Q.Two identical bars, one of paramagnetic material and other of diamagnetic material are kept in a uniform external magnetic field parallel to it. Draw diagrammatically the modifications in the magnetic field pattern in each case.

Concept: Magnetic response of materials

Paramagnetic materials have unpaired electrons that align with the external field, slightly enhancing it inside the material. Diamagnetic materials develop induced magnetic moments opposing the external field, slightly weakening it inside.

When a paramagnetic bar is placed parallel to a uniform field $\vec{B}_0$, the field lines bend inward and crowd closer together as they pass through the bar, so the field just inside it is slightly stronger than outside — the bar behaves like a very weak magnet aligned with the applied field. …

Why the field pattern changes

When you place any material in an external magnetic field, the atomic magnetic moments respond. The nature of that response — whether the material's internal dipoles align with or against the applied field — determines how the field lines redistribute themselves.

Paramagnetic materials have unpaired electrons whose magnetic moments tend to align parallel to the external field $\mathbf{B}_0$. This alignment is weak (thermal agitation fights it) but real: the material develops a small net magnetization $\mathbf{M}$ in the same direction as $\mathbf{B}_0$. The result: the magnetic field inside the bar becomes slightly stronger than outside, $B_{\text{inside}} > B_0$. Field lines are "pulled" into the material.

Diamagnetic materials have all electrons paired, so there's no permanent magnetic moment. But the external field induces tiny current loops (Lenz's law at the atomic scale) that create a magnetization $\mathbf{M}$ opposing $\mathbf{B}_0$. The field inside weakens: $B_{\text{inside}} < B_0$. Field lines are "pushed away" and bow around the bar.

The key is relative permeability $\mu_r$:

• Paramagnetic: $\mu_r > 1$ (slightly, typically $1.00001$ to $1.01$)
• Diamagnetic: $\mu_r < 1$ (slightly, typically $0.99999$ to $0.9999$)

Field lines prefer the path of higher permeability and avoid lower permeability.

What the sketch should show, step by step

1. Start with the uniform external field. Draw evenly spaced, straight, parallel lines representing $\mathbf{B}_0$, pointing (say) to the right — this is the undisturbed field, far from either bar.

2. The paramagnetic bar, placed parallel to the field. Because $\mu_r > 1$, the bar is a slightly better path for magnetic flux than empty space. As the lines approach the bar's near end they bend inward and enter it, running closer together inside the bar than they did outside — a visibly denser bundle of lines through the bar's length. They spread back out to their original spacing after leaving the far end. …