Electricity

• Live wire


  - The live wire is brown and carries a 230V potential.

  - It provides the alternating potential difference from the mains electricity supply.
• Neutral wire


  - The neutral wire is blue.

  - It is at or close to earth potential (which is 0V).

  - It completes the circuit, and carries current away from the appliance.
• Earth wire


  - The earth wire has green and yellow stripes.

  - It has a 0V potential and only carries a current if there is a fault.

  - It is a safety wire to stop the appliance becoming live.
• power = potential difference x current.
• energy transferred = power x time.
• Large power stations are more efficient than smaller power stations because they can burn lots of fuel at very high temperatures. This creates very hot steam that can turn the turbines.

  Steam turbines are more efficient at higher temperatures.
• The national grid


  is the network of cables and transformers that connect power stations with houses and shops
• Direct current (d.c)


  The electric current flows in only one direction.

  Batteries and cells are sources of direct current.
• Alternating current (a.c)


  The electric current repeatedly changes direction.
• Alternating p.d.


  A supply where the p.d. switches between positive a negative, reversing the direction of the current frequently.
• Direction p.d.


  A supply where the potential difference is fixed at a certain value, so the current flows in one direction only
• Earth wires


  - have a low resistance

  - are necessary in metal cased electrical appliances

  - connect to the ground
• An earth wire is a low resistance wire that connects the metal case to the ground. If the casing is touched by a wire inside the appliance, the current will flow through the earth wire. Because the resistance of the Earth wire is low, a very high current will flow through it, breaking the fuse in the device. This will stop the flow of current and make the appliance safe.
• Fuses and circuit breakers

  Fuses and circuit breakers protect us from electrical appliances by stopping the flow of electric current flow if it gets too high. This stops the appliance overheating.
• Fuses


  Fuses contain a thin wire that melts if the current flowing through an appliance is too high.

  This breaks the circuit and stops the flow of electric current.
• Circuit breakers


  Circuit breakers are special devices that switch off the flow of current if the current is higher than a certain value.

  Circuit breakers do the same job as fuses but can be used lots of times.

  Reusable
• Fuse ratings


  Fuse ratings tell us the level where a fuse will begin to melt. The fuse chosen should have a value just above the normal current that an electrical appliance uses.
• The purpose of fuses are

  - to stop appliances from overheating

  - to stop current flow if the current is too high

  - to melt of current is too high
• Electrical current


  It is the flow of electric charges.
• Coulombs (C)


  The charge is equal to the current multiplied by the time. (Q=I×t)
• Electrical insulators


  An electrical insulator is a material that doesn't allow electric currents (flow of electric charges) to pass through it.

  Plastic and glass are electrical insulators.
• Electrical conductors


  An electrical conductor is material that allows electric currents (flow of electric charges) to pass through it.

  All metals are electrical conductors.
• Electrical conductors II


  An electrical conductor has lots of charges that are free to move.

  In a metal, the charges that are free to move are electrons.

  The electric current through the metal is the flow of these free electrons.
• Electrical insulators II


  An electrical insulator has no free electrons.

  No charges are free to move and carry a current.
• An electrical conductor has lots of charges that are free to move.
• potential difference


  It is the difference in the amount of energy that charge carriers have between two points in a circuit.
• Potential differences I


  Potential difference (p.d.) is measured in volts (V) and is also called voltage.

  The energy is transferred to the electrical components in a circuit when the charge carriers pass through them.

  We use a voltmeter to measure potential difference (or voltage).

  1 Volt is the same as 1 Joule per Coulomb.
• Ammeters


  An ammeter measures the flow of current that passes through it.

  Ammeters have to be connected in series with the electrical component whose current you are measuring.
• Voltmeters


  Voltmeters measure potential difference (voltage) between two points in a circuit.

  Voltmeters must always be connected in parallel with the two points being measured.
• Resistance


  It is a measure of how difficult it is for current to flow through a component.
• When charge flows in an electric circuit, the size of the current is affected by two things:

  1. The resistance: If the resistance is increased, the current will decrease.

  2. The potential difference: If the potential difference is increased the current will increase.
• Ohm's Law


  The resistance between two points in a circuit is equal to the potential difference between those two points, divided by the current flowing through them.
• The longer the wire, the higher the resistance
• Factors that affect resistance

  1. Material - some are better conductors than others

  2. Length - a longer wire means a higher resistance

  3. Thickness - a thinner wire means higher resistance

  4. Temperature - a higher temperature means higher resistance, due to increased ion vibrations, more collisions
• Ohm's Law 2


  The current flowing through a wire is proportional to the potential difference (voltage) across it provided the temperature remains constant
• The resistance of a wire depends on

  the wire's length and the cross-sectional area of the circular end of the wire.
• Resistance (in ohms) is proportional to the length of the wire (in metres) divided by cross-sectional area (in metres squared).
• A filament lamp is connected to a power supply with a 4.7 m of wire. The lamp is dimmer than its specifications suggest. What could we do to the wire to make the lamp brighter?

  Reduce the length of cable if possible.

  Also consider swapping the cable for one with thicker internal wires.

  Both will reduce the resistance of the connecting wires. This will increase the current and the amount of electrical work that is done on the bulb.
• In an electrical circuit, the energy source (battery or power supply) makes a current flow.
• Charge flows from the energy source and through the rest of the circuit and its components, like lamps and heaters

  The flow of current transfers energy from the energy source to the components.

  
  

  The components then transfer energy to their surroundings.

  For example a lamp emits light into its surroundings.