4.2 Physics: Energy,Power and Resistance

• Potential difference is the energy transferred from electrical energy to other forms per unit charge

  V = W/Q

  E.m.f is the energy transferred from other forms e.g. chemical(battery) to electrical per unit charge

  
• Speed of electrons =

  eV = 1/2 M * V^2

  
• Resistance is the potential difference across a component divided by the current going through it

  R = V/I
• Ohms law:

  In a ohmic conductor the current is proportional to the potential difference at a constant temperature
• I/V characteristics:

  Resistor: Resistance is proportional Potential difference, Graph is a straight line through the gradient therefore obeys ohms law, is a ohmic conductor

  R is constant

  
• I/V Characteristics:

  Filament lamps: Potential difference does not obey ohms law as Pd is increasing at a faster rate than current

  Resistance increases because as current increases temperature increases therefore more collisions between the electrons and the positive ions

  
• I/V Characteristics

  Thermistor:

  Potential difference is not proportional to resistance as current is increasing at a faster rate than pd

  Therefore Resistance decreases because as the temperature increases the number density increases therefore more free conducting electrons
• I/V characteristics:

  V is not proportional to I therefore does not obey ohms law

  As R decreases as I increases at a faster rate than V

  1 = infinite resistance

  2. At the point of threshold pd starts to conduct

  3. above 3 diode has a small resistance

  
• 
  

  Resistivity =

  L = Lenght

  A = Cross sectional area

  
• P = V * I

  Substituting V = I*R in for I and V gives

  P = I^2 * R

  P = V^2/ R

  W = V * I * t
• Kilowatt hour is the amount of energy used by a 1kw device in one hour
• Answer:

  Resistance = resistivity * length/ cross sectional area

  Allow values between 220=230


  Question