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1.3:

Power and Energy

JoVE Core
Electrical Engineering
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JoVE Core Electrical Engineering
Power and Energy

言語

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Energy is the total work required to drive the current through the circuit.

A rise in voltage results in more work done, passing more current, resulting in increased energy transfer.

Power is the rate at which energy is supplied or absorbed over time, measured in watts.

Power can also be expressed as the product of the instantaneous voltage and current across an element.

The sign of power depends on the current's direction and the polarity of the voltage.

According to the passive sign convention, when the current enters a circuit element at the positive terminal of the voltage and exits the negative terminal, the element absorbs power.

Conversely, if the current enters the negative terminal and exits the positive terminal, the element supplies power.

The total power supplied to the circuit must equal the total power absorbed, validating the energy conservation law.

For an element, the energy absorbed or supplied is the integral of instantaneous power.

If a cassette player draws steady current, then using battery's parameters and power-energy relation, the player's runtime is calculated.

1.3:

Power and Energy

The power and energy delivered to an element are subjects of great significance in the field of electrical engineering. It is a well-known fact that a 100-watt light bulb emits more light than a 60-watt one. Therefore, power and energy calculations play a crucial role in the analysis of electrical circuits.

Power, defined as the time rate of expending or absorbing energy, is quantified in units called watts (W). The relation between power and energy is mathematically given as

Equation1

where "p" represents the power in watts (W), "w" denotes the energy in joules (J), and "t" signifies the time in seconds (s).

The power associated with the current passing through an element within a circuit is the product of the voltage across that element and the current flowing through it. When the current enters the circuit element at the positive terminal of the voltage and exits at the negative terminal, it adheres to the passive convention. In this convention, the voltage propels a positive charge in the same direction as indicated by the current. Consequently, the power computed by multiplying the element voltage by the element current represents the power received by the element. This power is occasionally referred to as "the power absorbed by the element" or "the power dissipated by the element." Importantly, the power received by an element can assume either positive or negative values, contingent upon the actual values of the element's voltage and current.

On the other hand, when the passive convention is not adhered to, the current enters the circuit element at the negative terminal of the voltage and exits at the positive terminal. In this case, the voltage drives a positive charge in the direction opposite to that indicated by the current. Therefore, when the element voltage and current do not comply with the passive convention, the power computed by multiplying these values represents the power supplied by the element. Similar to received power, supplied power can also be positive or negative, depending on the specific values of the element's voltage and current.

The relationship between the power received by an element and the power supplied by that same element is expressed as

Equation2

When the element voltage and current align with the passive convention, the energy received by an element is given by

Equation3

Energy is defined as the capacity to perform work and is expressed in joules (J).