Electricity

• Voltage = energy / charge
• Energy = charge x voltage
• voltage is the measure of how much energy is transferred by each coulomb of charge
• current is the rate of flow of charge
• Resistance is the measure of how much a component resists the flow of current
• v = IR
• for a fixed resistor, I - V graphs have a straight line through the origin and so are directly proportional
• For fixed resistors, I is directly proportional to V
• Ohmic if straight line through origin of I-V graph
• If an ohmic resistor gives a steeper gradient on I-V graph then it must have a lower resistance
• From an I-V graph you calculate resistance by picking a point and using ohms law. NO TANGENT.
• Filament lamps are non-ohmic as resistance isn’t constant
• Filament lamps I-V graph has a linear start then curves off ( rate of I increasing slows down as V increases )
• in a filament lamp, resistance increases with a higher P.D and higher current
• In a filament lamps I-V graph, you pick a point to find resistance and use Ohms law
• Why isnt resistance constant in filament lamps?

  delocalised electrons collide with ionic lattice and causes them to vibrate more (more obstruction/ collisions) and temperature increases meaning they gain KE and vibrate even more. More resistance.
• Diode has high resistance in reverse bias direction
• Diode has low resistance in forward bias direction
• Diodes only let current flow in one direction, around 1 volt.
• Light emitting diodes LEDs have arrows pointing out of the symbol
• Superconductors are materials that have zero resistance at their critical temperature
• Resistivity is the resistance of a cube of unit length sides
• Resistivity is measured in ohmic metres
• Experiment to find resistivity?

  • measure diameter of wire ( calculate mean, then c-s area
  • vary length of wire By moving crocodile clip.
  • calculate resistance for each length using ohms law
  • plot R-L graph and find gradient then times gradient by area for resistivity
• Measure diameter of wire using a vernier calliper
• resistivity = resistance x area / length
• total p-d is shared in series
• Current same in series
• Total resistance is the sum of resistance in series
• pd is the same in parallel
• Current is shared in parallel
• for Parallel

  1/Rt = 1/R1 + 1/R2
• Kirchhoffs second law states that the sum of EMFs must equal the sum of pd In a series circuit
• Potential dividers are when the total potential is shared throughout
• In potential dividers, the larger resistance takes the bigger share of voltage ( can calculate current of one and use it to find V for the other)
• Adding more resistors in parallel reduces total resistance as there are more paths for the current to take
• Kirchoffs first law states that charge and current must be conserved at any junction in a circuit ( I in = I out)
• Thermistors resistance decreases if temp increases
• Thermistors symbols have a line with a bend at the top going through the rectangle
• Light dependant resistors resistance decreases with light intensity increasinh