The voltage and power current are two basic parameters of the electrical circuit. However, only electric current and energy are not enough to determine the character of an electric object. We need to know how much electricity (electric power), something a region can handle.
We have all seen that a 60 watt electric lamp provides less than 100 watts electric light. When we pay for electricity consumption, we are actually paying for charges of electricity for a certain period of time. Electrical calculation is therefore very important in analyzing an electrical circuit or network.
Suppose, an element supplies or consumes an energy of dw joules for a time of dt second, then power of the element can be represented as,
This equation can also be rewritten as,
Hence, from the as the expression of voltage and current in the equation are instantaneous, the power is also instantaneous. The expressed power is time-varying.
So, the power of a circuit element is the product of voltage across the element and current through it.
As we have already mentioned, the circuit element can absorb or deplete energy. We represent the absorption of energy by placing a positive (+) sign on the energy output. Similarly, we put a dot (-) in it when representing the force brought in by a circle.
There is a simple relationship between the direction of the current power, the magnitude of the electrical power and the strength of the circuit element. We call this relationship a sign meeting that you did not do. When a current enters an object through its terminal voltage polarity, we set the positive (+) sign before the output voltage and current. This means that the element takes or consumes energy from the electrical circuit. On the other hand, when a current through an element leaves its terminal voltage polarity, we set a negative (-) mark before the current and current voltage output. This means that the element brings or energizes the electrical circuit.
Source: electrical4u