P5- Electric circuits

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Electrostatic forces

An atoms is made up of charged particles. It has a positive nucleus with negative electrons orbiting it. The nucleus is made up of neutral neutrons and positive protons.

  • Neutral objects have no overall charge.
  • There are electrostatic forces between charged objects.
  • Like charges repel, unlike charges attract.

(http://www.hk-phy.org/energy/power/nuclear_is/images/atom.gif)

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Static electricity

  • There is an electrostatic force of attraction between the positively charged nucleus and the negatively charged electrons in the atom.
  • The outermost electrons are less strongly attracted to the nucleus and can be removed by rubbing.
  • When two insulating objects are rubbed together they become charged, because electrons are transferred from one object to the other, the object that loses electrons becomes positively charged, the object that gains electrons becomes 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 shirt there can be a spark or 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 clouds and the earth as a flash of lightening.
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Conductors and insulators

Metals are good electrical conductors because they have free electrons. This means there are lots of charges free to move.
Plastics are electrical insulators because. There are few free electrons in plastics, so there are few charges free to move.

(http://ts1.mm.bing.net/th?&id=HN.608026395785887832&w=300&h=300&c=0&pid=1.9&rs=0&p=0)

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Moving charges

  • When a bulb is lit in a circuit there is an electric current.
  • The moving electrons or electric current transfer energy to light the bulb.
  • 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.
  • The bulb is converting the energy carried by the electrons into light and heat energy.
  • Electric current is the rate or flow of charge, or the charge flowing per second.
  • Electric current is measured in amperes or amps (A).
  • The more energy the charged particles receive from the power supply , the greater the current.
  • 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. An ammeter is connected in series.
A voltmeter is used to calculate voltage. A voltmeter is connected in parallel across a component in a circuit.
The unit of voltage is the volt (v).
The larger the voltage of the battery of a circuit, the bigger the current.
The voltage across a power supply is a measure of how much energy is supplied to a circuit.
The voltage across a component is the 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. It is measured in Watts (W).
Power (W) = Voltage (V) x Current (A)
The difference in energy per unit charge is known as 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 electrical form for each unit of charge.

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Electrical resistance

  • The more resistance in a circuit the lower the current.
  • All electrical components e.g. Lamps have some resistance to the flow of charge through them.
  • The greater the voltage across a resistor, the larger the current.
  • Resistance is the measure of how much a conductor opposes the current. It's unit is ohm.
  • Copper wires have such a low resistance that we can ignore it.
  • A variable resistor is a device that allows you to control the current by changing the amount of resistance wire in a circuit.

Resistance (ohms) = voltage (v) / current (amps)

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Ohms 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 (at constant temp).
The higher the resistance the lower the gradient.

(http://ts1.mm.bing.net/th?&id=HN.608051684550905174&w=300&h=300&c=0&pid=1.9&rs=0&p=0)

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Series circuits

  • Components connected in series are in a line.
  • The current is the same through all components connected in series.
  • The more cells connected in series the greater the potential difference (voltage).
  • 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.

(http://ts2.mm.bing.net/th?id=HN.608027482413730065&w=122&h=154&c=7&rs=1&pid=1.7)

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Parallel circuits

  • Components in parallel are each connected seperately 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 the 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, i.e. it is largest through the component with the 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.
  • (http://ts1.mm.bing.net/th?&id=HN.607992190663721920&w=300&h=300&c=0&pid=1.9&rs=0&p=0)
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Circuits

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 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, e.g. variable resistor, will cause a change in the potential difference across the components.

Cells connected in parallel will have the same potential difference as one cell on its own, but the amount of energy in the circuit will increase.

The potential difference across components connected in parallel is allways the same, and is equal to the pd of the battery.

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Thermisters and LDRs

A thermister is a semiconductor whose resistance changes with the temperature. A light dependant resistor (LDR) is a semiconductor whose resistance changes as the amount of light falling on it changes. In bright light the resistance will be low.

The graph shows the variation of resistance with temperature. At high temperature the resistance is low.

(http://t2.gstatic.com/images?q=tbn:ANd9GcSZoltmowCJGmCpjPP4ooVv4lKme0fMItQvFswvRkzNnd6FwxxS)The second graph show the variation in resistance with light intensity for an LDR.

(http://t2.gstatic.com/images?q=tbn:ANd9GcSjBqGy7BKYvc9VY4HzlEaZ8eS-JqyefzxMgInU3Iz5Vin3oorn6A)

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Metals and semiconductors

The graph shows how the current through a bulb varies with increasing potential difference. As the wire in a bulb gets hotter, its resistance increases.

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 increases.

(http://t2.gstatic.com/images?q=tbn:ANd9GcTUlAR2V3acchsFL0cg2Dlw5ms14E0FtxqHLyUoa0sdiZhd3k9n)

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Making an electric current

A magnetic field is a 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 (or when a wire is moved near a magnetic field). If the piece of wire is part of a circuit, a current will flow.

A voltage is allways induced when there is relative movement 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 around.

The current will increase if the speed of motion increases, a stronger magnet is used or there are more turns off 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:

  1. The strength of the magnet is increased.
  2. The number of turns in the coil is increased.
  3. An iron core is used inside the coil.
  4. The rate at which the coil is turned is increased.

The faster the rate of cutting the field lines, the larger the induced voltage.

Mains electricity is produced by generators in power stations that induce an alternating current.

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Generators

As a coil rotates in a uniform magnetic field it cuts the lines of 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 lines is at its maximum, so the induced voltage is at a peak.
  • As the coil rotates, it cuts field lines in a different direction, so the direction of the voltage alternates.

(http://ts1.mm.bing.net/th?&id=HN.607993208569988782&w=300&h=300&c=0&pid=1.9&rs=0&p=0)

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Distributing mains electricity

Direct current (d.c) allways flows in the same direction. Batteries produce d.c. current.

Alternating current (a.c.) changes direction at regular intervals.

In the UK, mains electricity is a.c. and is generated at 230 V and at a frequency of 50Hz. It is transmitted along cables held up by pylons at very high voltages.

Transformers are used to step up the voltage at the power station and step down the voltage near our homes. Transformers only work with a.c.

The UKs electricity supply is a.c. because it is easier to generate a.c. electricity in large amounts. Also many different fuels can be used in power stations. It is more efficient to transmit electricity at very high voltages.

(http://ts4.mm.bing.net/th?id=HN.608010336901989295&w=219&h=97&c=7&rs=1&pid=1.7)

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The magnetic effect of a current in motors

Motors are used in many electrical appliances that make things move, e.g. hair dryer, dvd player.

  • There is a magnetic field around wires carrying a 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 moves. 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 wires 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 the magnetic field is reversed.
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Motors

When a simple motor is placed in a uniform magnetic field, one side of the rectangular current-carrying coil is forced upwards, while 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.

The motor effect works because of the interaction between the magnetic field around the current carrying wire and the magnetic field of the permanent magnets.

The coil of wire 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.
  • The output voltage is across the secondary coil.
  • The larger of the two voltages will be across the coil with the most turns.

A step up transformer converts a low voltage input to a higher voltage output. The primary coil will have fewer turns than the secondary coil.

A step down transformer converts a high voltage input to a lower voltage output. The primary coil will have more turns than the secondary coil.

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Transformers

The alternating current in the primary coil creates an alternating magnetic 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 a alternating voltage willbe induced across the secondary coil.

If the number of turns in the secondary coil is doubled, the output voltage will be doubled.

The turns ratio is equal to the voltage ratio:

(http://ts1.mm.bing.net/th?id=HN.608029299188170831&w=235&h=131&c=7&rs=1&pid=1.7)

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