Electricity - Equations and Definitions

charge flow (C) = current (A) × time (s)

potential difference (V) = current (A) × resistance (Ω)

energy transferred (J) = charge (C) × potential difference (V)

power (W) = potential difference (V) × current (A) = (current (A))2 × resistance (Ω)

energy transferred (J, kWh) = power (W, kW) × time (s, h)

Charge - a fundamental property of matter which can be either positive or negative.  Charged particles exert forces on other charged particles at a distance called an electrostatic force, has units Coulombs, (C).


Static Electricity - the group of phenomenon caused by insulated objects becoming charged.


Electrical Field - an area of space in which charged particles experience a force.


Electric Field Lines - a representation of an electrical field with lines indicating the direction which a positively charged particle would move, i.e. from positive to negative.  The closer the lines the stronger the magnetic field.


Current - a measure of the charge past a point in a given time, has units Amperes (A) which is equivalent to one Coulomb per second.


Potential Difference (Voltage) - the difference in energy per unit of charge between two points in a circuit.  Has units Volts (V) which are equivalent to a Joule per Coulomb.


Resistance - a measure of an electrical component’s opposition to charge flowing within it.  Has units Ohms (Ω).


Ohm’s Law - current is proportional to voltage.  Is usually expressed as an equation V=IR, i.e. resistance is the constant of proportionality.


Chemical Cell - a component which supplies electromotive force (potential difference) to a circuit by a chemical electrolyte reacting at two electrodes.


Conventional Current - flows from positive to negative, although we know that the mobile charge is negative and so flows the opposite way.


Electrical Working - energy transferred due to forces between charged particle.


Series Circuit - a circuit with only one route from power supply back to power supply.  Charge must flow through each component in turn.


Parallel Circuit - a circuit with more than one branch.  Each branch has a direct route back to the power supply.


Kirchoff’s First Law - the sum of the currents into a point are equal to the sum of the currents out of the point.  Used to show how current is the same everywhere in a series loop and to show that currents in branches of a parallel circuit add up to the current through the power supply.


Kirchoff’s Second Law - the rises and falls of electrical potential sum to zero.  I.e. the energy supplied to the charge by the power supply is dissipated by the components.  The is used to show that in a series circuit the supplied voltage is shared between the components and that all branches in a parallel circuit have the same voltage.


Ohmic conductor - a conductor for which resistance is a constant through a large range of voltages and currents, i.e. a V vs I graph would have a constant gradient.


Non-ohmic conductor - a conductor for which resistance changes through a range of voltages or currents.  For example in most metallic conductors higher currents cause a higher resistance.  This results in a curved V vs I graph.


Variable Resistor - a component used to change the resistance in a circuit, usually consists of a length of wire which you can vary the length in the circuit.


Semiconductor - a material that conducts under some conditions and insulates under others.  Most commonly used material is silicon.


NTC Thermistor - (negative temperature coefficient) thermistors are components which have semiconducting properties dependent on temperature.  At higher temperatures they have lower resistances.


LDR - (light dependent resistors) components which have semiconducting properties dependent on light.  At higher intensities of light they have lower resistances.


Net resistance - resistance of combinations of resistances.  In series resistances add up, in parallel introducing new branches always reduces the overall resistance.


Potential Dividers - a circuit where the voltage is shared in the ratio of two or more resistances in series.


Sensing Circuits -  a potential divider circuit where one of the resistances is a semiconductor sensitive to its environment.  This causes a change in ratio of the two resistors and therefore a change in the voltage which can activate a component.