Electricity - E01 - E13

• What is current


  Current is the rate of flow of charge - flow of electrons

  I = change in charge (Q) / change in time (T)
• Positive current flows from the positive terminal to the negative terminal - conventionally

  whilst electrons are opposite, they are repelled from the negative terminal and attracted to the positive terminal
• How should an ammeter be connected

  -always in series

  -has a low resistance
• What is a coulomb


  A coulomb is the charge that flows past a point in one second when there is a current of 1 amp

  Q = It
• What is Kirchhoff first law

  Kirchhoff‘s first law states the sum of the currents into a junction equals the sum of the currents out
• What is an electrons charge

  1.6x10^-19 C
• Charge equation (e^-s)


  Q = Ne

  N = no of e^-s

  e = magnitude of charge of an e^-s (1.6x10^-19 C)
• in a series circuit the current is the same everywhere

  whilst in parallel the current in shared/split at junctions
• What is emf


  EMF is the amount of energy transferred from chemical energy into electrical energy per unit charge
• What is potential difference

  Potential difference is the energy transferred per unit charge from this electrical energy into other forms
• How should a voltmeter be connected

  -Must be in parallel

  -must have a very high resistance (so then the current goes just through the circuit not the voltmeter)
• What is Kirchoff’s second law

  Kirchoff’s second law states the sum of the emf in any closed loop and circuit is equal to the sum of the pd around any closed loop

  E = V
• What is a Volt


  a volt is the pd between two points when 1J of work done to move a charge of 1C

  or

  1V is the pd scores a component when 1J of energy is transferred per C of charge passing through the component
• Work done equation


  W = QV

  • W = work done
  • Q = charge
  • V = potential difference
• W = QV

  Ke = 1/2 m v^2

  eV = 1/2 m v^2

  W = Ke

  QV = 1/2 m v^2
• Power equations 


  P = IV

  P = change in work / change in time
• What is power


  Power is the rate of doing work
• What happens with an electron and its energy in a vacuum tube

  It is repulsed from the negative terminal and attracted to the positive terminal, as it travels from negative to positive it’s accelerating which increases its Ke

  -the enerdy transferred to the electron as it travels along the tube = QV =eV (this energy is KE)
• What happens with an electron and its energy in a fluorescent tube

  It travels from the negative terminal to the positive terminal. As they travel along they accelerate increasing its KE but collides with the Hg atoms, this process is repeated until the opposite terminal

  -the total energy transferred to the electron as it trsvles along the tube = QV (this energy is light and energ)
• What happens with an electron and its energy in a conductor

  Travels from the negative terminal to the positive terminal. As it travels along the electrons collide with the +ve ions. More Collins with +ve ions the more the +ve ions vibrate the more it heats up

  -the total energy transferred to the electron as it travels Along the conductor = QV (this energy is dissipated as heat in the conductor)

  • the e^-s in the I collide with the +ve ions, the +ve ions vibrate more, therefore the temperature increases
• What is resistance


  Resistance is the ratio of pd across a component to current through it

  R = V/I
• What is ohms law


  Ohms law states that current is directly proportional for an ohmic conductor if you keep it at a constant temperature
• Percentage uncertainty =

  ((absolute uncertainty)/(measured value)) x100
• The absolute uncertainty is the precision or 1/2 range

  (smallest division on the instrument)
• If quantities are X or divide = add percentage uncertainty

  if quantities are add or subtract = add absolute uncertainty
• Resistivity equation


  P = RA/L

  P = ohm metres
• The hotter a metal gets the greater the resistance to the flow of current
• about a resistor


  -I is directly proportional to V

  -R = constant

  -the higher the resistance = the less current let through
• About a diode


  -threshold voltage = 0.6V

  -one way
• About a filament lamp

  -ohmic at small current as temp is constant

  -resistance increases meaning the temperature increases (as the current increases)

  -the +ve ions vibrate more

  -more frequent collisions between electrons and the +ve ions

  -for the same increase in V the increase in I decreases (curved part of the line)
• About a thermistor 


  -NOT METALLIC

  -as temp increases, resistance decreases

  -this is because as the temp increases the energy is used to liberate more electron, so no. of charge carriers increases

  -thermistors are semi-conductors
• Resistors in series


  R(T) = R(1) + R(2) +R(3) + …
• Resistors in parallel


  1/R(T) = 1/R(1) + 1/R(2) + …
• What is a superconductor

  It is a material that has zero resistance below a very low critical temperature

  they are used to make:

  • high current wires
  • high magnetic fields
• Potential divider circuits divide the potential and are often used with thermistors or LDR’s as a sensing circuit
• Examples of semi conductors

  -thermistors

  -LDR
• What is photoelectric effect

  Where we have photons of light liberating the electrons
• what is photovoltaic effect


  Can generate a current
• What is photoconductive effect

  Making a material become more conducive when photons of light land on top of it