OCR GCSE P5

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Static electricity

An atom is made up of charged particles. It has a positive nucleus with negative electrons orbiting it. The nucleus is made up of neutrons and protons. There are electrostatic forces between charged objects. Like charges repel; unlike charges attract. 

There is an electrostatic force of attraction between the positively charged nucleus of an atom and the electrons that orbit it. The outermost electrons are less strongly attracted to the nucleus and can be removed by rubbing.Whe two insulating objects are rubbed together they become charged, because electrons are transferred from one object to another. The object that has lost electrons becomes positively charged and the object that gains electrons will become negatively charged.

When you brush your hair, individual hairs become similarly charged and repel each other, making your hair stick up. When you take off a nylon or polyester top, there can be a spark or a crackle over your head. This is caused by the electrons moving through the air from the negatively charged clothing to your positively charged hair. During a thunderstorm, charge builds up in the clouds. When the amount of charge becomes large enough to break down the insulation of the air, the charge flows between the cloud and the earth as a flash of lightning.

It is only the electrons which are transferred when objects are charged by friction.

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Conductors and insulators & Moving charges

Conductors and insualtors:

Metals are good electrical conductors beacuse they have free electrons. This means there are lots of charges free to move. Plastics are electrical insulators (do not conduct electricity). There are few free electrons in plastics, so there are few charges free to move.

Moving charges:

In a complete circuit there are free electrons in all the metal components and connecting wires. The cell (or battery) supplies energy to the electrons. The electrons carry a negative charge, so they will flow from the negative terminal of the cell towards the positive terminal. The flow of charge is the electric current.

Electric current is the rate of flow of charge, or the charge flowing per second. Current is measured in Amps and increases as power supply increases since this means that the charged particles receive more energy. In an electric circuit, charge is conserved and energy is transferred.

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Measuring current and voltage

An ammeter is used to measure current and is connected in series. A voltmeter is used to measure voltage and must be connected in parallel across a component in a circuit. The larger the voltage of a battery in a cicuit, the bigger the current. The voltage across a power supply is a measure of how much energy is supplied to the circuit. The voltage across a component is a measure of how much energy is transferred in the component. 

In a circuit, the charges (free electrons) are the energy carriers. They collect energy at the power supply and transfer energy at a component. Power is the rate at which energy is transferred. 

Power (W) = voltage (V) x current (A)

A voltmeter measures the difference in energy between the terminals of a battery or bulb.

The difference in energy per unit of charge is known as the potential difference (p.d.). This is the scientific term for voltage. A potential difference of 1 volt means that 1 joule of energy is transferred into or out of an electrical form for each unit of charge.

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Resistance

The more resistance in a circuit, the lower the current. The greater the voltage across a resistor, the larger the current. Resistance is a measure of how much a conductor opposes the current. Its unit is the ohm.

Resistance (in ohms) = voltage (V) / current (A)

Ohm's law:

A graph of voltage against current will give a straight line through the origin. This means that the current through a fixed resistor is directly proportional to the voltage across it. The higher the resistance, the lower the gradient. (Voltage must be on x axis and current on y axis)

(http://www.bbc.co.uk/staticarchive/f8b3ee75f7cebcc4e73075e5bde8f92e658fa8a7.gif)

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Series and parallel circuits

Components connected in series are in a line. The current is the same thorugh all components connected in series. The more cells connected in series, the greater the potential difference. The potential difference across the components adds up to the p.d. across the battery. The p.d. across each component will be in proportion to its resistance. The overall resistance will be the sum of all the individual resistances. 

Components in parallel are each connected separately to the power supply. The charge has a choice of pathways, so the current is shared between each branch. The current to and from the power supply is the sum of the current through all branches. Two or more resistors in parallel provide more paths for charges to move along than either resistor on its own, so the total resistance is lower. The current through each resistor is inversely proportional to its resistance (largest through component with smallest resistance). Work is done by the power supply to provide energy to the charged particles. A bulb uses the energy to do work to provide heat and light; a resistor uses the energy to do work to provide heat.

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Series and parallel circuits continued

In a series circuit, the work done on each unit of charge by the battery must equal the work done on it by the circuit components. More work is done by the charge moving through a large resistance than through a small one. Two or more resistors have more resistance than one on its own, because the battery has to move charges through both of them. A change in the resistance of one component (eg. variable resistor), will cause a change in the p.d. across all the components.

Cells connected in parallel will have the same p.d. as one cell on its own, but the amount of energy in the circuit will increase. The p.d. across components connected in parallel is always the same, and is equal to the p.d. of  the battery.

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Thermistors, LDRs, Metals & Semiconductors

Thermistors and LDRs:

A thermistor is a semiconductor whose resistance changes with temperature. In cooler temperatures, the resistance is higher than during warmer temperatures.

A light dependent resistor is a semiconductor whose resistance chages as the amount of light falling on it changes. In bright light, the resistance will be low. 

Metals and semiconductors:

In semiconductors, as the temperature or light intensity increases there are more free electrons to carry the current, so the current is higher.

The positive ions in the metal structure have more energy and vibrate more. The free electrons will collide more often. This means that they cannot move as fast, so the current decreases.

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Making an electric current

A magnetic field is the space around a magnet in which a magnetic force acts. The magnetic field is strongest where the field lines are closest together.

A voltage is induced when a magnet is moved near a piece of wire. If the piece of wire is part of a circuit, a current will flow. A voltage is always induced when there is relative motion between a magnet and a coil of wire.

The direction of the current is reversed when the motion of the wire is reversed, or the magnet is turned round. The current will increase if the speed of motion increases, a stronger magnet is used or there are more turns of wire in the coil. 

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Generators

A continuous supply of electricity is produced when there is continuous relative motion between a magnet and a coil of wire. The coils of wire continuously 'cut' the magnetic field lines so a voltage is induced. This is called electromagnetic induction. A larger voltage is induced if:

  • the strength of the magnet is increased
  • the number of coils in the wire is increased
  • the rate at which the coil is turned is increased
  • an iron core is used inside the coil

The faster the rate of cutting field lines, the larger the induced voltage. Mains electricity is produced by generators in power stations that induce an alternating voltage. 

As a coil rotates in a uniform magnetic field it cuts the lines of the magnetic field at different rates:

  • when the coil is at right angles to the field lines, it cuts no field lines so the induced voltage is zero.
  • when the coil is parallel to the field lines, its rate of cutting field lines is at a maximum, so the induced voltage is at its peak.
  • as the coil rotates, it cuts field lines in a different direction, so the direction of the voltage alternates.
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Distributing mains electricity

Direct current (d.c.) always follows in the same direction. Batteries produce d.c. electricity.

     Direct current:                                                          Alternating current:

the signal is a flat line at 1.5V (http://www.bbc.co.uk/staticarchive/a2119794c85f5bf689f2a6f067b02e9e32ba4530.gif)                                                 the signal is a wavy line (http://www.bbc.co.uk/staticarchive/ca9e2a6bb03c38a88e1c8c0821289d9249a0c21c.gif)

Alternating current (a.c.) changes direction at regular intervals. Transformers only work with a.c. and it is easier to generate a.c. electricity in large amounts, therefore, in the UK, mains electricity is a.c. and is generated at 230v and at a frequency of 50 Hz. Transporting the electrical energy from a power station at a high voltage and low current is more efficient. Most of the energy would be wasted as heat due to resistance in the power lines if this was not done.

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Motors (1)

Motors are used in any electrical appliances to make things move eg. hair dryers and DVD players. There is a circular magnetic field around a wire carrying electric current. If the wire is made into a coil, the magnetic field pattern becomes similar to that of a bar magnet. This is called an electromagnet

When a current flows in a wire that is in a magnetic field, the wire experiences a force. If the wire is free to move, it does so. This is called the motor effect. The force is largest when the current is at right angles to the magnetic field lines. The direction of the force is always at right angles to both the current in the wire and the magnetic field lines. No force is experienced when the current is parallel to the magnetic field lines. The direction of the force is reversed if either the current or magnetic field is reversed. When a simple motor is placed in a uniform magnetic field, one side of the rectangular current-carrying coil is forced upwards, whilst the other is forced downwards to produce rotation. The motor will turn faster if:

  • the current is increased
  • the number of turns on the coil is increased
  • the magnetic field is stronger
  • there is a soft iron core in the coil
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Motors (2)

The motor effect works because of the interaction between the magnetic field around the current carrying wire and the magnetic field of the permenant magnets. In the diagram below, the two magnetic fields reinforce each other above the wire and cancel each other out below the wire, so the wire is forced upwards. 

The coil of the motor is connected to the power supply using a commutator. The commutator swaps contacts with the coil every half turn to reverse the current through the coil. This keeps the motor turning. 

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Transformers

A transformer changes the voltage of an a.c. power supply. It consists of two separate coils around an iron core. The input voltage is fed into the primary coil and the output voltage is across the secondary coil. The larger of the two voltages will be across the coil with the most turns. 

The alternating current in the primary coil creates an alternating magnetic field around it. The magnetic, soft, iron core channels the magnetic field through the secondary coil. The alternating magnetic field will continuously cut through the wires in the secondary coil and an alternating voltage will be induced across it. 

Voltage across primary coil / voltage across secondary coil = number of turns in primary coil / number of turns in secondary coil

                                         (http://www.cyberphysics.co.uk/topics/magnetsm/electro/Transfromer/stepdown.gif)

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