05: Electricity

• Electric current (I)


  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 component, calculated by dividing the potential difference across a component by the current flowing through it
• Ohm's law


  • For an ohmic conductor, current is directly proportional to the potential difference across it, provided physical conditions are kept constant

  
• Ammeters have zero resistance, meaning they do not affect the measurement of current in a circuit
• Voltmeters have infinite resistance, meaning no current can flow through them, so their measurement of potential difference across a component is exact
• Resistivity (ρ)


  A measure of how easily a material conducts electricity, 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 increases
• As the temperature of a thermistor increases


  Its resistance decreases

  
• Superconductor


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


  • Power cables (reduce energy loss through heating to zero)
  • Strong magnetic fields (e.g. maglev trains, medical applications)
• Resistors in series


  The total resistance is the sum of the individual resistances
• Resistors in parallel


  The reciprocal of the total resistance is the sum of the reciprocals of the individual resistances
• Power (P)


  The energy transferred over time (rate of transfer of energy)
• In a series circuit, the current is the same everywhere in the circuit, and the battery p.d is shared across all elements
• In a parallel circuit, the sum of the currents in each parallel set of branches is equal to the total current, and the potential difference across each branch is the same

  
• When joining battery cells in series

  The total voltage is the sum of the individual voltages
• When joining identical battery cells in parallel

  The total voltage is equal to the voltage of one cell
• Kirchhoff's first law


  • The total current flowing into a junction is equal to the current flowing out of that junction
• Kirchhoff's second law


  • The sum of all the voltages in a series circuit is equal to the battery voltage
• Potential divider


  A circuit with several resistors in series connected across a voltage source, used to produce a required fraction of the source potential difference

  
• Electromotive force (emf, ε)


  The energy transferred by a cell per coulomb of charge that passes through it
• Internal resistance (r)


  Caused by electrons colliding with atoms inside the battery, resulting in some energy loss before electrons leave the battery
• Terminal p.d (V)


  The p.d across the load resistance R
• Lost volts (v)


  The p.d across the internal resistance r, equal to the energy wasted by the cell per coulomb of charge
• The emf of a battery can be measured by measuring the voltage across a cell using a voltmeter when there is no current running through the cell (open circuit)