Electricity Science Higher GCSE

• Resistance


  The measure of how difficult it is for current to flow, measured in ohms (Ω)
• Materials with low resistance

  Increase the current
• Materials with high resistance

  Decrease the current
• Resistance is caused by the collision of constantly flowing electrons with the vibrating metal ions in the wire
• If the temperature of the wire increases

  The metal ions vibrate more vigorously, increasing the number of collisions between the free electrons and the ions, which increases the resistance of the wire
• Power


  Proportional to current
• Variable resistor


  A device that allows you to change the resistance of a circuit
• Reducing the resistance or adding more cells

  Makes the bulb glow brighter
• Thermistor


  A device that changes its resistance when its temperature changes
• As the temperature of a thermistor increases

  Its resistance decreases, causing more current to flow and increasing the power output
• As the light intensity on an LDR increases

  Its resistance decreases, allowing more current to flow and increasing the power to the device
• If the potential difference increases

  The current increases
• If the resistance increases

  The current decreases
• Changing the polarity of a cell (flipping it around) results in a negative potential difference and current, but the current just flows in the opposite direction
• Fixed resistors (constant temperature)

  • Ohmic conductors - the current and potential difference are directly proportional
• Filament bulb


  • Initially an ohmic conductor, but as the current increases the resistance increases, so it becomes a non-ohmic conductor
• Diode


  • Allows current to flow in only one direction, with very high resistance in the reverse direction
• Required practical: Investigating I-V characteristics

  1. Independent variable: current in the circuit

  2. Dependent variable: potential difference across the component

  3. Control variables: thickness, length, material of filament/resistance of fixed resistor
• A resistor is ohmic when the relationship between current and potential difference is linear
• Direct current (d.c.)


  Current flows in one direction
• Alternating current (a.c.)


  Current constantly changes direction, produced by alternating voltages
• UK domestic mains supply is an AC supply of around 230V with a frequency of 50 Hz (50 cycles per second)
• The case of a plug is plastic because it is a good insulator that does not conduct electricity
• The pins of a plug are made of the alloy brass, which is harder and a better conductor than pure copper
• The wires in a plug are covered in plastic casing for insulation
• The fuse in a plug is connected to the live wire
• How a fuse works

  When too much current flows, the wire in the fuse gets hotter, melts, and the circuit breaks to prevent the device from being damaged
• The live wire brings AC current into the device at 230V


  The neutral wire is at 0V, creating a potential difference that causes current to flow through the device and out through the neutral wire
• The earth wire is connected to the metal case of the device


  It is at 0V and has very low resistance, providing a safety path for current
• In a normal circuit


  Current flows into the device through the live wire and out through the neutral wire
• In a short circuit (live wire touching neutral wire)

  The current flows directly from the live wire to the neutral wire, with a large potential difference and very low resistance, causing the wire to heat up and catch fire
• If the live wire touches the metal casing of the device

  A greater current flows from the live wire, through the metal casing and down the earth wire, as it has a 230V potential difference and very low resistance, causing the fuse to melt and break the circuit
• Short circuit


  Live wire touching neutral wire
• 2 core cable has fewer wires than 3 core cable
• Plastic cases don't conduct electricity, so an earth wire is not needed
• The earth wire is missing from a 2 core cable compared to a 3 core cable
• Appliance with double insulation does not require all three wires (live, neutral, earth) because the casing is not made of metal
• Mains electricity is 230V and 50Hz
• Current flows from live wire to neutral wire before reaching the toaster


  No current will flow through the toaster causing it not to work
• Earth wire


  To protect from electric shocks