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Q. No.A small permanent magnet is placed 'antiparallel' to a uniform magnetic field \(B.\) A null point is found at a distance \(r,\) on the axis of the magnet. Then, \(r\) is proportional to (nearly):
1. \(B^{-3}\)
2. \(B^{-2}\)
3. \(B^{-1/2}\)
4. \(B^{-1/3}\)
Subtopic: Bar Magnet |
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Two small paramagnetic samples are placed in an (otherwise uniform) strong external magnetic field \(B.\) If the two samples are placed with a separation that is along the direction of the external field \(B,\) then, the force exerted by the two samples on each other is:
| 1. | attractive. |
| 2. | repulsive. |
| 3. | zero. |
| 4. | any of the above depending on the external field \(B\) and the sample separation. |
Subtopic: Magnetic Materials |
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A current carrying circular loop of wire is placed in a magnetic field \(B\), which makes an angle \(\theta\) with the normal to the loop. The radius of the loop is \(r\), and the loop carries a current \(i\). The magnetic interaction energy of the current carrying loop is \(E_B\) and the torque on the loop has the magnitude \(\tau_B\).
Which of the following, is independent of \(\theta?\)
1. \(E_B\cdot\tau_B\)
2. \(\frac{E_B}{\tau_B}\)
3. \(E_B^2+\tau_B^2\)
4. \(E_B^2-\tau_B^2\)
Which of the following, is independent of \(\theta?\)
1. \(E_B\cdot\tau_B\)
2. \(\frac{E_B}{\tau_B}\)
3. \(E_B^2+\tau_B^2\)
4. \(E_B^2-\tau_B^2\)
Subtopic: Analogy between Electrostatics & Magnetostatics |
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Two short bar-magnets are placed at large distance from each other, with their axes aligned along the same line but in opposition direction to each other. A null point is formed between them at a distance \(r_1\) from the first magnet and \(r_2\) from the second. If the dipole moments of the magnets are \(P_1\) and \(P_2\), then:
1. \(\dfrac{r_1}{r_2}=\dfrac{P_1}{P_2}\)
2. \(\left(\dfrac{r_1}{r_2}\right)^2=\dfrac{P_1}{P_2} \)
3. \(\left(\dfrac{r_1}{r_2}\right)^3=\dfrac{P_1}{P_2} \)
4. none of the above is true.
1. \(\dfrac{r_1}{r_2}=\dfrac{P_1}{P_2}\)
2. \(\left(\dfrac{r_1}{r_2}\right)^2=\dfrac{P_1}{P_2} \)
3. \(\left(\dfrac{r_1}{r_2}\right)^3=\dfrac{P_1}{P_2} \)
4. none of the above is true.
Subtopic: Bar Magnet |
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A sample of magnetic material is placed in a magnetic field. As the magnetic field is increased, its magnetisation increases and finally, it becomes constant — even if the field is increased further.
Now, the temperature of the sample is decreased. The magnetisation:
Now, the temperature of the sample is decreased. The magnetisation:
| 1. | increases |
| 2. | decreases |
| 3. | remains unchanged |
| 4. | decreases first and then increases |
Subtopic: Magnetic Materials |
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Assume that the magnetic field of the earth is due to a small magnetic dipole, placed at the centre of the earth (radius: \(R\)). The magnetic field at the equator is \(B_e.\) The dipole moment of the dipole is:
1. \(\frac{\pi}{\mu_0}\left(B_eR^3\right )\)
2. \(\frac{2\pi}{\mu_0}\left(B_eR^3\right )\)
3. \(\frac{4\pi}{\mu_0}\left(B_eR^3\right )\)
4. \(\frac{2}{\mu_0}\left(B_eR^3\right )\)
1. \(\frac{\pi}{\mu_0}\left(B_eR^3\right )\)
2. \(\frac{2\pi}{\mu_0}\left(B_eR^3\right )\)
3. \(\frac{4\pi}{\mu_0}\left(B_eR^3\right )\)
4. \(\frac{2}{\mu_0}\left(B_eR^3\right )\)
Subtopic: Bar Magnet |
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When a diamagnetic material is placed in a magnetic field, the field strength within the material is:
| 1. | increased |
| 2. | decreased |
| 3. | unchanged |
| 4. | fluctuating with time: first increasing and then decreasing |
Subtopic: Magnetic Materials |
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The coercive force for a certain magnet is \(3 \times 10^3 ~\text{A/m}\). This magnet is placed within a solenoid having \(40~\text{turns/cm}\). What current should be passed through the solenoid so that the magnet is demagnetised?
1. \(0.75~\text{A}\)
2. \(75~\text{A}\)
3. \(1.33~\text{A}\)
4. \(133~\text{A}\)
1. \(0.75~\text{A}\)
2. \(75~\text{A}\)
3. \(1.33~\text{A}\)
4. \(133~\text{A}\)
Subtopic: Magnetization & Magnetic Intensity |
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A ferromagnetic material consists of domains in which the magnetic moments of the atoms are in the same direction within each domain. However, the domains are randomly oriented. A ferromagnetic material is placed in an external magnetic field. Then,
| 1. | all the domains grow in size. |
| 2. | all the domains shrink in size. |
| 3. | some domains grow in size, others shrink. |
| 4. | domains rotate in the magnetic field. |
Subtopic: Magnetic Materials |
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The magnetic field, at a point 10 cm away, from a short bar magnet is \(3 \times 10^{-4}\) T, when the magnet is placed in an end-on position. If the magnet is in a broadside-on position, the field will be:
1. \(6 \times 10^{-4}\) T
2. \(1.5 \times 10^{-4}\) T
3. \(3 \sqrt2 \times 10^{-4}\) T
4. \({3 \over \sqrt 2}\times 10^{-4}\) T
1. \(6 \times 10^{-4}\) T
2. \(1.5 \times 10^{-4}\) T
3. \(3 \sqrt2 \times 10^{-4}\) T
4. \({3 \over \sqrt 2}\times 10^{-4}\) T
Subtopic: Bar Magnet |
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