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Work, Power and Energy

Work Power Energy > Important Physics GK [PDF]

Work, Power and Energy General Knowledge

Work, Power, and Energy-related important General knowledge and basic concept with the definition for UPSE, IAS, SSC CGL, MTS, Railway, Banking, and other competitive exams.

In general sense, work means any kind of physical and mental activity. Someone study very hard in his study room to get good marks in the examination. This is a mental activity. We always say that he is hard-working on the study. But in mechanics, work is not like that. In physics, it is defined by others away.

What is work in Physics?

In physics or mechanics, when a body gets displaced by application of force on it, we say that work is done.

How much work is done?

Work done is measured as the product of displacement of a body and the force applied to it due to which it displaced. Due to the application of force F, an object displaced at a distance s, work done is –

W = Fs ; Which is the formula of work done.

Force and displacement are vector quantities. Dot vector product of force F and displacement s gives us the amount of work done. The dot product of two vectors is always scalar. Thus work is a scalar quantity.

If the direction of force and displacement is different, i,e. the direction of force and displacement make an angle θ between them, then according to vector dot product rule, work done is governed by –

W = |F| × |s| cosθ ; |F| is the magnitude of force and |s| is magnitude of displacement.

Work done – examples

If displacement is zero, the work done is zero. A man pushing a wall and get tired, he is not doing any work since the wall is in a stable position.

example of work done on an object

Again we see that if the angle between force and displacement is 90° then cosθ = cos90° = 0. The work done is zero. Earth is moving around the sun. The direction of gravitational force and displacement of the earth is 90°. Hence no work is done on or by the earth.

Both force and displacement are the same directions, the work done is maximum i.e. W = FS. SI and C.G.S system the unit of work done is Joule and erg respectively.

Work done – example problems:

Problem 1> 5 N force is applied on a cube and moves 8 m along the direction of a force. What amount of work is done on the cube?

Problem 2> 50 N forces making an angle 60° applied on an object. Calculate the amount of work done.

Please comment on your answers below.

What is Energy in Physics?

Capacity of doing work by an object is known as energy.

The unit of energy and work done is the same. SI unit of energy is Joule (J) and c.g.s unit of energy is erg. More units of Energy here.

1 joule = 107 erg.

Forms of Energy

Energy can be classified into 8 types.

Mechanical energy: Mechanical energy arises on an object due to the motion or rest (not absolute rest). There are two types of mechanical energy. One is kinetic energy and the other is potential energy.

> Kinetic energy: When an object is in motion with respect to another object, it constitutes mechanical energy. A particle moving with constant velocity, v, it’s kinetic energy will be (1/2)mv2, where m is the mass of an object.

Formula of kinetic energy Ek = (1/2)mv2

 > Potential Energy: Capacity of doing work developed in a body due to its position or configuration is called its potential energy (PE). PE arises when a body is confined to a force field. As an example, when a body of mass, m kept at a height, h from the earth surface, it constitutes mgh amount of energy in the body.

Formula of potential energy Ep = mgh. where g = gravitational acceleration.

Electrical Energy: Electrical energy stored in the electric field produced by a charged particle around it. This energy derived charged particles from power stations to our home supplying electric current. Every day we used electric energy for our daily life. Electric bulb, fan, computer, air conditioner, etc use electric energy.

Electric Energy density, Uelc = (1/2)μE2 ; μ = permittivity; E = electric field

Magnetic Energy: Magnetic energy stored in the magnetic field produced by the magnet. When an iron’s pin is kept near to a magnet, the magnet attracts it. Why? Because the field stored some amount of energy that helps to attract the iron’s pin towards the magnet. A big bar magnet exists inside the earth.

Magnetic Energy density, Umeg = (1/2)εB2 ; ε = permeability and B = magnetic field

From electrodynamics, we know that a changing magnetic field produces an electric field and a changing electric field produces a magnetic field.

Chemical Energy: In a molecule, atoms are closely held together. How the atoms are bonded? Energy called chemical energy helps to create such a bond that helps atoms to hold close and form a molecule. Thus Chemical energy is defined as the energy stored in the bonds of chemical compounds. When water is mixed with lime, the heat produced. The chemical energy stored in the lime comes out as heat energy. A dry cell uses its chemical energy to produce electric energy.

Light Energy: Energy stored in the light is called light energy. Green plants use light energy in the photosynthesis process to stored light energy into chemical energy inside glucose. A solar cell uses solar energy to produce an electric current.

Sound Energy: Energy stored in the sound is called sound energy. When we speak, our vocal cord vibrates and energy flows as a sound wave. When we stand near to a big DJ speaker our clothes start to vibrate. Why does it happen? Because sound energy is responsible for creating such vibration.

Nuclear Energy: Energy stored in the nucleus of atoms is nuclear energy. Protons are positively charged. The same charges repel each other. In the nucleus of atom protons also repel each other. So the nucleus must not exist. But in nucleus protons are held together. Who is responsible to hold protons together in the nucleus? The answer is nuclear energy, which strongly holds all the particles inside the nucleus of atoms. We can exert nuclear energy from the nucleus of Uranium or Thorium in the nuclear power plant. In an atom bomb, a huge amount of nuclear energy releases when it blasts.

Gravitational Energy: The energy stored in the gravitational field is called gravitational energy. When we lift some object up, gravitational energy starts to store in the object as potential energy. Due to gravitational energy, all the objects having mass in the universe attract each other. Sun attracts earth, or earth attracts the moon due to gravitational energy stored in the gravity or gravitational field.

What Is Power in Physics?

The definition of work says nothing about the time during which work is done. Suppose you move at a distance by walking or running. But you feel more tiredness when you run. Both cases work is done by the same amount. Then why we get tired in the case of running. The answer is given by introducing power. In the case of running, you required more power and hence you get tired. Another example of power is – a strong boy can lift up 100 kg weight but weak boy can not. Because a strong boy can generate more power to lift the weight up than weak boy.. Power is a physical quantity which is defined as follows.

Rate of doing work is known as power. Simply rate of change of work done with respect to time is called power.

Thus Power, P = (Work done)/Time = W/t.

Power is a scalar quantity. Its SI unit is Watt. 1 watt = 1 Joule/second. In so-called machinery system unit of work is Horse Power (HP).

1 HP = 746 watt.

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