Energy, Power and resistance

• A conductor has a very high number density, insulators however have a much lower valve and semiconductors are inbetween a conductor and insulator
• emf is the energy supplied to the charge carries (cells and batteries)

  pd is the energy transferred by the charge carries (bulbs, resistors)
• emf (E) = charge (Q) / the energy transferred (W)
• energy transfer (W) = charge (Q) x voltage (V) or charge (Q) x emf (E)
• resistance (R) = pd (V) / current (I)
• Ohms law states that the current is directly proportional to the potential difference across a component.
• Overtime the resisance of a circuit will increase because the temperature of the wires increases because of the current flowing through them. This then results in the current decreasing
• When the temperature in a wire increases the positive ions inside the wire have more energy so vibrate with greater amplitude therefore resulting in more frequent collisions causing an increase in resistance
• A component that obeys ohms law is referred to as an ohmic conductor eg fixed resistors
• Resistance and variables

  • resistance is directly proportional to length
  • resistance is inversely proportional to cross sectional area
• Energy and power

  • electrical power (P) = pd (V) x current (I)
  • electrical power (P) = resistance (R) x current squared (I^2)
  • electrical power (P) = energy (W) / time (t)
• 1 Kwh = 3.6 x 10^6 J
• In a thermistor, as the temperature increases the resistance decreases

  In a LDR, as the temperature increases the resistance decreases
• One volt is the pd across a component when 1J of energy is tranferred per unit charge

  $1V =$$1JC^-1$
• Thermistors have a negative temperature coefficient because they behave like some semiconductors where as the temperature increases the number density of charge carriers also increases decreasing the resistance