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, given that physical conditions (e.g temperature) are kept constant
• Ammeters can be assumed to have zero resistance, meaning they will not affect the measurement of current in a circuit at all, and 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, 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 critical temperature, has zero resistivity
• Series resistors


  The total resistance is the sum of the individual resistances
• Parallel resistors


  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 the circuit, therefore the total sum of the voltages across all elements is equal to the supply p.d
• 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 across the cells is equal to the sum of the individual voltages of the cells
• When joining identical battery cells in parallel


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


  • The total current flowing into a junction is equal to the current flowing out of that junction
• Kirchoff'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, which remains constant
• Internal resistance (r)


  Caused by electrons colliding with atoms inside the battery, therefore some energy is lost before electrons even leave the battery
• Electromotive force (emf/ε)


  The energy transferred by a cell per coulomb of charge that passes through it
• Terminal p.d (V)


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


  The p.d across the internal resistor 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, which means it is in an open circuit