Electricity

• Q


  Charge = property of matter (Coulamb)
• Density of electrons


  • ↑ Conductor
  • Insulator
• Electric current


  The flow of charge per unit time or the rate of flow of charge
• Potential difference (V)


  The energy transferred per unit charge between two points in a circuit
• Resistance (R)


  A measure of how difficult it is for charge carriers to pass through a device or component, and is measured by dividing the potential difference across a component by the current flowing through it
• Resistance with increased temp


  • ↑ Collisions
  • more resistance
• Conventional Current


  Movement of positive charge
• Current-voltage characteristics


  • Electrons repelled by negative
  • Electrons all start drifting towards positive
• Ohm's law


  For an ohmic conductor, current is directly proportional to the potential difference across it, given that physical conditions (e.g. temperature) are kept constant
• Ohmic conductor


  • Current-voltage graph is a straight line through the origin
• Semiconductor diode


  • Must consider forward and reverse bias
  • Forward bias allows current to flow easily past threshold voltage
  • Reverse bias has extremely high resistance, only small current can flow
• Transverse wave demonstration


  1. Shaking a string vertically

  2. Waves seen on a string when attached to a fixed point
• Filament lamp


  Contains a length of metal wire, which heats up as current increases, therefore the resistance of this component increases as current increases
• For very low currents, Ohm's law is obeyed. However, as the current increases in either direction the graph begins to curve due to the increasing resistance.
• Ammeters


  Can be assumed to have zero resistance, meaning they will not affect the measurement of current in a circuit at all
• Voltmeters


  Can be assumed to have infinite resistance, meaning no current can flow through them, meaning their measurement of potential difference across a component is exact
• Resistivity (ρ)


  A measure of how easily a material conducts electricity, it is defined as the product of resistance and cross-sectional area, divided by the length of the material
• As the temperature of a metal conductor increases


  Its resistance will increase
• As the temperature of a thermistor increases


  Its resistance decreases
• Superconductor


  A material which, below a certain temperature (critical temperature), has zero resistivity
• Applications of superconductors


  • Power cables (reduce energy loss)
  • Super efficient wires
  • Strong magnetic fields (e.g. maglev trains, MRI)
• Series resistors


  Total resistance = R1 + R2 + R3 + ...
• Parallel resistors


  Total resistance = 1 / (1/R1 + 1/R2 + 1/R3 + ...)
• Calculating total resistance of a circuit

  1. Find resistance of parallel combinations

  2. Use series rule to add remaining resistors
• Power (P)


  The energy transferred over time (rate of transfer of energy)
• Calculating energy transferred and current in a lamp

  Use P = VI to find energy transferred

  Use I = V/R to find current
• Series circuit


  Current is the same everywhere in the circuit

  Battery p.d. is shared across all elements
• Parallel circuit


  Sum of currents in each parallel branch equals total current

  Potential difference across each branch is the same
• Joining battery cells in series


  Total voltage = sum of individual cell voltages
• Joining identical battery cells in parallel


  Total voltage = voltage of one cell
• Kirchhoff's first law


  Total current flowing into a junction equals current flowing out
• Kirchhoff's second law


  Sum of all voltages in a series circuit equals battery voltage
• Potential divider


  Circuit with resistors in series, used to produce a required fraction of the source potential difference
• Internal resistance (r)


  Caused by electrons colliding with atoms inside the battery, resulting in energy loss
• Electromotive force (emf, ε)


  Energy transferred by a cell per coulomb of charge that passes through it
• Measuring emf of a battery

  Measure voltage across cell with no current running (open circuit)
• Calculating current in a circuit with known emf, lost volts and resistance


  Use ε = IR + Ir to find current
• Calculating lost volts and internal resistance of a battery


  Use ε = V + v to find lost volts

  Use ε = IR + Ir to find internal resistance