P5: Electric Currents

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What is Static Electricity?

  • When you rub your hair on a balloon, your hair becomes charged.
  • Electrons move from your hair, to the balloon
  • Each hair is Positively charged, like charges repel, so the hairs get as far away from each other as possible.
  • There are attractive forces between opposite charges, positively charged hair is attracted to the negatively charged balloon.
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What is an electric current ?

  • Electic current- A flow of charge (electrons).
  • Metal conductors have many charges (electrons) that are free to move.
  • Insulators do not conduct electricity- they have few charges that are free to move
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What is electrical power?

  • When electric charge flows a through a component in a circuit, Work is done by the power supply; Energy is transferred from the power supply to the component and its surroundings.
  • Power- Measure of rate an electrical power suppply transfers energy to an appliance or its surroundings. 
  • Power (Watts, W) = Voltage (Volts, V) x Current (Amperes, A)
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Resistance?

  • Bigger Resistance = Smaller Current.
  • Current through metal conductor is proportional to the voltage across it:
  • Resistance (Ohms) = Voltage (Volts, V) / Current (Amperes, A)
  • Resistors get hotter when electric current passes through them, Moving electrons bump into stationary ions in the wire.
  • 2 Resistors in Series have more resistance than 1 on its own; The battery must push charges through both resistors.
  • 2 Resistors in parallel have a smaller total resistance than 1 on its own; There are more paths for the electric charges to flow along.
  • The resistance of Light-dependent resistors (LDR) change with light intensity; Its resistance in the dark is greater than in the light.
  • Resistance of a Thermistor changes with temperature; hotter temp = lower resistance.
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How do Series and Parallel circuits work?

  • Voltage = Push on the charges in a circuit.
  • Potential difference (p.d.) = Voltage.
  • Greater p.d. between 2 points in a circuit, more work done to make charges move between the points.
  • Voltmeter (connected in parallel)  is used to measure Potential Difference.
  • Adding an extra battery in parallel gives no greater p.d. but in series it does; more push is given to the charges in series.
  • 3 components in series;
  • Same current through all, p.d. across components = p.d. across the battery, p.d. is greater along the component with the greatest resistance.
  • 3 components in parallel;
  • Current near the battery = total current through components, Current is largest through component with the smallest resistance.
  • The p.d. across each component = the p.d. across the battery.
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What is electromagnetic induction?

  • Electromagnetic induction- Move a magnet into coil of wire, voltage is induced across ends of coil.
  • Join up ends of the coil to make a circuit, a current flows.
  • You can induce a voltage in the opposite direction by: 

- Moving the magnet out of the coil.

- Moving the other pole of the magnet into the coil.

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How is mains electricity generated?

  • Generators make electricity by electromagnetic induction.
  • In generator, a magnet or electromagnet turns near a coil of wire and induces a voltage along the ends of the wire.
  • The direction of the votage changes as the magnet rotates.
  • Increase the voltage size by:

-Turning the magnet faster   - Adding more loops to the coil   -Putting an iron core inside the coil   -Making the magnetic field stronger.

  • Alternating current (a.c.)- the magnet in a generator turns all the time and its magnetic field constantly changes direction.
  • Direct current (d.c.)- the current from a battery always is in the same direction.
  • Mains electricity is a.c. (230v)  as;
  • It is easier to generate than d.c.
  • It can be distributed more efficiently as less energy is wasted as heat.
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What are transformers?

  • If current in a wire coil changes, its magnetic field changes and induces a voltage in a nearby coil.
  • Tranformer- 2 coils of wire wound onto an iron core, it changes the size of an alternating voltage.
  • Voltage across primary coil (v1) / Voltage across secondary coil (v2) = Number of turns on primary coil (n1) / Number of turns on a secondary coil (n2).
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How are electric motors made?

  • If current is flowing through a wire or coil it can exert a force on:

-a nearby permanent magnet.

-another current-carrying coil nearby.

For current carrying wire in a magnetic field:

  • If magnet's lines of force are at 90' to the wire, the wire experiences a force at 90' to the current direction and the lines of force of the field.
  • If the magnet's line of force are parallel to the wire, the wire experiences no force.

The motor effect results in a turning force on a rectangular coil in a uniform magnetic field. Adding a Commutator allows the coil to rotate continuously; the Commutator swaps the current direction enery time the coil is vertical

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