Electrical Quantities and Units
Electricity is described using several physical quantities. To read a technical specification, interpret a device’s measurements, or understand the results of a simulation, it is important to know what these quantities represent and in which units they are expressed.
- Electric voltage
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Voltage, denoted by \(U\), represents the potential difference between two points in a circuit. It indicates how much the electric charges are "pushed" to move. Its unit is the volt \((\mathrm{V})\). Examples: an AA battery provides approximately \(1.5\ \mathrm{V}\); the household power grid in Europe provides approximately \(230\ \mathrm{V}\).
- Electric current
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Current, denoted by \(I\), represents the flow of electric charges through a conductor. Its unit is the ampere \((\mathrm{A})\). A current of \(1\ \mathrm{A}\) means that a certain amount of charge flows through the conductor every second. Examples: a few milliamps \((\mathrm{mA})\) in an electronic circuit board, several amps in a heating appliance.
- Electric power
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Power, denoted by \(P\), indicates the rate at which energy is supplied or consumed. Its unit is the watt \((\mathrm{W})\). A power of \(1000\ \mathrm{W}\) (\(1\ \mathrm{kW}\)) means that the system consumes or supplies 1000 joules of energy per second. Examples: a \(10\ \mathrm{W}\) LED bulb, a \(400\ \mathrm{W}\) solar panel, a \(2\ \mathrm{kW}\) radiator.
- Electrical energy
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Energy, denoted by \(E\), measures the total amount of electricity produced or consumed over a certain period of time. In the International System, the base unit is the joule \((\mathrm{J})\), but in electricity, the watt-hour \((\mathrm{Wh})\) and especially the kilowatt-hour \((\mathrm{kWh})\) are very commonly used. Example: A consumption of \(5\ \mathrm{kWh}\) means that, in total, the equivalent of \(5\ \mathrm{kW}\) was used for one hour.
Key Points – Main Electrical Quantities
- Voltage: symbol \(U\), unit volt \((\mathrm{V})\)
- Current: symbol \(I\), unit ampere \((\mathrm{A})\)
- Power: symbol \(P\), unit watt \((\mathrm{W})\)
- Energy: symbol \(E\), units joule \((\mathrm{J})\), watt-hour \((\mathrm{Wh})\), kilowatt-hour \((\mathrm{kWh})\)
Prefixes (m, k, M)
In practice, the values encountered can be very small or very large. We therefore use prefixes to simplify the notation of units:
| k (kilo) | _ | _ | Unit | _ | _ | m (milli) |
|---|---|---|---|---|---|---|
| 1 | ||||||
| 1 | 0 | 0 | 0 | |||
| 0, | 0 | 0 | 1 |
- \(\mathrm{m}\) (milli) \(= 10^{-3}\) → \(1\ \mathrm{mA} = 0.001\ \mathrm{A}\)
- \(\mathrm{k}\) (kilo) \(= 10^{3}\) → \(1\ \mathrm{kW} = 1000\ \mathrm{W}\)
- \(\mathrm{M}\) (mega) \(= 10^{6}\) → \(1\ \mathrm{MW} = 1\ 000\ 000\ \mathrm{W}\)
Energy Conversion