if there are more turns to cut through.

A bicycle dynamo acts like a mini electrical generator. The bicycle wheel spins the wheel on the dynamo, which in turn makes a magnet spin inside a coil in the dynamo. The induced voltage power's the bikes lights.

We have previously seen how an electric pulse can be induced by a relative movement between a magnetic field and a metal wire. A continuous supply of electricity can be induced by relative rotation of a magnet and a coil. In a model generator, a coil rotates continously in a magnetic field. The poles of the magnets are on their large flat sides. The North and South poles are positioned so that they face each other. There is a magnetic field between the poles of the magnets. Imagine straight parallel lines between the magnets. When the coil is made to rotate, the magnetic field lines "cut through" wire. A voltage is induced across the ends of the coil. This is called ELECTROMAGNETIC INDUCTION.

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## The size of the induced voltage

A model generator can be used to investigate the size of the voltage and current produced. The results show that the size of the current produced depends on the rate at which the coil rotates, that is the rate at which the magnetic field lines are cut. It also depends on the number of turns of the coil, the strength of the magnet and whether there is an iron core in the coil.

But current depends on the induced voltage. So this experiment also tells us that the size of the induced voltage can be increased by: increasing the strength of the magnet, increasing the number of turns in the coil, increasing the rate at which the coil is turned and placing an iron core inside the coil.

The model generator produces an ALTERNATING CURRENT (AC). The direction of an alternating current changes at regular intervals. Our mains electricity in UK is AC generated at 230V and a frequency of 50Hz, which means the direction of the current changes 50 times a second. This is the frequency of rotation of the coils in the power stations.

The size of the voltage induced in a generator depends on the rate at which the coil in the generator cuts through the magnetic field lines. In the model generator, the magnetic field is the same strength at all points between the poles. This is called a UNIFORM field and is shown on the diagrams by parallel, evenly spaced lines.

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## The magnetic effect of a current

Varying PD will result in an alternating current when the circuit is completed. The frequency of the alternating current will be the same as the frequency of the rotating coil that induced it.

The first house in UK was fitted with carbon filament lights in 1880. These electric light bulbs were the first kind of practical, safe light bulbs, invented by Thomas Edinson. But the house had to run it's own generator. The first council-owned power station was opened in Bradford in 1889. The National Grid supply network was not developed until the 1930s.

In all electric appliances, there is a MOTOR, which has an axle that is attached to the part that needs to move. In a hairdryer it is attatched to the fan, in a DVD player it rotates the disc drive, and in a food mixer it is attatched to the blades in a bowl. Motors convert electrical energy to kinetic energy.

To understand how a motor works, we need to start with the magnetic effect of a current. When an electric current flows through a wire, it produces a circular magnetic field around the wire. If the wire is made into a coil, the magnetic field pattern becomes similar to the field around a bar magnet. We know there is a magnetic field because, near the wire or coil, a force is exerted on a magnet, or on another current-carrying wire or coil.

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## The motor effect

When a current flows through a wire in a region where there is another magnetic field the wire experiences a force. If it is free to move it moves, this is called the MOTOR EFFECT.

The biggest force is felt when the magnetic field and the current are perpendicular (at 90o) to each other. No force is felt if the current - carrying wire is along a magnetic field line.

The force is bigger if the current if larger or if the magnetic field is stronger.

The force on the current-carrying wire in a magnetic field is at right angles to both the magnetic field and to the current in the wire. If either are reversed, the direction of the force - and hence the movement - is reversed.

A motor uses the motor effect but is assembled so that the force gives continuous rotation. It is like a generator but is used in reverse. The coil is connected to a power supply and a force is produced that makes the coil turn. This can be understood by considering a rectangular coil. Each side of the coil in the field experiences a force: on one side the force is upwards and the other side it acts downwards. This makes the coil turn. The motor will turn faster if: the current is bigger, there are more turns on the coil, the magnets are stronger or there is a soft iron core in the coil.

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## Explaining how a motor works

The motor effect works because of the magnetic field around the current-carrying wires and the magnetic field of the permanent magnet combine.

The fields reinforce each other above the wire and cancel each other out below the wire. Why does the motor keep turning? The left hand side of the coil experiences an upward force and the right hand side experiecnes a downward force. When the sides reach the top and bottom (that is, when the plane of the coil is vertical) the fixed COMMUTATOR (a metal ring split into two halves) swaps contacts with the coil to reverse the current through the coil. This happens each half cycle and ensures that the turning effect is always in the same direction and the coil rotates continuously.

My laptop needs 19.5V but when I charge it up I plug it into the 230V mains. Why doesn't it blow up? The black box on the mains lead is a transformer. This is a device that changes one voltage to another. The laptop transformer changes 230V to 19.5V so that it is safe for the laptop to use.

A transformer will convert mains voltage (230V) to 3V. It is made up of two separate copper wire coils with iron core inside and surrounding the cois. The mains AC current is fed into the PRIMARY COIL on the left. The smaller voltage is induced in the SECONDARY COIL on the right, and this is connected to the appliance.

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## Stepping up and stepping down

In a circuit, ten turns of wire are wound on a soft iron core and connected to an AC power supply. This is the primary coil. The secondary coil has 25 turns and is connected to a small bulb. The two coils have been clipped together so that the soft iron makes a complete loop. Another small bulb has been connected across the primary supply.

When the alternating voltage is applied, both bulbs light. The brighter secondary bulb indicates that the secondary voltage is greater than the primary voltage. This is a STEP UP transformer.

If the coils are reversed so that the one with the larger number of turns is the primary coil and the 10 turn coil as the secondary the transformer will decrease the voltage and the secondary bulb will be dimmer. This is a STEP DOWN transformer.

The coils of a transformer are not connected, so how can a current in one coil induce the current of a nearby coil? The AC voltage in the primary coil creates an ever-changing magnetic field around it. The magnetic soft iron core channels the field through the secondary coil. This repeatedly changing magnetic fiel cuts through the secondary turns and an AC voltage is produced across the ends of the secondary coil. A changing current in one coil of the transformer will cause a changing magnetic field in the iron core. This in turn induces a changing PD across the other transformer coil.

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## Alternating current

If the number of turns in the secondary coil is doubled, then the output voltage will be doubled. If the number of turns in the secondary coil is halved, then the output voltage will be halved. The turns ratio is equal to the voltage ratio:

Voltage across primary coil / voltage across secondary coil

=

Number of turns in primary coil / Number of turns in secondary coil

A house could run on battery power, and then there would be no ugly power lines going across the country. But the battery would have to be replaced frequenctly when it ran out. Using a battery, it would not be possible to transform voltages to all the different voltages that our appliances use, as batteries can only produce direct current.

In some circuits the current flows in one direction all the time. This is called DIRECT CURRENT or DC. Torches use DC and so does the electrical system in a car. In other circuits, the current keeps changing direction. This is called ALTERNATING CURRENT or AC. Mains appliances, such as TVs and hair dryers, run on AC. The mains electricity in UK has a frequency of 50 Hz. This means it changes direction 50 times a second.

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## Distributing mains electricity

There are good reasons for using alternating electricity as our electricity supply. Generators generate alternating electricity. They produce far more power than other forms of electricity production. Solar panels large enough to generate an equivalent amount of power would take huge amounts of land (and Sun). An equivalent battery would be huge, expensive and impractical. Another advantage of generators is that many different fuels can be used to turn the large turbines.

Another important reason for using alternating electricity as our mains supply is that, both at the power station and at the consumer end, transformers are used to achieve the required voltage. Transformers only work with alternating electricity. At the power station transformers are used to step up to the high voltage used for transmission. Pylons and aluminium cables are used to transmit electricity across the country. There is less energy lost at high voltages, making the transmission of electricity across the country more efficient. Outside a town you may see substations. Here, transformers change the high transmission voltage to low domestic voltage. By using several transformers different voltages can be delivered to various consumers. Factories can have 30kV and 230V can be delivered to homes. Some appliances, such as computers, use DC. The AC can be converted to DC. POWER is the rate at which energy is transferred. The formula for ELECTRICAL POWER is: power = voltage x current. Power is measured in watts, currents in amps, and voltage in volts. A current is lower at a higher voltage.

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