Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

WORK, ENERGY AND POWER
(A) WORK, ENERGY AND POWER, THEIR MEASUREMENTS AND UNITS

WORK
In our daily parlance, the word 'work' is used for some sort of exertion
(physical or mental) or for various activities such as writing, reading or
eating; we say that we are doing work
...
Work is said to be done only when a body moves under
the influence of a force
...


MEASUREMENT OF WORK
The amount of work done on a body depends on two factors:
(i) the magnitude of the force applied, and
(ii) the displacement produced by the force
...
e
...


Work is a scalar quantity
...
If under some circumstances, a force displaces the body

in a direction other than the direction of force, then we can determine the
amount of work done by the force in either of the two ways:
(1) by finding the component of displacement of the body in the direction of
force, or

(2) by finding the component of force in the direction of displacement
...


Special cases
Case (i): If the displacement is in the direction of force, i
...
, θ = 0° deg then
cos 0° = 1
W=FxS
The work done is positive
...
e
...

W=0
Hence the work done is zero
...
e
...

The work done is negative
...
Therefore, the
work done by the force of gravity on the body is
W = FS = mgh
Thus, work done by the force of gravity is same whether the body comes
down from a certain height using stairs or slope or a lift
...
I
...
I
...
It is abbreviated as J
...


Bigger units of work are kilo-joule (kJ), mega-joule (MJ) and giga-joule (GJ),
where 1 kJ = 10³ J, 1 MJ = 10⁶ J and 1 GJ = 10⁹ J
...
G
...
unit: The C
...
S
...

Define 1 erg
1 erg of work is said to be done when a force of 1 dyne displaces a body
through a distance of 1 cm in its own direction
...

Power is a scalar quantity
...

If work W is done in time t, then
Power P = Work done W / Time taken t
Or P = W/t
P =F *v

Thus power spent by a source depends on the following two factors:
(1) the amount of work done by the source, and
(2) the time taken by the source to do the said work
...
I
...
I
...

If 1 joule of work is done in 1 second, the power spent is said to be 1 watt
...
e
...
G
...
Unit: The C
...
S unit of power is erg per second (erg s-1)
...
It is related to the S
...
unit watt as below:
1 H
...
= 746 W = 0
...

The energy of a body is its capacity to do work
...

(1) The S
...
unit of energy is joule (J) and the C
...
S
...


(2) The bigger units of energy are:
(i) Watt hour: One watt hour (1 Wh) is the energy spent (or work done) by a
source of power 1 W in 1 h, i
...
,
1 watt hour (Wh) = 1 watt x 1 hour
= 1 J s–¹ x 3600 s
= 3600 J = 3
...
i
...
,
1 kilowatt hour (kWh) = 1 kilowatt × 1 hour
= 1000J s-¹ x 3600s
= 3
...
6 MJ

(3) Calorie: Heat energy is usually measured in calorie
...

It is related to joule as 1 J = 0•24 calorie or 1 calorie = 4•18 J
...

1 eV is the energy gained by an electron when it is accelerated through a
potential difference of 1 volt
...
It is in two forms:
(1) potential energy, and
(2) kinetic energy
...

It is usually denoted by the symbol U
...
The potential energy possessed by a body due to the force of
attraction of Earth on it, is called its gravitational potential energy
...
On removal of
the force, it regains its original form due to the property of elasticity
...


GRAVITATIONAL POTENTIAL ENERGY AT A HEIGHT (U = mgh)
The work done W on the body in lifting it to a height h is

W = Force of gravity (mg) x displacement (h) = mgh
Gravitational potential energy U = mgh

KINETIC ENERGY (K)
The energy possessed by a body due to its state of motion is called its
kinetic energy
...


Examples: A fast moving stone has the capacity of breaking a window pane
on striking squ it because it has kinetic energy
...
have kinetic energy
...
It can be
calculated by finding the amount of work needed to be done by an opposing
force to stop the body
...
Then its
kinetic energy K = 1/2 m v²
momentum p = mv

Work Energy Theorem

When a force is applied in the direction of motion of a body, it accelerates
the motion and thus increases the kinetic energy of the body
...


Forms of kinetic energy
A body can have three types
(1) Translational kinetic energy: The motion of a body in a straight line_path
is called translational motion and the kinetic energy of the body due to
motion in a straight line is called translational kinetic energy
...


(3) Vibrational kinetic energy: If a body moves to and fro about its mean
position, the motion is called vibrational motion
...


CONVERSION OF POTENTIAL ENERGY INTO KINETIC ENERGY
Potential energy changes into kinetic energy whenever it is put to use
...
When the hammer is made to fall on a nail fixed
upright on a wooden piece, the nail begins to penetrate
...


(2) A wound up watch spring has elastic potential energy which is equal to
the work done in bringing it to the wound-up state
...


DIFFERENT FORMS OF ENERGY
Nature has provided us energy in various forms namely, (1) solar energy, (2)
heat (or radiant) energy, (3) light energy, (4) chemical (or fuel) energy, (5)
hydro energy, (6) electrical energy, (7) nuclear energy, (8) geo-thermal
energy, (9) wind energy, (10) sound energy, (11) magnetic energy, and (12)
mechanical energy, etc
...

Solar energy cannot be used directly to perform work, because it is too
diffused and it is not always available uniformly
...

have been invented to make use of solar energy
...

(ii) A solar furnace has a large aperture parabolic mirror which focuses the
sun rays on a small area so that a high temperature is achieved
...

(iii) Solar cells are devices used to convert solar energy directly into
electrical energy
...
However, the efficiency of a solar cell to convert solar energy into
electrical energy is very low
...
The steam obtained on heating water possesses heat
energy and it has the capacity to do work
...


(3) Light energy: It is a form of energy in the presence of which other
objects are seen
...
Moon reflects
the sunlight at night
...
also give light energy along with heat energy
...
These fuels contain chemical energy stored in them
...


(5) Hydro energy: The energy possessed by fast moving water is called
hydro energy
...
For this, water is stored in dams built across the rivers high
up in the hills and the water from the dams is allowed to flow down through
channels
...


(7) Nuclear energy: The energy released due to loss in mass during the
processes of nuclear fission and fusion, is called nuclear (or atomic)
energy
...


(8) Geo thermal energy: The energy released in nuclear disintegrations in
the interior of Earth gets stored deep inside the Earth and is called geo
thermal energy
...


(9) Wind energy: The energy possessed by the fast moving air is called wind
energy
...
In rural areas for past

many years, wind mills were used to pump out the underground water and
to grind grains
...
It is sensed
by our ears
...


(11) Magnetic energy: The energy possessed by a magnet due to which it
can attract iron filings, is called magnetic, energy
...


(12) Mechanical energy: The energy possessed by a body due to its state of
rest or of motion, is called mechanical energy
...
A body at a height, a moving body, a stretched
bow, etc
...


CONVERSION OF ONE FORM OF ENERGY INTO THE OTHER FORM
In our daily life, we require energy in various forms
...

Now let us consider some more examples of conversion of one form of
energy into another form
...
When water
flows down, its potential energy decreases and kinetic energy increases
...


(2) Electrical energy to mechanical energy
In an electric motor, when an electric current is passed in a coil, freely
suspended (or pivoted) in a magnetic field, a torque acts on the coil due to
which it rotates
...


(3) Electrical energy to heat energy
In electric appliances such as heater, oven, geyser, toaster, etc
...


(4) Heat energy to electrical energy
In a thermocouple, when two junctions of two different metals are kept at
different temperatures (one junction is kept hot, while the other cold), a
current flows in the thermocouple
...


(5) Electrical energy to sound energy
A loudspeaker when in use, receives electrical energy in form of electrical
signals from the microphone and changes it into sound energy
...


(6) Sound energy to electrical energy
A microphone converts the sound energy into electrical energy in form of
varying electric signals
...


(8) Chemical energy to electrical energy
When current is drawn from an electric cell, the chemical energy stored in it
changes into electrical energy
...
Similarly in a kerosene lamp when the
oil soaked in its wick burns, the chemical energy changes into light energy
...


(11) Electrical energy to light energy
When an electric bulb glows on passing an electric current through it, the
electrical energy changes into heat and light energies
...

In a solar cell, light (or solar) energy changes into electrical energy
...


(14) Chemical energy to heat energy
When fuel such as wood, coal, bio-gas, etc
...
In a steam engine, the chemical energy of coal on
burning changes into the heat energy of steam
...


(15) Chemical energy to mechanical energy
In automobiles, while in motion, the chemical energy of petrol (or diesel)
changes into mechanical energy (or kinetic energy)
...
Thus, electrical
energy changes into magnetic energy
...


(18) Mechanical energy to heat energy
When water falls from a height, the potential energy stored in water at that
height changes into the kinetic energy of water during the fall
...


(C) CONSERVATION OF ENERGY

PRINCIPLE OF CONSERVATION OF ENERGY
The principle of conservation of energy is one of the fundamental principles
of nature
...
It only changes from one form to another
...
The sum of all forms of
energy in the universe remains constant
...
e
...
e
...


THEORETICAL VERIFICATION OF K + U = CONSTANT FOR A FREELY
FALLING BODY
Let a body of mass m be falling freely under gravity from a height h above
the ground (i
...
, from position A in Fig
...
At each point of motion, the sum of
potential energy and kinetic energy remains unchanged
...
Then u = 0, S = x, a = g
From equation v² = u² + 2aS
V12 = 0 + 2gx = 2gx

◆ Kinetic energy K = 1/2 mv12
= 1/2 m x (2gx) = mgx

At position C (on the ground):
Let the velocity acquired by the body on reaching the ground be v
...
When a body is whirled in a circle, the work done by it is:

(a) Positive
(b) negative
(c) zero
(d) infinite

2
...
One electron volt is equivalent to:

(a) 1
...
6 x 10¹⁰ J
(c) 2
...
6 J

4
...
If 10 N of force is applied to an object, but the object does not move, then
how much work being done by the force?

(a) Zero
(b) 10 J
(c) 20 J
(d) 10 N

6
...
Water stored in a dam possesses :

(a) no energy
(b) electrical energy
(c) kinetic energy
(d) potential energy

8
...
A horse does 5000 J of work in 100 s
...
An aeroplane is flying at an altitude of 10,000 m at a speed of 300 km/hr
...
When the speed of a moving object is doubled, then its kinetic energy:

(a) remain same

(b) decreases
(c) is doubled
(d) becomes four times

12

---