Otis Singleton
Electricity is a foundational concept in physics, and every electric parameter has its own unit of measurement, standardised under the International System of Units (SI). The four fundamental units in electricity are: Volt (V), Ampere (A), Ohm (Ω), and Watt (W). Each of these units is used to measure a specific electrical parameter, such as current, charge, voltage, resistance, power, or energy. For example, the unit of measurement for electric charge is the Coulomb (C). Understanding these units and their relationships is essential for both theoretical and practical applications, such as calculating electricity bills or solving physics problems.
| Characteristics | Values |
|---|---|
| Unit of Electricity | SI Units |
| Electric Current | Ampere (A) |
| Electric Charge | Coulomb (C) |
| Electric Potential | Volt (V) |
| Resistance | Ohm (Ω) |
| Electric Power | Watt (W) |
| Energy | Kilowatt-hour (kWh) |
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What You'll Learn
Current: Ampere (A)
The ampere, often shortened to amp, is the unit of electric current in the International System of Units (SI). One ampere is equal to one coulomb (approximately 6.24 x 10^18 elementary charges) moving past a point per second. The unit is named after French mathematician and physicist André-Marie Ampère (1775-1836), considered one of the fathers of electromagnetism.
Ampere's force law states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force was used in the formal definition of the ampere, giving the vacuum magnetic permeability (magnetic constant, μ0) a value of exactly 4π x 10^-7 henries per metre (H/m, equivalent to N/A2). The SI unit of charge, the coulomb, was then defined as "the quantity of electricity carried in one second by a current of one ampere".
The 2019 revision of the SI defined the ampere by taking the fixed numerical value of the elementary charge e to be 1.602176634 x 10^-19 when expressed in the unit C, which is equal to A·s. This means an ampere is an electric current equivalent to 1019 elementary charges moving every 1.602176634 seconds. Prior to the redefinition, the ampere was defined as the current passing through two parallel wires one metre apart that produces a magnetic force of 2 x 10^-7 newtons per metre.
Ampere is used to measure the rate of flow of electric charge. A higher ampere rating means more electricity can flow through a circuit. For example, large appliances like air conditioners, washers and dryers will be connected to 30-amp circuits, while most outlets in a home will be powered by 15-amp or 20-amp circuits. If too many appliances are connected to the same circuit, the breaker will "trip" and cut off power to protect the wiring from overheating.
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Charge: Coulomb (C)
The unit of electric charge in the International System of Units (SI) is the Coulomb (C). It is defined as the quantity of electricity carried in one second by a current of one ampere. In other words, one ampere is equal to the electric current that corresponds to the flow of one coulomb per second.
The relationship between the Coulomb and the ampere can be expressed as follows: 1 C = 1 A x 1 s (or C = A x s). This means that if a current in a circuit is 1 A, 1 C of charge passes through a point in the circuit every second. The Coulomb can also be defined in terms of the elementary charge, which is fixed at 1.602176634 x 10^-19 C. This value serves as a defining constant in the SI.
The Coulomb is named after Charles-Augustin de Coulomb. As with every SI unit named after a person, its symbol starts with an uppercase letter (C) but follows standard capitalisation rules when written in full. The International Electrotechnical Commission (IEC) approved the Coulomb as the unit of electric charge in 1881.
The Coulomb can be modified by adding a prefix that multiplies it by a power of 10. For example, one faraday is equal to 9.648533212 x 10^4 coulombs, while one ampere-hour equals 3600 C. The Coulomb is a fundamental concept in physics, helping us understand how we measure electrical quantities and how electric circuits function.
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Voltage: Volt (V)
The volt, with the unit symbol 'V', is the unit of measurement of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). One volt is defined as the electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. It can also be defined as the potential difference between two points that will impart one joule of energy per coulomb of charge that passes through it.
The volt is named after Alessandro Volta, an Italian physicist who invented the electric battery. In 1800, Volta developed the so-called 'voltaic pile', a forerunner of the battery, which produced a steady electric current. The volt was defined in 1893 as 1⁄1.434 of the emf of a Clark cell, and this definition was abandoned in 1908 in favour of a definition based on the international ohm and international ampere. In 1948, the entire set of "reproducible units" was abandoned.
The voltmeter is used to measure the voltage between two points in a system. Voltage, also known as electrical potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to move a positive test charge from the first point to the second point.
A common use of the term "voltage" is in describing the voltage dropped across an electrical device, such as a resistor. The voltage drop across the device can be understood as the difference between measurements at each terminal of the device with respect to a common reference point (or ground).
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Resistance: Ohm (Ω)
Resistance is measured in ohms, represented by the Greek letter omega (Ω). It is one of the four fundamental units in electricity, alongside volts (V), amperes (A), and watts (W). These units are standardised under the International System of Units (SI).
Ohm's law states that resistance is the opposition to the flow of electric current in a circuit. In other words, it is a measure of how much a material resists the flow of electric current. The higher the resistance, the lower the current for a given voltage. This relationship between resistance, voltage, and current is described by Ohm's law: V = I * R, where V is voltage, I is current, and R is resistance.
The symbol Ω is used to denote the unit ohm. For example, 1 MΩ equals one million ohms. Resistance is a property of the material, with a conductor having low resistance. For a wire, the smaller the diameter, the greater the resistance.
Resistance is also related to power, which is measured in watts (W). Power is the rate at which electrical energy is consumed or converted. In a circuit, power can be calculated using the formula P = V * I, where P is power, V is voltage, and I is current. This formula can be rearranged to P = I^2 * R, combining Ohm's law with Joule's first law (P = I^2 * R).
Understanding these units and their relationships is essential for comprehending how electric circuits function and for practical applications such as calculating electricity bills or solving physics problems.
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Power: Watt (W)
Watt (W) is the unit of power in the International System of Units (SI). It is used to quantify the rate of energy transfer or energy consumption. One watt is defined as the energy consumption rate of one joule per second. In other words, one watt is the rate at which work is done when an object's velocity is held constant at one meter per second against a constant opposing force of one newton.
In terms of electricity, one watt is the rate at which electrical work is performed when a current of one ampere (A) flows across an electrical potential difference of one volt (V). This is equivalent to the definition of one volt-ampere. However, the volt-ampere is used for a different quantity than the real power of an electrical circuit.
The watt unit is named after James Watt, the 18th-century Scottish inventor, mechanical engineer, and chemist who improved the Newcomen engine with his own steam engine in 1776. This invention became fundamental for the Industrial Revolution.
The watt is used to measure power in electrical circuits, electronic devices, and household appliances. For example, a 40-watt light bulb uses 0.04 kW of electrical energy in an hour. The watt is also used to calculate electricity consumption for billing.
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