Electricity

• Electric current (I)

  Flow of charge per unit time

  I = ΔQ/ΔT
• Potential difference (V)

  The energy transferred per unit charge

  V = W/Q
• Resistance (R)

  Measure of how difficult it is for charge carriers to pass through a component

  R = V/I
• Ohm's law


  For an ohmic conductor, current is directly proportional to the potential difference
• Current voltage graphs for...

  an ohmic conductor:

  
• Current-voltage graph for...

  a diode

  
• Current-voltage graph for...


  a filament lamp

  
• Usually ammeters can be assumed to have...

  0 resistance
• Usually volt meters can be assumed to have...

  infinite resistance (so no current can flow so the measurement of pd across a component is exact)
• Resistivity (ρ)


  Measure of how easily a material conducts electricity

  The product of resistance and cross-sectional area divided by the length of the material
• Resistivity is also dependent on...

  environmental factors such as temperature
• Equation for resistivity 


  ρ = RA/L
• When the temperature of a metal conductor increases...

  Its resistance will increase
• Thermistors

  Opposite from metal conductors. As temperature increases, resistance decreases because increasing temperature causes electrons to be emitted from atoms, so number of charge carriers increases so current increases causing resistance to decrease.
• Temperature-resistance graph of...



  a thermistor

  
• Application of a thermistor...

  In circuits as a temperature sensor e.g. turn the heating on below a certain temperature. It can trigger an event to occur when the temperature reaches a certain value.
• Superconductors


  Materials which below a certain temperature (known as the critical temperature) has zero resistivity .
• Most critical temperatures...


  lie close to 0k (-273 degrees C)
• Graph of resistance and temperature for a superconductor

  
• Applications of superconductors


  Power cables - reduce energy loss through heating during transmission.

  Strong magnetic fields which would not require a constant power source. Used in maglev trains and in MRI
• Adding resistors in series...

  Rtotal = R1 + R2 + R3
• Adding resistors in parallel...

  1/Rtotal = 1/R1 + 1/R2 + 1/R3
• Power equations


  P = IV = 1^2R = V^2/R
• P = E/T and P x t = E so


  E = VIt
• In series...


  Current is the same everywhere

  pd is shared
• In parallel...


  Current is shared

  pd in each branch is the same
• When joining cells in series

  Vtotal = V1 + V2 + V3
• When joining cells in parallel


  Vtotal = V1 = V2 = V3
• In DC circuits...

  Charge and energy are always conserved
• Kirchhoff's first law


  total current flowing into a junction is equal to the total current flowing out of that junction (no charge is lost)
• Kirchhoff's 2nd law 


  Sum of all the voltages in a series circuit is equal to the battery voltage (no energy is lost)
• Potential divider


  A circuit with several resistors in series connected across a voltage sourced used to produce a fraction of the source pd
• Light dependent resistor

  Resistance decreases as light intensity increases
• Internal resistance (r)


  Is caused by electrons colliding with atoms inside the battery.

  can be represented by a resistor inside the battery

  
• Electromotive force


  energy transferred by a cell per coulomb of charge that passes through it
• emf = E/Q
• Rtotal = R + r
• emf = IR + Ir = I(R + r) = V + Ir
• terminal pd


  pd across the terminals of a battery
• r = 


  the lost volts = energy wasted by the cell per coulomb of charge