In daily life, from moving to completing a task, humans keep on getting some work done all the time. However, how to define the term ‘work’ in Physics? A man is walking, is this Work? Someone pushing a wall, will this be called a work done? Well, in real life, perhaps, both the examples look like the cases where some work is getting done, but in Physics, we only take the Work is done when some displacement is shown. Hence, if a man walks a certain distance, some work is done, but if someone is trying to push a wall, no work is done in this case as no displacement is seen.

### Work

Work can be explained applying force along some displacement. In simple words, work is force times displacement. The SI unit of work is joules. There can be different ways as to how force is being applied on an object, for example, force can be applied in the direction of displacement, this is known as positive work. When force is applied opposite to the direction of displacement, the respective force is known as negative force.

W = F × dWhere, W = work done

F = Force applied

d = displacement

There are cases when the work done is not linear but at certain angle, in those cases, the angle also comes in the picture and the formula slightly changes. For instance, if the force is applied at an angle θ above the ground, the formula for work becomes,

**W = F.d cosθ**

### Energy

The term often confuses in physics since the definition of energy is “The ability to do work”, The definition may look confusing but is very easy to understand. When someone says “they do not have enough energy to do work” this statement can be understood as the definition itself. Energy is that ability that helps in applying force in order to do some work. It is simply that force that causes things to move **“The capacity to do work is known as Energy”**

One very important fact about energy to note is that even though, energy occurs in many forms and has so many types, from kinetic energy to potential energy to solar energy, etc,** Energy can never be created nor destroyed, it can only be transferred from one form to another.**

The SI Unit of energy is Joules. Apart from joules, other units of Energy are ⇢ Calorie, Horsepower, Kilowatt (kW)-Power, Kilowatt-hour (kWh).

### What is the Commercial Unit of Energy?

Energy is associated with electricity, commonly known as Electrical energy, Its unit is watt. However, when a large amount of this energy is required, which is, in large industries and colonies, it is measured in kWh (Kilowatt hour). One Kilowatt-hour can be defined as the energy produced by an object/machine in one hour when working at a very constant rate of 1 kW.

The other smaller units are watt-hour and horsepower, but **The SI unit of energy is Joules and the Commercial unit of energy is kilowatt-hour (kWh). **Energy is a scalar quantity.

Note: Board of Tradeunit is a famous term used for 1 kWh

**Relationship between the SI unit and the Commercial unit of energy**

Since the units are defined differently in order to easily explain the smaller and larger amount at the same time. The units of the same quantity can be converted into each other and are always related to each other through some factor.

The commercial unit of energy is separately defined and explained as Electricity is a very big part of human life, and technology as a whole depends all on electricity, Humans look around and everything they see work on Electricity, hence, it is necessary to provide a separate unit in order to measure very large amounts of energy.

The Commercial unit of energy is related to the SI unit in the following manner,

The unit Watt is one of the units of energy used to define smaller amount of electrical energy, It is related to The SI Unit as,

1 Watt = 1Joule/sec

Now, 1kWh = 1000Wh

It can also be written as,

1 kWh = 1000 W × 1h

1kWh = 1000 W × 3600seconds

1kWh = 1000 (Joules/sec) × 3600 seconds

1kWh = 3600000 Joules

1kWh = 3.6 × 10^{6}JoulesHence, there is 3.6 × 10

^{6}joules of energy in 1kWh

### Sample Problems

**Question 1: How much kWh of energy is present in 1Joule?**

**Solution:**

It is known that, 1kWh = 3.6× 10

^{6 }JoulesUsing Unitary method,

1 Joule = 1/3.6×10

^{6}(kWh)1Joule = 2.77× 10

^{-7 }kWh

**Question 2: How is Power and Energy-Related to Work?**

**Answer:**

Power is defined as the rate of doing Work

P= Work done/Time (J/s)

Energy is defined as the capability or the ability to do work.

**Question 3: An Electric Iron of 1000W is used for 1 Hour every day. Find the Electrical Energy Consumed by the Iron in a week?**

**Solution:**

1 week= 7day= 7 days

1hour every day= 7 Hours in total.

Power consumed in 1 hour= 1000W

Energy = Power × Time

Energy = 1000W × 7 Hr

Energy = 1kW × 7Hr

Energy = 7 kWh

Hence, The Electrical energy consumed by the electric iron in a week is 7kWH.

**Question 4: Find the resistance of an Electric bulb marked as 220V, 120W.**

**Solution:**

In Electrical bulb, Power is given as,

P = V

^{2}/RWhere, P= Power, V=Voltage, R= Resistance

R = V

^{2}/PR = 220

^{2}/120R = 403.33 ohms

**Question 5: An Electric Iron Converts,**

**1. Electrical energy into mechanical energy**

**2. Electrical energy into potential energy.**

**3. Electrical energy into Kinetic energy.**

**4. Electrical energy into Heat energy.**

**Answer:**

The job of an Electric iron is to produce heat in order to iron clothes. Hence, Electric Iron converts Electrical energy into Heat Energy. Hence, Option 4 is the correct answer.

**Question 6: A bulb rated as 220V, has a resistance of 10ohms. Is turned on for half an hour every day for a month. How much amount of electrical energy is consumed in that month, answer in terms of a** **commercial unit of energy.**

**Solution:**

Power consumed by electric bulb= V

^{2}/RPower= 220

^{2}/10Power= 4840W

Time for which the bulb glowed,

T= 1/2× 30× 24= 15× 24= 360 hrs

Energy is defined as,

Energy= Power × Time

Energy= 4840W × 360hr

Energy = 4.8kW × 360hr

Energy = 1728 kWh