# Moving Charges and Magnetism Class 12 Notes Chapter 4

**1. The space** in the surroundings of a magnet or a current-carrying conductor in which its magnetic influence can be experienced is called magnetic field. Its SI unit is Tesla (T).

**2. Oersted experimentally** demonstrated that the current-carrying conductor produces magnetic field around it.

When key K is closed, then deflection occurs in the compass needle and vice-versa,

**3. Biot-Savart’s Law** According to this law, the magnetic field due to small; current-carrying element dl at any nearby point P is given by

**4.** The relationship between μ_{0}, ε_{0} and c is

where, c is velocity of light, ε_{0} is permittivity of free space and μ0 is magnetic permeability.

**5.** Magnetic field at the centre of a circular current-carrying conductor/coil.

**6.** Magnetic field at the centre of semi-circular current-carrying conductor.

**7.** Magnetic field at the centre of an arc of circular current-carrying conductor which subtends an angle 0 at the centre.

**8.** Magnetic field at any point lies on the axis of circular current-carrying conductor

**9. Magnetic field** due to straight current-carrying conductor at any point P at a distance r from the wire is given by

**10.** The following figure shows the graphical representation of variation of B with distance from straight conductor.

**11. Ampere’s Circuital Law** The line integral of the magnetic field B around any closed loop is equal to μ_{0} times the total current I threading through the loop, i.e.

Magnitude of magnetic field of a straight wire using Ampere’s law

**12. Maxwell introduced** the concept of displacement current.

**13. Magnetic Field due to a Straight Solenoid**

(i) At any point inside the solenoid,

B = μ_{0}nI

where, n = number of turns per unit length.

(ii) At the ends of the solenoid,

B = 1/2 μ_{0}nI

**14. Magnetic Field due to Toroidal Solenoid**

(i) Inside the toroidal solenoid,

B =μ_{0}nI, here, n =N/2πr ,N= total number of turns

(ii) In the open space, interior or exterior of toroidal solenoid,

B= 0