“Electric Power” Voltage and current are two basic parameters of a circuit. However, only the voltage and the current are insufficient to express the behavior of an electrical circuit. Basically, we need to know how much electrical energy a circuit element can process.
We have all seen that a 60-watt electric lamp produces less light than a 100-watt electric lamp. When we pay the electricity bill for using electricity, we are actually paying the electricity bill for a period of time. Therefore, the calculation of the electrical power for the analysis of a circuit or a power grid is essential. Electric Power
Power is the rate of energy supplied or consumed by an electric element with respect to time. Electric Power
Assuming that an element supplies or consumes energy of dw joules for a period of dt seconds, then the performance of the element can be represented by:
This equation can also be rewritten as,
Since the expression of voltage and current in the equation is instant, therefore, power is instant too. The expressed performance varies over time.
The performance of a circuit element is therefore the product of the voltage across the element and the
current flowing through it. Electric Power
As already mentioned, a circuit element can either absorb or deliver energy. We represent the absorption of power by giving the expression of power a positive sign (+). We also put a negative sign (-) when we represent the power delivered by the circuit element.
Passive Sign Convention
There is a simple relationship between the direction of current, the polarity of the voltage and the sign of the performance of a circuit element. We call this simple relationship with a passive sign convention. When a current enters an element through its positive voltage polarity terminal, we put a positive sign (+) in front of the product of voltage and current. This means that the element absorbs or uses energy from the circuit. On the other hand, if the current flowing through the element leaves its terminal with positive voltage polarity,
a negative sign (-) is placed in front of the product of voltage and current. This implies that the element is supplying or feeding the circuit.
Imagine a resistor connected between two circuit terminals. Although the rest of the circuit is not shown here in the figure.
The polarity of the voltage drop across the resistor and the direction of
the current through the resistor is shown in the figure below. The resistor consumes vi watts of power when the current i amps enters the resistor through its positive side of the falling voltage v volts as shown.
Imagine a battery connected between two circuit terminals. Although the rest of the circuit is not shown here in the figure.
The polarity of the voltage drop across the battery and the direction of
current through the battery is shown in the figure below. The battery delivers 4 watts of power when 1 ampere current enters the V-volt battery through its terminal with positive polarity as shown. Electric Power