Physics P1 Electricity

• Potential difference


  Volts (V)
• Current


  Amp (A)
• Resistance


  Ohms (Ω)
• Ohmic conductors produce a straight line I-V graph that goes through the origin
• Non-ohmic conductors


  • Resistance changes with current
  • Includes lamps, diodes, thermistors, LDRs
• Diodes


  Electrical components that only allow current to flow in one direction
• Thermistors


  Resistors where resistance varies with temperature
• LDRs (Light Dependent Resistors)


  Resistors where resistance varies with light intensity
• Measuring resistance


  1. Set up circuit with ammeter and voltmeter

  2. Use equation R = V/I
• Standard circuit diagram symbols

  • Switch open
  • Switch closed
  • Cell
  • Battery
  • Diode
  • Resistor
  • Variable resistor
  • LED
  • Lamp
  • Fuse
  • Voltmeter
  • Ammeter
  • Thermistor
  • LDR
• Series circuit


  One loop of wire
• Electric current


  Flow of electric charge, usually electrons
• Ohmic conductor
• Mains electricity in UK
• Wiring in 3-core cable

  • Live (brown)
  • Neutral (blue)
  • Earth (yellow/green)
• Live wire


  Carries alternating potential difference from supply
• Neutral wire


  Completes the circuit
• Earth wire


  Safety wire to stop appliance becoming live
• Live wire can be dangerous even with open switch
• Power


  When electrical appliances are connected into a circuit energy is transferred to the appliance. The rate at which energy is transferred to the appliance is the power rating of the appliance.
• To calculate the power of an electrical component

  1. Power (W)= Potential Difference (V) x Current (A)

  2. Power = (current)2 x Resistance
• Power Equations


  • Power (P)
  • Potential difference (V)
  • Current (I)
  • Resistance (R)
• Example: Calculating current in a microwave oven

  1. State the equation: P= VI

  2. Rearrange: I = P/ V

  3. Substitution: I= 800 / 230

  4. Answer: I= 3.5 A (to 1 decimal place)
• Example: Calculating power loss in an overhead powerline

  1. State the equation: P = (I)2 x R

  2. Substitution: P = (400)2 x 27.5

  3. Answer: P = 4.4 MW or 4 400 000 W
• Everyday electrical appliances


  • They are designed to bring about energy transfers
  • The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance
  • They convert electrical energy from ac mains, or from batteries into more useful forms
• Some common energy transfers from electrical energy

  • Motors converting electrical energy into kinetic energy
  • Lightbulbs converting electrical energy into light energy
  • Electric heaters converting electrical energy into heat energy
• As with any energy transfer, some energy will be transferred usefully and some energy will be wasted (converted into forms that are not useful)
• Work


  Work is done when charge flows in a circuit
• To calculate the amount of energy transferred by electrical work

  1. Energy transferred (J) = Power (W) × Time (s)

  2. Energy transferred = Charge flow (C) x Potential difference (V)
• Using Equations for Energy Transferred

  • Example 1: Energy transferred by a 5 kW electric motor switched on for 2 minutes
  • Example 2: Calculating charge flow for an 8 kW electric motor running for 1.5 minutes
• The National Grid


  A system of cables and transformers linking power stations to consumers e.g. homes, shops, factories
• How electricity is transferred using the National Grid

  1. Step-up transformers are used to increase the potential difference from the power station to the transmission cables

  2. Step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use in homes
• Why transformers are used in the National Grid

  • Electric current generates heat as it moves through electrical wires
  • Transmitting at very high potential difference and low current means less energy is wasted as heat, making the whole system more efficient
• A lower potential difference is used in the home as it is safer, so a step-down transformer is used near homes and offices
• Pylons carry overhead power cables