electricity

• for electrical charge to flow through a closed circuit, the circuit must include a source of potential difference
• charge flow = current x time (Q = I x t)
• the greater the rate of flow of charge, the greater the current
• in series, the current is the same at every point and in every component
• in parallel, the total current is the sum of the current in each branch
• the current through a component depends on both the resistance of the component and the potential difference across the component
• the greater the resistance of a component, the smaller the current (for a given potential difference across the component)
• voltage = current x resistance
• the potential difference is the energy transferred per Coulomb of charge
• potential difference = work done (same as energy transferred )/ charge
• electrical current is the rate of flow of electrical charge
• a cell makes one end of a circuit positive and the other end negative, and this sets up a potential difference.
• electrical resistance is the amount of potential difference required to cause one amp of current to flow through a component
• good conductors have a low resistance and insulators have a high resistance
• in an ohmic conductor (where resistance is constant), the current and voltage are directly proportional, provided that temperature is constant. This creates a linear IV graph. A steeper line means a smaller resistance (easier for current to flow)
• in filament lamps, as the current increases, the temperature increases and therefore the resistance increases.

  • all solids are made up of vibrating particles
  • and as the temperature increases, these particles gain energy and so vibrate more
  • when they have more energy, the charged particles (current) collide with the wire particles more.
  • this impedes their flow, so the current decreases
  • a decrease in current leads to an increase in resistance (V = I x R) because pd is constant
  • so an increase in temperature leads to an increase in resistance
• diodes are semiconductors that only allow current to flow in one direction. The diode has a very high resistance in the reverse direction, meaning that the current cannot flow.
• LDR - the resistance of an LDR decreases as the light intensity increases
• thermistor - the resistance of a thermistor decreases as the temperature increases
• thermistors are found in: ovens, refrigerators, fire alarms etc
• LDR's are found in: street lights, security lights, burglar alarm circuits
• series:

  • potential difference is split between components
  • current stays the same in components
  • total resistance is the sum of all the components resistances
• parallel:

  • potential difference is the same between branches
  • current is split between the different branches
  • total resistance is smaller than the smallest resistor
• Mains electricity is an alternating current supply. In the UK the domestic electricity supply (Mains) has a voltage of 230V and frequency of 50Hz
• in alternating current the current continuously changes direction, meaning there is a forwards and a backwards flow.
• in direct current, the current is constantly flowing in the same direction
• cells and batteries supply direct current
• live wire (brown):

  • carries alternating pd from mains supply to the appliance
  • 230V
  • dangerous as even if mains circuit is off, current still may be flowing
  • if the live wire touches the appliance without the Earth wire, it can cause electrocution
• neutral wire (blue):

  • 0V
  • forms the opposite end of the circuit to the live wire to complete the circuit
• earth wire (green and yellow):

  • 0V, only carries a current if there is a fault
  • a safety wire to stop the appliance from becoming live
  • it is connected to the earth and casing.
  • it provides a low resistance path to the ground, so in the event of a fault the live current passing through the case will follow this path to the ground instead of passing through a person.
• everyday electrical appliances are designed to bring about energy transfers. The amount of energy an appliance transfer depends on how long the appliance is switched on for and the power of the appliance
• work is done when charge flows in a circuit. the energy transferred is equal to the work done
• power is the energy transferred (or work done) per second.
• power is directly proportional to current and voltage.
• power loss is proportional to resistance, and the square of the current
• energy is transferred from chemical potential in batteries to electrical energy in wires to the form of energy in the device they power
• energy transferred (joules) = power (watts) X time (in seconds)
• energy transferred = charge flow (Coulombs) x potential difference
• in a series circuit, the current is the same at all positions since the charge only has one path to flow through
• in a parallel circuit, the current is split between different branches because when the current reaches a junction it splits