# CBSE Class 12 Physics Notes Chapter 4 Moving Charges and Magnetism

## What is Lorentz Force?

When the total force on a charge c which is moving with a velocity v in the presence of electric field E and magnetic field B is termed as the lorentz force.

F= q(v * B + E) (This acts normal to v and the work done by it is zero)

### Cyclone Frequency

A charge c executes a circular orbit on a plane normal to B which is the uniform magnetic field. This frequency of uniform circular motion is called as cyclone frequency. This frequency is unaffected by the radius and speed of the particle. It can be determined with the help of a machine known as cyclotron which is used to accelerate the particles which are charged.

### Biot-Savart law

According to this law, the magnetic field dB due to the presence of an element dl which is carrying a continuous flow of current I at the point P at a distance r from the current element is to obtain a total field at P and integration of this vector expression over the entire length of the conductor.

### Ampere’s Circuital Law

According to this law, an open surface S if bounded by a loop C, then the current I passing through S is determined by

$\oint_{c}B.dl = \mu _{0}I$

The sign of I is determined by the right-hand rule. Non-parallel currents repel and parallel currents attract each other.

### Important Questions

1. Calculate the magnetic field at the coil’s center if the radius and current flowing in a circular coil of wire having 200 turns are 20 cm and 2 A respectively.?
2. Examine magnitude of the field at a point 60 cm from the wire if the Current flowing in a straight wire is 45 A.?
3. Find out the direction and magnitude at a point 6 m east of the wire in which the current of 70 A is flowing and is passing in the north and south direction.?