Electricity formulae - At a glance:
| Physical quantity | Symbol | Definition | Formula | SI Unit | Unit Symbol |
| Electric charge | \(Q\) or \(q\) | Basic fundamental properties of all matter | \(Q\ =\ ne\) | \(coulomb\) |
\(C\)
|
| Electric current | \(I\) | Rate of flow of electric charge through a conductor | \(I\ =\ \frac{Q}{t}\) | \(ampere\) | \(A\) |
| Potential difference | \(V\) | Work done to move unit charge between two points | \(V\ =\ \frac{W}{Q}\) | \(volt\) | \(V\) |
| Ohm's law | - | States that current is directly proportional to voltage at constant temperature | \(V\ =\ IR\) | \(volt\) | \(V\) |
| Resistance | \(R\) | Opposition offered by a conductor to the flow of current | \(R\ =\ \frac{V}{I}\) | \(ohm\) | \(\Omega\) |
| Resistivity | \(\rho\) | Intrinsic property of a material that opposes current flow | \(\rho\ =\ \frac{RA}{L}\) | \(ohm-metre\) | \(\Omega\ m\) |
| Resistance of a conductor | \(R\) | Resistance depends on material, length, and area of the conductor | \(R\ =\ \frac{\rho\ L}{A}\) | \(ohm\) | \(\Omega\) |
| Equivalent resistance in series | \(R_s\) | Total resistance when resistors are connected end-to-end | \(R_s\ =\ R_1\ +\ R_2\ +\ R_3\ +\ . . .\) | \(ohm\) | \(\Omega\) |
| Equivalent resistance in parallel | \(R_p\) | Combined resistance when resistors are connected across the same voltage | \(\frac{1}{R_p}\ =\ \frac{1}{R_1}\ +\ \frac{1}{R_2}\ +\ \frac{1}{R_3}\ +\ . . .\) | \(ohm\) | \(\Omega\) |
| Joule's law of heating | \(H\) | Heat generated when current flows through a resistor |
\(H\ =\ I^2Rt\)
\(H\ =\ VIt\)
\(H\ =\ \frac{V^2t}{R}\)
|
\(joule\) | \(J\) |
| Electric Power | \(P\) | Rate at which electrical energy is consumed or converted |
\(P\ =\ VI\)
\(P\ =\ I^2R\)
\(P\ =\ \frac{V^2}{R}\)
|
\(watt\) | \(W\) |
| Electrical energy | \(E\) | Energy consumed by an electrical appliance when it operates | \(E\ =\ P\ \times\ t\) | \(watt-second\) | \(Ws\) |
Important constants and conversions:
| Quantity | Definition | Relation |
| Electronic charge | Charge carried by a single electron | \(e\ =\ -\ 1.6\ \times\ 10^{-16}\ C\) |
| \(1\ coulomb\) | The quantity of charge transferred in one second by a current of one ampere | \(1\ C\ =\ 6.25\ \times\ 10^{18}\ electrons\) |
| \(1\ ampere\) | Current due to flow of one coulomb charge per second | \(1\ A\ =\ 1\ C/s\) |
| \(1\ volt\) | Potential difference when one joule of work is done per coulomb | \(1\ V\ =\ 1\ J/C\) |
| \(1\ ohm\) | Resistance allowing one ampere current at one volt | \(1\ \Omega\ =\ 1\ V/A\) |
| \(1\ watt\) | Power when one joule of energy is used per second | \(1\ W\ =\ 1\ J/s\) |
| \(1\ kilowatt\) | Commercial unit of power | \(1\ kW\ =\ 1000\ W\) |
| \(1\ kilowatt-hour\) | Commercial unit of electrical energy | \(1\ kWh\ =\ 3.6\ \times\ 10^6\ J\) |