IGCSE Physics Section B (Electricity)

IGCSE Physics Section B notes :)

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Hazards

  • Frayed cables, any damaged insulation exposes live wires
  • Long cables as they are more likely to get damaged
  • Damage to plugs or any insulating casing on any mains operated devices
  • Water around electric socket or mains operated devices
  • Pushing metal objects into mains sockets without socket cover

Other safety features:

  • Insulation- all mains wiring is double insulated preventing separate conductors from touching as well as someone from touching a live wire.
  • Appliances with a metal outer casing that the user might touch might be earthed.
  • Fuses are fitted in plugs so if voltage is too high, circuit breaks.
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Electrical power

Power = Current X Voltage

W = A X V

Power is the rate at which electrical energy is converted in an appliance.

AC = alternating current( change direction continously )

DC = direct current ( one direction )

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Electric Charge

Materials that conduct electricity well are known as conductors, they are usually metallic. Materials that do not, are called insulators.

The atom is made up of neutrons (neutral) protons (positive) and electrons (negative)

In solids the position of atoms are fixed and therefore the nuclei are also fixed. The surrounding electrons can move from place to place under certain conditions (transfer of electrons)

Static electricity was discovered through friction.

Unlikes charges attract, Like charges repel.

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Uses of static electricity (Photocopier)

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/ph_elect27.gif)

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Others

A defibrillator is a machine that can be used by paramedics to restart your heart if it stops.

Two paddles with insulated handles are charged from a high voltage supply. They're put in good electrical contact with the patient’s chest. It's important that no one else gets a shock, which is why the paddles have insulating handles and the operator tells any one nearby to 'stand clear' before charge is passed through the patient to make the heart contract.

Car manufacturers can save money by using charged paint spray guns. They work because like charges repel and unlike charges attract.

The spray gun is charged positively, which causes every paint particle to become positively charged. Like charges repel and the paint particles spread out. The object to be painted is given a negative charge and so attracts the paint particles. The advantages of using this system are that less paint is wasted, the object receives an even coat and the paint covers awkward ‘shadow’ surfaces that the operator cannot see.

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Electrical circuit symbols

(http://www.bbc.co.uk/ks3bitesize/science/images/circuit_symbols.gif)

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Series

Components that are connected one after another on the same loop of the circuit are connected in series. The current that flows in each component connected in series is the same.

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/ph_elect02.gif)

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Parallel

The diagram shows a circuit with two lamps connected in parallel. If one lamp breaks, the other lamp will still be lit.

Because a parallel circuit has more paths for charges to flow along, the current is bigger, and the resistance of the whole circuit is smaller.

(http://www.bbc.co.uk/schools/gcsebitesize/science/images/ph_elect03.gif)

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Potential difference

An ammeter in series with a lamp (http://www.bbc.co.uk/schools/gcsebitesize/science/images/ph_elect04.gif)

Potential difference

A 'potential difference' across an electrical component is needed to make a current flow in it. Cells or batteries often provide the potential difference needed.

Measuring potential difference:

  • Potential difference is measured in volts, V.
  • Potential difference 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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Current

Measuring current:

  • Current is measured in amperes.
  • Amperes is often abbreviated to amps or A.
  • Current flowing in a component in a circuit is measured using an ammeter.
  • The ammeter must be connected in series with the component.

An ammeter in series with a lamp (http://www.bbc.co.uk/schools/gcsebitesize/science/images/ph_elect04.gif)

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PD Series

The potential differences across resistors in series must add up to the battery voltage. This is because the total energy transferred by the battery must equal the amount of energy transferred to the other components in the circuit. Energy is always conserved.

The energy is transferred from the cell to the electric charge moving through the circuit. The charge then transfers energy to the components (bulbs, resistors, etc).

More energy is transferred by charge flowing through a larger resistance than through a smaller one.

This is why a large and a small resistor connected in series have different voltages across them. The large resistance has a larger voltage because more energy is being transferred as the charge flows through it.

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PD Parallel

In parallel circuits, the voltage across each component is the same as the voltage of the battery. Each component in parallel has the same current as it would have if it were connected to the battery without the other components present.

This means that a higher resistance in parallel with a smaller resistance would have less current in it, as the same voltage will cause less current in a larger resistance than in a smaller one.

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