Electric power equations (Begineers, 2024)

Apart from voltage and current, electric power is another crucial factor concerning electronic circuits.

Power is the rate at which work is done or energy is transformed into a circuit. Simply put, it means how much energy is used for an instant of time. Various equations are used to calculate the power consumed by different components in the circuit. The product of voltage and current is also called power. The SI unit of power is watt (W).

In this article, we will explore what electric power is and how we derive the equations that govern it.

Electric power

In electronic circuits, the applied voltage makes the charges flow. This flow of charges generates current so it means work is done by moving charges, this work generates power that will be used by the circuit to function.

So electric power is basically, how much work is done or energy is transmitted in an electronic circuit.

When we plug a device into an electrical outlet, such as a phone charger or a television, it draws energy to function. This energy is supplied by the electricity flowing through wires, which powers the device.

So electric power is defined as,

The rate at which electrical energy is transferred or consumed in a circuit. It represents how quickly energy is being used or converted by electronic devices.

The electrical energy transferred in a circuit per unit of time is power. By definition, it means if more energy is transferred in unit time in the circuit the power will be high.

Let’s see an example of a water tank to understand it better. Imagine you have a water tank connected to a pipe. The water tank is at a certain height, and the water flows through the pipe.

Think that the pressure of water is voltage, as it makes the water flow the same as the voltage making electric charges move. The flow of water is the current.

Power is like the speed at which water is being delivered. If you have a high flow rate (current) and strong pressure (voltage), you’re delivering a lot of water quickly.

So electric power tells us how fast energy is being used, and just like water flow, it depends on both the pressure (voltage) and the amount of flow (current).

Quantity

Electric power is a scalar quantity. This means it has only magnitude and no direction, unlike vector quantities, which have both magnitude and direction.

Electric power simply measures how much energy is used or delivered per unit of time, without any concern for direction, making it a scalar.

Types of power

There are two main types of electric power: AC and DC. The difference between these two is the direction of current flow.

1. DC power

The DC power is the product of voltage and current. It is the type of power in which current flows in one direction.

Batteries, solar cells, and fuel cells are common sources of DC power. In electronics, DC power is used to power up the devices.

2. AC power

AC is a type of electrical power where the current periodically reverses direction. AC power is the most common form of electricity used in homes and businesses.

It is further divided into three types:

Active power: It is the power that does useful work in a circuit. It is the power that is used by the device to operate. It is represented by “P”

P=Vrms*Irms*cos(θ)

Reactive power: It is the power that does not do any useful work but is needed to maintain the magnetic and electric fields in devices like motors, transformers, and inductors. It flows back and forth between the source and the load without being consumed. It is represented by “Q”

Q=Vrms*Irms*sin(θ)

Apparent power: It is the total power supplied to the circuit. It includes both active and reactive power. You can think of it as the total amount of power the source needs to deliver. The “S” symbol is used to represent apparent power.

Equations of power

The rate at which work is done in an electric circuit is called electric power or power.

Electric power (P) = Work done in the circuit (W)/ Time (t)

In the above when voltage (V) is applied in a circuit, the current (I) starts flowing in the circuit through a resistor (R) in time ‘t’. From this, we see that work is done by moving the electric charge in the circuit.

So this work which has to be done in moving the charge in unit time is called electric power. The work done is equal to

W=Q*V

Where ‘W’ shows work done, ‘Q’ charge, and ‘V’ voltage across the circuit.

Q = I*t

After putting a charge Q in the above equation, work becomes

W=(It)*V

Now power is equal to

P=W/t

P= (IV)t/t

P=IV ………………..(A)

As you see the power is the function of current and voltage. Here ‘P’ represents power, ‘V’ and ‘I’ voltage and current respectively. There are also some other equations or formulas to calculate power.

As from Ohm’s law, we know.

V=IR

If we put voltage in the power equation (A), the equation becomes

P=I(IR)

P=I^2R

Ohm’s law also defines current which is equal to

I=V/R

Now put the current in the equation (A) of power.

P=(V/R)V

P=V^2/R

The above formulas are equally valid for calculating power in the circuit. The known quantities define which formula should be used.

Unit of power

The SI unit of power is watt (W). The power of 1W is consumed in the circuit if the voltage of 1V causes a current of 1A to flow through it. So

1W=1V*1I

For larger power, there are bigger units such as kilowatts (kW) and megawatts (mW).

Factors of power

The power consumed in the circuit depends upon two main factors. These factors are:

The current (I) flows in the circuit.
The voltage (V) across the two ends of a component.

A proportionate rise in either of these factors will result in a higher power. That’s why the formula of power is in terms of these two variables.

Example of power calculation

Let’s discuss an example of how to calculate power in the circuit.

In the above circuit, we have the value of voltage and resistor. To find power we must know how much current flows in the circuit.

So from Ohm’s law,

V=I*R

I=V/R

As in the circuit V=9V and R=10Ω. so current is equal to

I=9V/10Ω

I=0.9A

Now current is known to us, we can now find power that begins used by the resistor. As power equals

P=I*V

P=0.9A9V

P=8.1W

From this, we got to know the power that has been consumed in the circuit by the resistor is 8.1W.

You can practice on different circuits to find the electric power that is consumed by various components.

Power rating

When you start working with electronic components and circuits you will see a term called power rating. You must be wondering what is meant by power rating.

Every component in an electronic system is made to withstand a certain level of power to function correctly and prevent damage and overheating.

So the power rating is the maximum amount of power that the component can safely dissipate or handle without damage.

Before connecting a component in a circuit, understanding this rating is essential to ensure safety and reliability.

Conclusion

Electric power is a fundamental concept that underpins our understanding of basic electronics and circuits. The power in the circuit is defined as the electric energy transfer per unit time.

Power is defined in terms of voltage (V) and current (I). If the voltage is high, the current will be high so power produced in the circuit will be high.

The electric power formula P=VI can be used to determine the amount of electric power transferred in a circuit. Other formulas are derived using Ohm’s law.

Electric power is a scalar quantity because its magnitude alone can be used to characterize it; direction is not necessary.

Power depends upon two main factors: current and voltage. By increasing them you can increase the power in the circuit.

The SI unit of power is watts (W) and some bigger units like kilowatts (kW) and megawatts (mW) are also used.

This was all about power and its various equations, I hope it will be helpful.

Thank you and stay blessed…