Beatrice Lang
Electric power is the rate of electrical energy transfer by an electric circuit per unit of time. The unit of electric power is the watt, defined as one joule per second. In alternating current (AC) circuits, the direction of the voltage and current periodically reverses. Circuits can either function in an idle, active, or maximum state of power consumption. Power is the function of current and voltage, and the current is a varying factor in determining power. The amount of electrical energy consumed by a circuit over a period of time is measured in watt-hours.
| Characteristics | Values |
|---|---|
| Unit of energy | Kilowatt-hours |
| Unit of power | Watt |
| Power consumption in steady-state circuit | No increase in kinetic energy, energy is consumed through resistance |
| Power consumption in non-steady-state circuit | Voltage source with no resistance, increasing current |
| Power consumption in alternating current (AC) circuits | Positive or negative, depending on the direction of power flow |
| Power consumption in circuits with energy storage | Some energy is temporarily stored and returned to the circuit, resulting in periodic reversals of energy flow direction |
| Power consumption in resistors | Voltage drop over the resistor is equal to the battery voltage drop |
| Power consumption in ideal voltage sources | No energy loss, raises the potential of charges in the circuit |
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What You'll Learn
Electric power is the rate of electrical energy transfer
Electric power is the rate at which electrical energy is transferred by a circuit per unit of time. It is measured in watts (W), and its equation is given as Power = work done ÷ time taken. The potential difference, or voltage, of a supply is a measure of the energy given to the charge carriers in a circuit, and it is measured in volts (V). The current, or the movement of electric charges, is measured in amps (A). One watt is equal to one joule per second (J/s).
The rate of energy transfer in a circuit is determined by the potential difference or voltage, and the current. Electrical power can be delivered as a low current with a high voltage or a high current with low voltage. A high current will have a much higher heating effect on the transmission wires than a low current. Hence, transmitting energy at a high voltage and low current will keep the wires cooler and waste less energy.
The power consumed by a circuit is dependent on the state of power consumption. Circuits can function in an idle, active, or maximum state of power consumption. For example, an alarm controller remains in sleep mode, with a minimum current until it is activated by an interrupt. It then enters an active state, where it consumes more power by reading biometric thumbprints and comparing them within its database. Upon validation, it triggers the relays, activates buzzers, and communicates with the central server. These simultaneous actions result in the maximum power consumption of the device.
In circuits with energy storage elements, some of the energy flowing into passive components is stored temporarily and returned to the circuit each cycle. This results in periodic reversals of the direction of energy flow. The portion of energy flow (power) that, averaged over a complete cycle of the AC waveform, results in a net transfer of energy in one direction is known as real power or active power. The amplitude of that portion of energy flow (power) that results in no net transfer of energy but instead oscillates between the source and load in each cycle due to stored energy, is known as the absolute value of reactive power. The product of the RMS value of the voltage wave and the current wave is known as apparent power. The ratio of real power to apparent power is called the power factor.
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Power consumption measurement
Measuring power consumption in an electric circuit is important for improving the lifespan of electronics and keeping power supply and electronics in good working order. Circuits function in idle, active, and maximum states of power consumption. For example, an alarm controller would remain in sleep mode with a minimum current until activated. Upon activation, it consumes more power by reading biometric thumbprints and comparing them within its database. Once activated, it triggers the relays, buzzers, and communicates with the central server, resulting in maximum power consumption.
To measure power consumption, it is crucial to understand the relationship between power, current, and voltage. Power is the function of current and voltage, and in a circuit, the voltage remains relatively constant, while the current is a varying factor in determining power. An ammeter is a quick way to measure power consumption, but it is limited as it does not provide information on peak power consumption, duty cycle, or duration of various levels of power consumption.
To perform a more in-depth analysis, an oscilloscope and a current probe can be used to measure the current flowing into the circuit. These readings can be stored and used for further analysis to gain a better understanding of the current consumption of the circuit.
Additionally, power consumption can be calculated by summing up power over a time period, such as converting watts to watt-seconds, which equals joules. This calculation provides insight into the energy consumed through the circuit for the duration that the power is on.
For per-circuit metering in homes, devices like IoTaWatt can be used to measure voltage output, and current sensors can be employed to monitor energy usage.
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Power sources and loads
On the other hand, a load is a device that extracts energy from a system, converting electricity back into motion, light, heat, or sound. Typical electrical loads include resistors, lamps, and electric motors. These devices resist the flow of electric current, resulting in a voltage drop. In a hydraulic system, for example, a pump acts as a power source by infusing energy into the system, while a restrictor acts as a load by extracting energy and restricting fluid flow.
The distinction between sources and loads is important for understanding and troubleshooting electric circuits. For instance, in a 4-20 mA signal "loop" circuit, a 2-wire transmitter regulates the electrical current based on physical variables like pressure, temperature, or flow rate. Despite its role in setting the current value, the 2-wire transmitter is considered an electrical load rather than a source.
The behaviour of a circuit changes when a load is added. For example, adding a load to a voltage source creates a closed circuit, allowing charge to flow and causing a voltage drop. In alternating current (AC) circuits, the direction of voltage and current periodically reverses, but the definition of sources and loads remains consistent. The current flows from lower to higher potential in a source and from higher to lower potential in a load.
The rate of transfer of electrical energy within a circuit is known as electric power, measured in watts. Electric power can be supplied by generators or batteries and is delivered to homes and businesses through the electric power industry's electrical grid. This power can then be utilised for various applications, such as motion, light, or heat, depending on the load connected to the circuit.
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Energy storage elements
The most common electrical energy storage elements include capacitors, supercapacitors, and rechargeable batteries, such as lithium-ion batteries. Capacitors, originally known as 'condensers', are passive two-terminal electrical components that can store energy electrostatically. They are widely used in electronic circuits to block direct current while allowing alternating current to pass. The voltages and currents in a circuit containing capacitors depend not only on the present values but also on previous values. Capacitors are also used to maintain the power supply in electronic devices while batteries are being changed, preventing the loss of information in volatile memory.
Rechargeable batteries, such as lithium-ion batteries, are commonly used in portable electronic devices due to their wide energy density, long life cycle, lack of memory effect, and suitability for long-scale energy storage applications. Lead-acid batteries hold the largest market share of electric storage products, offering low-cost, low-maintenance energy storage. However, they have a quick discharge rate, resulting in a shorter lifespan and lower energy density.
Other energy storage solutions have been proposed, such as boron, silicon, and zinc, as well as the use of organic compounds like norbornadiene, which can store solar energy as chemical bonds. Hydrogen and methane, produced through the conversion of electricity, are also forms of stored energy that can be utilised in transportation and the natural gas grid.
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Energy conversion
Electric power is the rate of transfer of electrical energy within a circuit. Its SI unit is the watt, defined as one joule per second. Electric power is usually produced by electric generators but can also be supplied by sources such as electric batteries.
In alternating current (AC) circuits, the direction of the voltage and current periodically reverses. Electric power can flow into or out of a component, and a convention is needed for which direction represents positive power flow. Electric power flowing out of a circuit into a component is defined to have a positive sign, while power flowing into a circuit from a component is defined to have a negative sign. Passive components have positive power consumption, while power sources have negative power consumption. This is called the passive sign convention.
In circuits with energy storage elements such as inductance and capacitance, in addition to the energy being consumed by resistances, some of the energy flowing into passive components is stored temporarily and returned to the circuit each cycle. This may result in periodic reversals of the direction of energy flow. The portion of energy flow (power) that, averaged over a complete cycle of the AC waveform, results in a net transfer of energy in one direction is known as real power (also referred to as active power). The amplitude of that portion of energy flow (power) that results in no net transfer of energy but instead oscillates between the source and load in each cycle due to stored energy, is known as the absolute value of reactive power.
In an electric circuit with a resistor, there is an energy conversion to other forms. The voltage drop over the resistor is always equal to the battery voltage drop. The power formula (P = I·V) and Joule's first law (P = I^2·R) can be combined with Ohm's law (V = I·R) to calculate power consumption.
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Frequently asked questions
Electric power is consumed in an electric circuit. This is the rate of transfer of electrical energy within a circuit.
The SI unit of electric power is the watt, defined as one joule per second. Thousands, millions, and billions of watts are called kilowatts, megawatts, and gigawatts, respectively.
Power is the function of current and voltage. As the voltage for a circuit is constant, the current is a varying factor when it comes to determining power.
Circuits either function in an idle, active, or maximum state of power consumption. An example of this is an alarm controller that remains in sleep mode with a minimum current until it is activated.
