Otis Singleton
The standard units for measuring electricity are based on the International System (SI) of units. The SI unit of electric potential difference is the volt (V), which is the energy of 1 joule consumed when an electric charge of 1 coulomb flows in a circuit. The unit of electric current is the ampere (A), which measures the amount of electrical charge that flows in an electrical circuit per 1 second. The unit of electric resistance is the ohm (Ω). The unit of electric power is the watt (W), which measures the rate of energy consumed.
| Characteristics | Values |
|---|---|
| Standard unit of electricity | Defined systematically, starting with the ampere, which comes from the charge and coulomb of an electron |
| SI unit of electricity | Watt (W), named after James Watt, inventor of the steam engine |
| SI unit of electric potential difference | Volt (V) |
| SI unit of electric resistance | Ohm (Ω) |
| SI unit of electric current | Ampere (A) |
| SI unit of energy | Joule (J), named after James Prescott Joule |
| Unit of electrical energy for utility bills | Kilowatt-hour (kWh) |
| Unit of electrical power | Watt (W) |
| Unit of electrical current | Ampere (A) |
| Unit of electrical voltage | Volt (V) |
| Unit of electrical resistance | Ohm (Ω) |
| Unit of electrical energy | Decibel-milliwatt or dBm (logarithmic scale referenced to 1mW) |
| Unit of electrical power | Decibel-watt or dBW (logarithmic scale referenced to 1W) |
| Unit of capacitance | Farad |
| Unit used in atomic physics, particle physics, and high-energy physics | Electronvolt (eV) |
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What You'll Learn
The standard unit of electricity is the Watt
The Watt is equal to one joule per second or one kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. In electromagnetism, 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 means that one Watt is equivalent to the volt-ampere, though this is used for a different quantity from the real power of an electrical circuit.
The Watt is used to measure the power of electrical appliances, such as a light bulb, which consumes 100 watts of power per hour. Radio stations also use Watts to report the power of their transmitters, referring to the effective radiated power. This refers to the power that a half-wave dipole antenna would need to radiate to match the intensity of the transmitter's main lobe.
The Watt is one of the four basic units of electricity, along with Volts, Amps, and Ohms. Volts are the unit of voltage, Amps are the unit of electrical current, and Ohms are the unit of resistance. Other electrical units are derived from these SI base units.
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Volts, Amps, Ohms, and Watts are the four basic units
Volts
The volt (V) is the unit of electric potential, also known as electromotive force. It represents the potential difference between two points of a conducting wire carrying a constant current. Volts measure the potential for energy to travel, analogous to water pressure in a plumbing system. Voltage is the amount of work required to move an electric charge from one point to another.
Amps
The ampere (A) is the unit of electrical current. It measures the amount of electrical charge that flows in an electrical circuit per second. Current is the rate of flow of electric charge, analogous to the flow rate of water in a pipe.
Ohms
The ohm (Ω) is the unit of electrical resistance. It measures the opposition to the flow of current in a circuit, analogous to the size of a water pipe.
Watts
The watt (W) is the unit of power. It measures the rate at which electrical energy is transferred or consumed, or the amount of work done. Watt-hours (Wh) are used to measure the total energy consumption over time.
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Voltage is the difference that causes current to flow
The SI unit of electricity is the Watt, named after James Watt, the inventor of the steam engine. The standard units of electrical measurement for voltage, current, and resistance are the Volt [V], Ampere [A], and Ohm [Ω] respectively.
Voltage is the difference in charge between two points that causes current to flow. It is the measure of potential energy between two points in a circuit, or the "voltage drop". When a voltage source is connected to a closed-loop circuit, the voltage will cause a current to flow around the circuit. The greater the voltage, the greater its ability to "push" electrons through.
In a battery, the negative end tries to attract charge carriers, while the positive end tries to push them out. The voltage between these two points will not result in a charge flow unless there is a continuous path for the charge carriers to move through.
The voltage will cause a uniform flow of charge carriers through a circuit, known as a current. Current is the continuous and uniform flow of charge around a circuit. It is measured in amperes or amps and has the symbol I.
The relationship between voltage, current, and resistance is described by Ohm's Law. Resistance is a material's tendency to resist the flow of charge or current. It is measured in ohms and is the opposition to the flow of current in a circuit.
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Current is the rate of flow of electric charge
Electrons are the charge carriers in most metals and they follow an erratic path, bouncing from atom to atom, but generally drifting in the opposite direction of the electric field. The speed they drift at can be calculated from an equation. Typically, electric charges in solids flow slowly. For example, in a copper wire of cross-section 0.5 mm2, carrying a current of 5 A, the drift velocity of the electrons is on the order of a millimetre per second.
To create a net flow of charge, the particles must also move together with an average drift rate. For a net current to flow, more states in one direction than the other must be occupied. For this to occur, energy is required. Current density is the rate at which charge passes through a chosen unit area.
The unit used to measure current is the ampere, or "amp" for short. The ampere is defined as the charge and the coulomb of an electron.
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Resistance is the opposition to the flow of current
Resistance, measured in ohms (Ω), is the opposition to the flow of current in an electrical circuit. It is influenced by the material's properties, length, cross-sectional area, and temperature. All materials resist current flow to some degree, but they generally fall into one of two categories: conductors and insulators. Conductors, such as metals, offer very little resistance and allow electrons to move easily. On the other hand, insulators like rubber, paper, glass, wood, and plastic have high resistance, restricting the flow of electrons.
The relationship between voltage, current, and resistance is described by Ohm's Law, which states that voltage (V) is equal to the current (I) flowing through it times the resistance (R)—that is, V = I x R. This equation can be rearranged to solve for current (I = V/R) or resistance (R = V/I). Resistance measurements are often taken to indicate the condition of a component or a circuit. For example, if the resistance is abnormally high, it could be due to damaged conductors caused by burning or corrosion. Conversely, if the resistance is abnormally low, it could be the result of insulators damaged by moisture or overheating.
Resistance also plays a critical role in controlling the flow of current in electrical systems. An increase in resistance will result in increased heat generation. In three-phase motor circuits, even a slight increase in resistance can significantly affect the balance of the circuit. Resistance is also an important indicator of installation errors, poor connections, and failed components. Technicians often determine resistance by measuring voltage and current and applying Ohm's Law.
The SI units for voltage, current, and resistance are volt (V), ampere (A), and ohm (Ω), respectively. These units are derived from the International System (SI) and are used to measure electrical expressions in electric circuits, electronics, and electrical appliances.
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Frequently asked questions
The SI unit of electricity is the Watt, which is equal to one ampere under one volt of pressure.
The standard units of electrical measurement for voltage, current, and resistance are Volt [V], Ampere [A], and Ohm [Ω] respectively.
The unit of electrical energy used for utility bills is the kilowatt-hour (kWh).
The electrical unit of electrical current is the Ampere, which measures the amount of electrical charge that flows in an electrical circuit per second.
The unit of electrical power is the Watt, which measures the rate of consumed energy.
