Electric Power and its Concepts

Key Concepts

1. Energy

2. Power

3. Work done

4. One watt

5. One joule

6. Electrical power

Introduction

“A fan of 260 W uses 260 J of electrical energy every second and produces 260 J of wind and heat energy”. In this session, you are going to learn about the meaning of this statement. We are going to read about power, energy, and the formula which relates power, work done and time taken. 

Explanation:

Power 

It is the rate of doing work. Work is done when the three conditions are satisfied 

1. Force is applied  

2. Displacement is there 

3. Force is not perpendicular to the displacement  

The SI unit of work is Joule (J). One Joule is the unit for measuring energy, named for the British scientist James Prescott Joule (1818–1889). 

Power = Work done/ time taken 

Power and Energy 

Power is also defined in terms of energy. It is defined as the rate of change in energy. Energy is the ability to do work. Energy and work done both have the same unit, that is Joule (J).  

There are different forms of energy. One of them is electrical energy.  

The rate of transformation of energy is also defined as power.  

Power = Change in energy/ time taken 

Mathematically,  

P= F.ds/Δt= W/t

Figure 2: Relation between power, work done and time taken 

After the Scottish inventor James Watt (1736–1819). SI unit of power is Watt (W). 

1 W

=1 J1 s=1 J1 s

One Joule (J) of energy transformed in one second (s) is called one Watt (W) of power. A few prefixes of Watt are used as multiples and submultiples of SI unit such as: 

1 microwatt = 1 μW =

1 × 10−61 × 10−6

J 

1 milliwatt = 1 mW =

1 × 10−31 × 10−3

J 

1 kilowatt = 1kW=

1 × 1031 × 103

J 

1 horsepower = 746 Watt 

Electrical Power 

It is the rate of change in electrical energy in other forms of energy. The law of conservation of energy is followed. The law of conservation of energy states that energy is neither created nor destroyed. It can only be converted from one form to another. 

Such as, a fan of 260 W uses 260 J of electrical energy every second and produces 260 J of wind and heat energy. 

Questions and answers 

Question 1: What is the electrical power conversion in the case of a 60 W bulb? 

Answer: 

A 60 W bulb uses 60 J of light energy per second and produces 60 J of light energy.  

Question 2: Convert the following into SI unit of power.  

4 μW, 2 mW, 3 kW 

Answer: 

A few prefixes of Watt are used as multiples and submultiples of SI unit such as: 

4 microwatt = 4 μW = 4

 × 10−6 × 10⁻⁶J 

2 milliwatt = 2 mW =

2 × 10−32 × ¹⁰⁻³J 

3 kilowatt = 3 kW=

3× 1033× 10³J

Summary

• Power is the rate of doing work or conversion of energy.

• Electric power is the rate of change in electrical energy into other forms of energy.

• SI unit of power is Watt (W).

• One joule is the ability of a body to displace an object by one meter by applying a force of one newton.

• One joule (J) of energy transformed in one second (s) is called one watt (W) of power.