Energy, power & resistance

• Kirchoff's 2nd law --> In any circuit the sum of the electromotive force is equal to the sum of the pd in a closed loop. This is a result of the conservation of energy.
• In a series circuit, the emf will be split so that the total pd across each component is equal to the emf
• Electromotive force (emf) --> used to measure work done to charge carriers when they gain energy as they pass through a cell/ power supply. It is defined as the energy transferred from chemical to electrical energy per unit charge. Also measured in Volts
• Potential difference --> Used to measure work done by charge carriers which lose energy as they pass through components in a circuit. Defined as the energy transferred from electrical energy to other forms per unit charge.
• Emf (e) = Work done (W) / charge (Q)

  W= eV --> E = eV

  eV = 1/2 mv^2
• Electron gun --> When electrons are subjected to a pd & gain energy. Made to accelerate so the energy gained is KE. As W=VQ, the energy gained by a single electron (with charge e) will be eV. This is converted to W=eV
• V is directly proportional to I when temperature is constant
• Electrical power --> the rate of energy transfer, and is measured in watts or JS^-1
• Power = work done / time
• As pd across a component is equal to work done per unit charge, we can rewrite work done as pd multiplied by charge

  • P = VQ / t
  • P= VI
  • P = I^2R
  • P= V^2/ R
• As power is the energy transferred per unit time, we can calculate the energy transferred, W, by a component by multiplying the power to the component by the time the component is used for

  • W = Pt --> W = VIt
• 1 Kilowatt hour --> the energy transferred by a device with a power of 1 kilowatt when it is operated for a time of 1 hour