Which of the following is the correct representation of magnetic field lines?

If a magnetic needle is made to vibrate in uniform field $H$, then its time period is $T$. If it vibrates in the field of intensity $4H$, its time period will be:

A bar magnet of length $l$ and magnetic dipole moment $M$ is bent in the form of an arc as shown in the figure. The new magnetic dipole moment will be:

Figure shows two small identical magnetic dipoles $a$ and $b$ of magnetic moments $M$ each, placed at a separation $2d$, with their axes perpendicular to each other. The magnetic field at the point $P$ midway between the dipoles is:

The unit of pole strength is:
1. $\text{Am}^2$
2. $\text{Am}$
3. $\frac{\text{A}^2}{\text{m}}$
4. $\frac{\text{A}^2}{\text{m}^2}$

Two equal bar magnets are kept as shown in the figure. The direction of the resultant magnetic field, indicated by arrowhead at the point $P$ is: (approximately)

1.		2.
3.		4.

A vibration magnetometer placed in a magnetic meridian has a small bar magnet. The magnet executes oscillations with a time period of 2 s in the earth's horizontal magnetic field of 24 $\mu$T. When a horizontal field of 18 $\mu$T is produced opposite to the earth's field by placing a current-carrying wire, the new time period of the magnet will be:

1. 1 s

2. 2 s

3. 3 s

4. 4 s

Two identical bar magnets are fixed with their centres at a distance $d$ apart. A stationary charge $Q$ is placed at $P$ in between the gap of the two magnets at a distance $D$ from the centre $O$ as shown in the figure.