Electric Power

Definition of Electric Power


What is meant by electric power? Power is commonly defined as the rate at which the work is done. When this is done with respect to time and in an electrical circuit, it is known as electric power. Alternately, electric power is defined as the rate at which electric energy is transferred across an electrical circuit per unit time. Electric power is versatile - it can be produced in generators in our houses and can be supplied to electric batteries used in devices.


Unit of Electric Power


When you push or pull something through some distance, you do some work, and it is represented by Joule. Power is actually the rate at which any work is done. Watt is the unit for power. It is the rate at which work is done or how fast some work is done. It is the derived unit in the metric system.


The Unit of Electrical Power is Watt. 


One Watt is one Joule of work being done on an object per second. It is defined as Joule per second. Watt is represented as W. If you do 75 Watts of work per second, it means you do 75 joules of work every second. When we calculate power, we simply take the work and the amount of time it takes for us to do the work. 

Voltage is the electromotive force or potential difference between two points, which signifies the amount of work required to move a charge between two points. The unit of voltage is Volt. Ampere is represented by A and is the unit of electric current. 

The power consumed in an electric circuit is known as one Watt when one ampere of current is seen flowing through that electrical circuit. In this case, the potential difference of 1 volt is applied across it. 

If you want to denote a larger unit of electric power, you can use kilowatt (equal to 1000 watts).

You can also use gigawatt and megawatt for larger units of electric power.


The Formula of Electric Power


To calculate power, the most simple equation is work done divided by time.

(1) P = W/t

W is work done; t is time

However, the above is used mostly for mechanical power. For electric power, another equation used is when we calculate work through the amount of charge and the potential difference through which the charge is moving. 

(2) W= qV

 Where q= total charge used and V=voltage

When we substitute (1) in (2), we understand that power is now the charge multiplied by the voltage divided by the time.

(3) P = qV/t

Additionally, we know that current is the charge per second that passes through the circuit at any given point in time. 

(4) q = It

Where q = total charge and I = current (ampere)

Now, when we substitute (4) in (3), we understand that power is actually the current multiplied by time multiplied by voltage, divided by time. In this case, time gets canceled from the numerator and denominator, to give us the final equation, which is :

P = IV

Here, P is the power, V or voltage is the potential difference in the circuit, and I is the electric current.

Power can also be written as 

P=V2/R or I2R

Where V is the voltage, R is the resistance, and I is the electric current.

These can be obtained when we apply the Ohms law, which says that electric current is proportional to voltage and inversely proportional to resistance.


Electric Power Problems and Answers


Q1. A 280 V – 10 A electric lamp is used for 30 minutes. How much energy does it require?

A1. In the above question, 

V or Voltage = 280 Volt

I or electric power = 10 Ampere

T or Time = 30 minutes = 30 x 60 seconds = 1800 seconds

So, Electric power can be calculated.

P = IV

P = 10 x 280

P = 2800 Volt Ampere = 2800 Watt = 2800 Joule/second

However, electric energy is Electric power multiplied by time.

Electric Energy = P x t = 2800 Joule/second x 1800second = 5040000 Joule

= 5040 Joule

Q2. The energy used by the iron for 2 minutes is 18 kJ, at a voltage of 250 volts. How large is the current in the iron?

A2. Here, t = 2 minutes = 120 seconds

Energy = 18 Kilojoule = 18000 Joule

Voltage = 250 V

Electric power = P = W/t = 18000 Joule/120 seconds = 150 Watt

Therefore, Electric current (I) = P/V = 150 / 250 = 0.60 Ampere

FAQ (Frequently Asked Questions)

Q1. What are the Types of Electric Power?

There are three major types of electric power - active power, reactive power, and apparent power. 

  • Active Power: This type of electric power is also known as real power or actual power or true power. It is the power that is actually used in real life. We use this power in all kinds of gadgets and devices in the house. It is really transferred to the load, such as the induction motor or transformer or generator. The unit in which actual power is calculated in watts.

  • Apparent Power: The apparent power is that which is not used very often or lies idle. It is generally denoted by the letter S with its SI unit as volt-amps or VA.

  • Reactive Power: The power developed within the circuit reactance is known as reactive power (Q). It is measured in volt-ampere reactive (VAR)

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Q2. What are Some Prerequisites to Know Before Studying Electric Power?

What is essential for you to know while studying electric power is the concept of power, which is the formula of work done by time. Electric potential is the amount of work done in bringing a positive unit charge from infinity to a given point. So, the electric potential is the work done when divided by charge.  Another critical concept to keep in mind is that of current, which is the rate of flow of charge. We should remember that power is the potential difference multiplied by the current.