## Video Lectures

Physics topics would be explained here by various videos.Videos are taken from youtube for the educational purpose only. If you find any problem with publishing the links to guide students feel free to write at abhishek@physicsclass.in

## Conduction of heat

Conduction is method of heat transfer without the motion of particles. Here particles vibrate and transfers energy through the conductor.

All matter is made up of atoms and molecules. These atoms are in different type of motion(translation, rotation, vibration ).motion of the molecule create heat or thermal energy.

Conduction is the transfer of heat between substances that are in direct contact.

The better the conductor more rapidly heat will be transferred.

## Convection of heat

Thermal energy is transferred  from hot places to cold places by convection. This occurs when liquid and Gases of warmer areas rises to cooler areas and replaces.this results in continuous circulation pattern .water boiling in a pan and heating of atmosphere are the examples of convecation.

## Pascal law

If there is a change in pressure at point inside a liquid then this change in pressure is equally distributed in all the directions.

hydraulic lifts and hydraulic  brakes are the applications of pascal law.

## Archimedes principle

It is said that great scientist archimedes discovered the principle while measuring the purity of the Crown of King. Here a animated story is taken from you tube.

If an object partially or completely immersed in a fluid then it experience an upward force (buoyant force) and it is equal to the weight of the fluid displaced.

## Right hand thumb rule

Prediction of direction of field (B), given that the current I flows in the direction of the thumb.

## Fleming's left hand rule

Whenever a current  carrying conductor comes under a magnetic field, there will be force acting on the conductor and on the other hand, if a conductor is forcefully brought under a magnetic field , there will be an induced current  in that conductor. In both of the phenomenons, there is a relation between magnetic field, current force. This relation is directionally determined by Fleming Left Hand rule and Fleming Right Hand rule respectively. 'Directionally' means these rules do not show the magnitude but show the direction of any of the three parameters (magnetic field, current, force) if the direction of other two are known. Fleming Left Hand rule is mainly applicable for electric motorand Fleming Right Hand rule is mainly applicable for electric generator. In late 19th century, John Ambrose Fleming introduced both these rules and as per his name, the rules are well known as Fleming left and right hand rule.

## Fleming right hand rule. ...

Fleming's right hand rule is applicable for electrical generators. As per Faraday law of electro magnetic induction  , whenever a conductor is moved in an electromagnetic field, and closed path is provided to the conductor, current gets induced in it.  According to Fleming right hand rule  the thumb, fore finger and middle finger of right hand are stretched perpendicular to each other as shown in the figure at right, and if thumb represents the direction of the movement of conductor, fore-finger represents direction of the magnetic field, then the middle finger represents direction of the induced current.

## Force on a moving charge in a magnetic field

• We know that force acting on any charge of magnitude q moving with velocity v inside the magnetic field B is given by
F=q(v X B)
and this is the magnetic force on charge q due to its motion inside magnetic field.
• If both electric field E and magnetic field B are present i.e., when a charged particle moves through a reagion of space where both electric field and magnetic field are present both field exert a force on the particle and the total force on the particle is equal to the vector sum of the electric field and magnetic field force.
F=qE+q(v X B)
• This force in equation is known as Lorentz Force.
• Where important point to note is that magnetic field is not doing any work on the charged particle as it always act in perpendicular direction to te motion of the charge
Motion of Charged Particle in The Magnetic Field

• As we have mentioned earlier magnetic force F="(vXB)" does not do any work on the particle as it is perpendicular to the velocity.
• Hence magnetic force does not cause any change in kinetic energy or speed of the particle.
• Let us consider there is a uniform magnetic field B perpendicular to the plane of paper and directed in downward direction and is indicated by the symbol C in figure shown below.

• Now a charge particle +q is projected with a velocity v to the magnetic field at point O with velocity v directed perpendicular to the magnetic field.
• Magnetic force acting on the particle is
F=q(v X B) = qvBsinθ
Since v is perpendicular to B i.e., angle between v and B is θ=90 Thus charged particle at point O is acted upon by the force of magnitude
F=qvB
and the direction of force would be perpendicular to both v and B
• Since the force f is perpendicular to the velocity, it would not change the magnitude of the velocity and the peffect of this force is only to change the direction of the velocity.
• Thus under the action of the magnetic force of the particle will more along the circle perpendicular to the field.
• Therefore the charged particle describe an anticlockwise circular path with constant speed v and here magnetic force work as centripetal force. Thus
F=qvB=mv2/r
where radius of the circular path traversed by the particle in the magnetic in field B is given as
r=mv/qB
thus radius of the path is proportional to the momentum mv pof the charged particle.
• 2πr is the distance traveled by the particle in one revolution and the period T of the complete revolution is
T=2 πr /v
From equation
r/v=m/qB
time period T is
T=2πm/qB
and the frequency of the particle is f="1/T=qB/2πm"
• From equation we see that both time period and frequency does not dependent on the velocity of the moving charged particle.
• Increasing the speed of the charged particle would result in the increace in the radius of the circle. So that time taken to complete one revolution would remains same.
• If the moving charged particle exerts the magnetic field in such a that velocityv of particle makes an angle θ with the magnetic field then we can resolve the velocity in two components

vparallel : Compenents of the velocity parallel to field
vperpendicular :component of velocity perpendicular to magnetic field B
• The component vpar would remain unchanged as magnetic force is perpendicular to it.
• In the plane perpendicular to the field the particle travels in a helical path. Radius of the circular path of the helex is

r=mvperpendicular/qB=mvsinθ/qB

## force on current carrying conductor in magnetic field

ersted's experiment shows that a current carrying wire exerts a force on a magnetic needle and deflects it from its usual north-south position. The reverse must also be true, which was proved by the French scientist Andre Marie Ampere, who suggested that a magnet must also exert an equal and opposite force on the current carrying conductor. The above mentioned concept can be best understood by way of a demonstration as explained below.

 Procedure Observation A small aluminium rod AB (5 cm in length) is connected to the wires and suspended horizontally as shown in the fig A strong horse-shoe magnet is placed in such a way that the magnetic field is directly upwards and is placed vertically The rod AB gets displaced. The rod AB is connected in series to a battery, a key and a rheostat Switch on the current

 Procedure Observation Repeat the experiment by changing the direction of flow of current. The rod AB gets displaced in the reverse direction.

 Procedure Observation Repeat the experiment by reversing the direction of magnetic field. The rod AB gets displaced in the reverse direction.

Conclusion :

A current carrying conductor experiences a force when placed in a magnetic field. The direction of force is reversed when the direction of current in the conductor is reversed.

The force acting on the current-carrying conductor can be changed by changing the direction of the magnetic field.

## Click on the link above to view the animation

Faraday's laws of of electromagnetic induction explains the relationship between electric circuit and magnetic field. This law is the basic working principle of the most of the motors, generators, transformers, inductors etc.

Whenever a conductor is placed in a varying magnetic field an EMF gets induced across the conductor (called as induced emf), and if the conductor is a closed circuit then induced current flows through it.
Magnetic field can be varied by various methods -
1. By moving magnet
2. By moving the coil
3. By rotating the coil relative to magnetic field

Faraday's second law of electromagnetic induction states that,  the magnitude of induced emf is equal to the rate of change of flux linkages with the coil. The flux linkages is the product of number of turns and the flux associated with the coil.

Consider the conductor is moving in magnetic field, then
flux linkage with the coil at initial position of the conductor = NΦ1  (Wb) (N is speed of the motor and Φ is flux)
flux linkage with the coil at final position of the conductor = NΦ2      (Wb)
change in the flux linkage from initial to final = N(Φ1 - Φ2
let  Φ1 - Φ2 = Φ
therefore, change in the flux linkage = NΦ
and, rate of change in the flux linkage = NΦ/t
taking the derivative of RHS
rate of change of flux linkages = N (dΦ/dt)

According to Faraday's law of electromagnetic induction, rate of change of flux linkages is equal to the induced emf

So, E = N (dΦ/dt)  (volts)

### Phenomenon Of Mutual Induction

Alternating current flowing in a coil produces alternating magnetic field around it. When two or more coils are magnetically linked to each other, then an alternating current flowing through one  coil causes an induced emf across the other linked coils. This phenomenon is called as mutual induction.

### Lenz's Law

Lenz's  law of electromagnetic induction states that, when an emf is induced according to Faraday's law, the polarity (direction) of that induced emf is such that it opposes the cause of its production.

Thus, considering Lenz's law

E = -N (dΦ/dt)  (volts)

The negative sign shows that, the direction of the induced emf and the direction of change in magnetic fields have opposite signs.