Imagine a world without any
electricity. Imagine your home now… no televisions, no air conditioners, no
light, no refrigerator, no machines, no mobile phones, no electronic gadgets to
assist you, no apps, no facebook, no twitter… The only form of electricity available
to us would be that of lightning.
Ohh.. what a boring life…
But life would be very close to
nature. Sun and fire will be the only source of light. Watching birds, animals
would be your favorite pass time. Crude handmade gadgets like magnetic compass,
hammer, knife would be your apps. Pigeon would serve as twitter, FB…
So my dear friend, electricity is
an integral part of our modern life. The study of electricity is necessary so
that such a vast knowledge is not lost overtime. It must be passed to the next
generation.
I am doing my duty as I am passing
the story to electricity to you..
Take a piece of copper metal. If
you view the copper at the microscopic level you would find a lot of empty
space with copper atoms suspended in space in the cloud of electrons. Most of
the electrons in the cloud are orbiting to some copper atoms or the other. Some
electrons in the cloud are wandering from one atom to the other, colliding with
other atoms and electrons. At this microscopic level, everything happens very
fast. Electrons are moving at 106m/s, atoms are vibrating very
rapidly with a frequency of ……..
Suddenly, in the macroscopic, outer
world, a human connected the metal plate to a battery.
In the inner atomic world, there is
a change in the scene now. The cloud of wandering electrons start moving towards positive terminal of the
battery with a very slow speed of 1mm/s. The number of wandering electrons are
however enormous. About 1022 in few grams of copper.
We call this flow of electrons as
electric current.
When such moving electrons are made
of pass through a filament of electric bulb, it heats up to approx 2700°C and
starts glowing.
This is how we use electricity of light a bulb. Watch the video from YouTube to understand electricity in a more detail.
Ohm's Law is a relationship between potential difference applied across a conductor to the electric current flowing in it. You must have experienced the following situations in daily life: (1) Fluctuations in voltage which leads to dimming or brightening of electric bulb. (This is the reason why we use stabilizers for ACs - to prevent any damage due to voltage fluctuations ) (2) While playing with LEDs, torch bulb, electric cells and connecting wires you must have observed that by increasing the number of cells in the circuit the LEDs glows brighter. (3) A 12V battery provides a large amount of current than a 3V battery. This suggests that there must be a relation between voltage (potential difference) and electric current. This relationship is known as Ohm's Law. Ohm's law was given by George Simon Ohm in the year 1827. This is a relation between potential difference (V) applied across the ends of a conductor to the electric current (I) flowing in the conductor According the Ohm's Law, the electric current flowing in a conductor is directly proportional to the potential difference (V) applied across the ends of a conductor provided the temperature of the wire remains the constant. Mathematically, I ∝ V or it can also be written as V ∝ I removing the proportionality sign and introducing a constant V = R I Where R is a constant known as the resistance of the given conductor.
Graph:
The V–I graph is a straight line that passes through the origin of the graph, as shown in Figure. Thus, \[\frac{V}{I}\] is a constant ratio.
Resistance Physically, resistance is defined as a property of a conductor to resist the flow of charge through it. The SI unit of resistance is ohm (Ω) (Greek symbol omega) A conductor is said to have a resistance of 1Ω when a potential difference of 1V is applied across the ends of a conductor and a current of 1A flows through it. \[R=\frac{V}{I}\] \[1Ω=\frac{1V}{1A}\] A fan regulator is actually a variable resistor. It changes the resistance of the circuit and thereby we can control the speed of the fan.
To test your knowledge on Ohm's Law, attempt the following test.
1. Name and state the law which relates the current in a conductor to the potential difference across a conductor and the current flowing through it.
2. Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?
3. When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.
4. (a) How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200 Ω? (b) How much current will an electric heater coil draw from a 220 V source, if the resistance of the heater coil is 100 Ω?
5. The potential difference between the terminals of an electric heater is 60 V when it draws a current of 4A from the source. What current will the heater draw if the potential difference is increased to 120 V? 6. The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below – I (amperes) 0.5 1.0 2.0 3.0 4.0 V (volts) 1.6 3.4 6.7 10.2 13.2 Plot a graph between V and I and calculate the resistance of that resistor.
7. Keeping the potential difference constant, the resistance of a circuit is doubled. By how much does the current change?
We know that force is a push or a pull which tends to change
the state of motion of an object i.e. if an object is initially at rest then an
applied force tends to bring it in motion and if an object is already in motion
then a force acting on it can bring the object at rest or change its speed or
change the direction of motion.
The relationship between force and motion can be best
understood with the Newton’s Laws of motion.
It will be better if we try to understand the Galileo’s law
of inertia first.
Law of Inertia:
The tendency of an object to resist any change in its state of rest or uniform
motion along a straight line is called inertia. In other words, if an object is
at rest it continues to stay at rest and if an object is in motion it continues
to remain in motion.
The measure of inertia is mass of an object. Higher the mass
of an object, higher is its inertia i.e. higher is its tendency to resist any
change in its state.
Inertia is of 3
types:
(1)Inertia
of rest
(2)Inertia
of motion
(3)Inertia
of direction
Newton’s First Law of
Motion:
Newton’s first law is simply the law of inertia or we can
say extension of law of inertia.
An object continues to be in a state of rest or uniform
motion in a straight line unless acted upon by an external unbalanced force.
The difference between Galileo’s law of inertia and Newton’s
first law of motion is that Galileo was not able to identify the cause or agent
that can bring the change in state of motion or rest. Whereas Newton’s identify
that unless an external force acts on an object its state cannot be changed.
Examples of Newton’s
first law:
(a)When
a bus starts suddenly from rest passengers tends to fall backwards.
(b)When
a moving bus stops suddenly passengers tends to fall forward.
(c)An
athlete continues to run further even after crossing the finishing line.
(d)When
a carpet is beaten with a stick dust particles comes out.
(e)If
we hit a striker on a pile of carom coins only the bottom most coin will move
with the striker.
Before we start second law of Newton let us discuss an
important physical quantity known as momentum.
Momentum: The
product of mass and velocity of an object is known as momentum. It is denoted
by letter p and it is a vector quantity.
P = mv
The momentum simply gives us the idea of quantity of motion
an object possesses. A bullet of small mass moving with very high velocity
possesses huge momentum because of its velocity. Whereas a truck moving with
slow speed possesses large momentum due to its huge mass.
Newton’s 2nd
Law
The first law of Newton’s identifies the force that it is
the cause of motion or change in state of motion. The Newton’s 2nd
law gives the mathematical expression for that force.
The rate of change of momentum of an object is directly
proportional to the applied unbalanced force in the direction of force.
Consider an object of mass m moving with velocity u. A
constant force F acts on an object for a small time t such that its velocity
increases from u to v.
\[Change\ in\ momentum = mv - mu \] \[Rate\ of\ change\ of\ momentum = \frac{mv - mu}{t}\] According to second law, the rate of change of momentum is directly proportional to the applied unbalanced force. \[\frac{mv-mu}{t}∝F\] \[F∝\frac{mv-mu}{t}\] \[F∝m\frac{v-u}{t}\] \[F∝ma\] \[F=kma\]
Where k is a constant. The unit of Force is so chosen such that the value of k becomes 1. Thus \[F=ma\]
The SI unit of force is newton (N). 1N is that force which when applied to a mass of 1 kg produces an acceleration of 1 m/\(s^2\) in it.
Examples
of Newton’s 2nd law
(a)In
cricket, while catching a fast moving ball, the fielder gradually lowers his
hands with the ball. This is done to increase the time in which the final
velocity of the ball becomes zero. This reduces the rate of change of momentum
thereby decreasing the force exerted by ball on the hands of the player.
(b)A
karate player moves his hands very fast while breaking a pile of tiles.
(c)Delicates
items are wrapped in thermocol.
(d)In
a high jump, the athletes are made to fall on a cushion bed.
Newton’s
Third Law of motion
For every action, there is equal and
opposite reaction.
It means when one object exerts a force
on another object then the second object instantaneously exerts equal and
opposite force on the first. These two forces are equal in magnitude but
opposite in direction.
The action and reaction forces act on
different object (not on the same object).
Examples
of Newton’s Third law
(1)When
a gun is fired the bullet moves in the forward direction and the gun recoils in
the opposite direction. Here gun applies force on the bullet in the forward
direction and the bullet apply equal and opposite force on the gun in the
backward direction.
(2)When
we walk on the road we apply force on the road in the backward direction and
the road pushes us in the forward direction.
(3)When
we jump we apply force on the ground in downward direction and the ground apply
equal and opposite force on us in the upward direction.
(4)During
the flight of a bird, the bird pushes the air downward with its wings and the
air applies equal and opposite force on the wings in the upward direction.
(5)While
swimming, we apply force on the water in the backward direction. It is reaction
force of water that pushes us in the forward direction.
