Magnetic Field

Q. Three similar magnetic south poles, each of strength $10\text{Am}$ are placed at the corners of an equilateral triangle of side $20\text{cm}$. The magnetic force on one of the pole is:

1. $0.25\times {10}^{-3}\text{N}$
2. $2\times {10}^{-3}\text{N}$
3. $2.5\sqrt{3}\times {10}^{-4}\text{N}$
4. $5\times {10}^{-3}\text{N}$

Q. Which of these statements are wrong?

1. Magnetic field lines form closed loops.
2. Electric field lines form closed loops.
3. The tangent to magnetic field lines gives the direction of magnetic field at that point.
4. The closer the magnetic field lines the stronger is the magnetic field strength.

Q. The pole strength of $12$cm long bar magnet is $20$ A m having Magnetic moment $20\times 0.12A{m}^2$.The magnetic induction at a point $10$cm away from the centre of the magnet on its axial line is $[\frac{{\mu }_0}{4\pi }={10}^{-7}H{m}^{-1}]$

1. $2.20\times {10}^{-3}$ T
2. $1.17\times {10}^{-2}$ T
3. $1.17\times {10}^{-3}$ T
4. $2.20\times {10}^{-2}$ T

Q. At the centre of which of the following four circular rings is the magnetic field strongest, for equal magnitude of current?

1. $3$
2. $1$
3. $4$
4. $2$

Q. A very long magnet is placed vertically with one pole on the table. A neutral point was found at $20cm$ from the pole. What is the pole strength if the vertical component of earth's field is $0.4\times {10}^{-4}Wb{m}^{-2}$?

1. $16A-m$
2. $8A-m$
3. $4A-m$
4. None of these

Q. Derive an expression for magnetic induction at a point on the equitorial line of a bar magnet.

Q. Equal current I flows in two segments of a circular loop in the direction. Radius of the loop is r. The magnitude of magnetic field induction at the centre of the loop O is :

1. zero
2. $\frac{{\mu }_{0}I\theta }{4xr}$
3. $\frac{{\mu }_{0}I}{4xr}(\pi -\theta )$
4. $\frac{{\mu }_{0}I}{2xr}(\pi -\theta )$

Q. The magnetism is minimum ...................in a bar magnet.

1. near the ends
2. near the middle
3. at the ends
4. at the middle

Q. A part of a long wire a carrying a current i is bent into a circle of radius r as shown in figure. The net magnetic field at the centre O of the circular loop is

1. $\frac{{\mu }_{0}i}{4r}$
2. $\frac{{\mu }_{0}i}{2r}$
3. $\frac{{\mu }_{0}i}{2\pi r}(\pi +1)$
4. $\frac{{\mu }_{0}i}{2\pi r}(\pi -1)$

Q.

A bar magnet is placed with its north pole pointing towards north. The neutral point is at a distance $20cm$ from the center of the magnet. The net magnetic induction at a distance $10cm$ on the axial line of the magnet is:

1. $3B_H$
2. $7B_H$
3. $15B_H$
4. $17B_H$