Electricity

• Current is the rate of flow of charge.
• Potential difference is the energy transferred per unit charge between two points in a circuit.
• Resistance is a measure of how difficult it is for charge carriers to pass through a component.
• Ohm's law states that for an ohmic conductor, current is directly proportional to the potential difference across it given that physical conditions are constant.
• An IV graph of an ohmic conductor is a straight line through the origin.
• The IV graph of a semiconductor diode is zero until a specific threshold voltage where it rapidly increases.
• The IV graph of a filament lamp is ohmic through the origin but flattens horizontally as voltage increases.
• An ideal ammeter has zero resistance.
• An ideal voltmeter has infinite resistance.
• Resistivity is a measure of how easily a material conducts electricity.
• The equation for resistivity is P = (R A)/L.
• As the temperature of a metal conductor increases, its resistance will increase as the atoms of the metal gain kinetic energy and move faster, increasing collisions with the charge carriers and thus causing them to slow down.
• As the temperature of a thermistor increases, its resistance will decrease as more conduction electrons will be emitted from atoms, thus increasing the number of charge carriers and so increasing the current.
• A superconductor is a material with zero resistance below a certain critical temperature, which is usually close to absolute zero.
• A superconductor may be used for power cables to reduce energy loss through heating to zero and also for strong magnetic fields which would not require a constant power source. This has medical and high speed transport applications.
• The total resistance of resistors in series is the sum of individual resistances.
• The total resistance of resistors in series is 1/ the sum of individual 1/ resistances.
• Voltage, resistance and current are connected by the equation V=I R.
• Power is the rate of transfer of energy and has the equation P=E/t.
• Power is related to voltage, current and resistance by three equations: P = V I = V^2/R = I^2 R.
• In a series circuit, current is the same at all points and the total sum of the voltages across all elements is equal to the supplied voltage.
• In a parallel circuit, the potential difference across each branch is the same and the sum of the currents in each parallel set of branches is equal to the total current.
• Kirchoff's first law is that the total current flowing into a junction is equal to the total current flowing out that junction.
• Kirchoff's second law is that the sum of all voltages in a series circuit is equal to the battery voltage.
• A potential divider is a circuit with several resistors connected across a voltage source used to produce a required fraction of the source potential difference which remains constant.
• Batteries have an internal resistance which is caused by electrons colliding with atoms inside the battery, therefore causing some energy to be lost even before the electrons have left the battery.
• Electromotive force (emf) is the energy transferred by a cell per coulomb of charge that passes through it, and has the equation emf = E/Q.
• The total resistance in a circuit is the internal resistance of the cells plus the load resistance of the components.
• Emf is the product of the total resistance and the current of the circuit and this can be represented in an equation: emf = I R + I r.