Topic 2 - Electricity

• Resistance is the opposition in an electrical component to the movement of electrical charge through it. Resistance is measured in ohms.
• Conductors have a low resistance. Insulators have a high resistance.
• RP:

  1. Change by reducing the length of the wire by 10 cm each time down to a minimum length of 10 cm.
  2. Plot a graph of resistance against length for the resistance wire.

  
• RP: Relationship between current and potential difference in a resistor vs bulb vs diode


  Resistor: directly proportional to each other

  Bulb: curved 'S' shape graph: the more energy that is put into the bulb, the harder it is for the current to flow - the resistance of the bulb increases.

  Diode: graph only moves at positive p.d: only allows current to flow in one direction. If the potential difference is arranged to try and push the current the wrong way no current will flow as the diode’s resistance remains very large.
• Series circuit:

  • Current: a series circuit is one loop; all electrons in that loop form one current. An ammeter will measure the same current wherever.$I1 =$$I2 =$$I3$
  • Potential difference: the potential difference supplied is equal to the total of the potential differences across all other components$Vs=$$V1+$$V2$
• Parallel circuit:

  • Current: the current will split as it leaves the cell and passes through one or other of the loops. An ammeter placed in different parts of the circuit will show how the current splits.
  • Potential difference: since potential difference is used to measure changes in energy, the potential difference supplied is equal to the potential differences across each of the parallel components$Vs=$$V1=$$V2$
• RP: Resistance in series vs parallel

  • In series, the resistance of the network is equal to the sum of the two individual resistances.
  • In parallel, the resistance of the network is less than either of the two individual resistances.
• Power is the energy transferred each second, measured in watts (W). $Power =$$work done ÷ time taken.$
• AC vs DC:

  • An alternating current regularly changes direction.
  • A direct current flows in only one direction.
• Live wire - Copper wire coated with brown plastic along which the current enters the device

  Fuse - A glass or ceramic canister containing a thin wire that melts if the current gets too high

  Neutral wire - Copper wire coated with blue plastic that also connects to the cable in the wall and completes the circuit

  Earth wire - Copper wire coated in striped plastic that provides a path for current to flow from the case of the device to the ground if there is a fault
• National Grid - The network that connects all of the power stations in the country to make sure that everywhere has access to electricity.
• Step-up transformer - a transformer that increases the voltage of the ac supply.
• Step-down transformer - a transformer that decreases the voltage of the ac supply.
• Objects that are charged can affect other charged objects using the non-contact forces of static electricity.
• When a polythene rod is rubbed with a duster, the friction causes electrons to gain energy. Electrons gain enough energy to leave the atom and ‘rub off’ onto the polythene rod.
• A charged object will experience non-contact force from another charged object. The type of force will depend on the type of charge (positive or negative) on the two objects.
• All charged objects have an electric field around them, which shows how they will interact with other charged particles.
• An electric field is a region where charges experience a force.
• Field lines point away from positive charges and towards negative charges.
• With a radial field around a positive charge, other positive charges are repelled away. Therefore, the arrows are pointing away from the central positive charge. This is what happens with the example of the Van de Graaff generator.

  However, if a negative charge is placed in that field, it would attract the positive charge and feel a force in the opposite direction to the field lines.

  
• If the field is strong enough, charges can be forced though insulators such as air and a spark will occur. This is what happens during a lightning strike. It may also happen if a charged person touches a conductor.