3 Marks Question

Question 13 Marks

Can we have magnetic hysteresis in paramagnetic or diamagnetic substances?

Answer

No, magnetic hysteresis is the lagging of intensity of magnetization (I) behind magnetising force (H). When diamagnetic and paramagnetic materials are placed in a magnetic field they get weekly magnetised. Also, they lose their magnetization as the magnetic field is removed (low retentively). Therefore, they do not form magnetic hysteresis curve.

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Question 23 Marks

What are the advantages of using soft iron as a core, instead of steel, in the coils of galvanometers?

Answer

The material used as a core in the moving coil galvanometer undergoes cycle of magnetization for long period. Therefore, low hysterisis loss is the first requirement for such material. In soft ron core, area under the hysteresis curve is small thus loss of energy is less as compared to steel. Further, it is easily magnetized by the magnetizing field, which increase the magnetic field and hence sensitivity of galvanometer.

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Question 33 Marks

The magnetic intensity Hat the centre of a long solenoid carrying a current of 2.0A, is found to be 1500A/m. Find the number of turns per centimetre of the solenoid.

Answer

$\text{B}=\mu_0\text{ni},$
$\text{H}=\frac{\text{B}}{{\mu_0}}$
$\Rightarrow\text{H}=\text{ni}$
$\Rightarrow1500\text{A/m}=\text{n}\times2$
$\Rightarrow\text{n}=750\text{turnes/meter}$
$\Rightarrow\text{n}=7.5\text{turnes/cm}$

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Question 43 Marks

A rod is inserted as the core in the current-carrying solenoid of the previous problem.

What is the magnetic intensity H at the centre?
If the magnetization I of the core is found to be 0.12Nm, find the susceptibility of the material of the rod.
Is the material paramagnetic, diamagnetic or ferromagnetic?

Answer

H = 1500A/m

As the solenoid and the rod are long and we are interested in the magnetic intensity at the centre, the end effects may be neglected. There is no effect of the rod on the magnetic intensity at the centre.

I = 0.12A/m

We know $\overrightarrow{\text{I}}=\text{X}\overrightarrow{\text{H}}$

X = Susceptibility

$\Rightarrow\text{X}=\frac{\text{I}}{\text{H}}=\frac{0.12}{1500}=0.00008=8\times10^{-5}$

The material is paramagnetic.

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Question 53 Marks

Why is it not possible to make permanent magnets from paramagnetic materials?

Answer

Permanent magnets are made from the material that are easily magnetized and retain the magnetization even reverse magnetizing field is applied (high coercivity). Paramagnetic materials get small magnetization, if they are placed in magnetic field, they lose their magnetization easily if the reverse field is applied. Hence, they are not used to make permanent magnet.

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Physics 3 Marks Question

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