Very Short Question Answers

Q1. What is the importance of oersted’s experiment?

Ans.

Moving charges or currents produce a magnetic field in the surrounding space.

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Q2. State ampere’s law and biot-savart law.

Ans.

Ampere’s law: The line integral of magnetic field $B$ along a closed path  is equal to $\varepsilon_{\circ}$ times the total current enclosed by the path.
 $\oint B.dl =\mu_{\circ}i$
Biot-savarts law:The magnitude of the magnetic field $dB$.
i) Directly proportional to the current     $dB\propto i $
ii) Directly proportional to the length of the element    $dB\propto dl$
iii) Directly proportional to the sine of the angle between the current and the element.  $dB\propto sin\theta $
iv) Inversely proportional to the square of the distancer. $dB\propto \frac{1}{r^{2}}$
   $$\therefore \, dB=\frac{1}{4\pi \varepsilon_{\circ} }\, \frac{i \,dl \,sin\theta}{r^{2}}$$

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Q3. Write the expression for the magnetic induction at any point on the axis of a circular current – carryingcoil. Hence, Obtain an expression for the magnetic induction at the centre of the circular coil.

Ans.

$1) \, B=\frac{\mu_{\circ}\,niR^{2}}{2\left(R^{2}+x^{2}\right)^{\frac{3}{2}}} $

$2) At\, the \, centre\, of \, the\,coil\,(x=0)\, B=\frac{\mu_{\circ}\,ni }{2R} $

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Q4. A circular coil of radius ‘r’ having N turns carries a current ‘i’. what is its magnetic moment?

Ans.

Magnetic moment $M=N\,i\,A=N\,i\,(\pi r^{2})$

where $A=\pi r^{2}$

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Q5. What is the force on a conductor of length L carrying a current ‘i’ placed in a magnetic field of induction B?when does it become maximum?

Ans.

$F = B\,i\,L\, sin\theta \:\: $ if $\,\theta=90°$ the force becomes maximum.$F_{max}= B\,i\,L\,$

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Q6. What is the force on a charged particle of charge ‘q’ moving with a velocity ‘v’ in a uniform magnetic field ofinduction B? when does it become maximum?

Ans.

Force $F = B\,v\,q\, sin\theta \:\:$ If $\,\theta=90° $the force becomes maximum. $F_{max}=  B\,v\,q\, $

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Q7. class="MsoListParagraph" style="mso-add-space: auto; text-indent: -.25in; line-height: normal; mso-list: l0 level1 lfo1; margin: 0in 0in .0001pt .25in;">Distinguish between ammeter and voltmeter.

Ans.

AMMETER	VOLTMETER
It is used to measure current in circuits.	It is used to measure the potential difference between two points in circuit.
It is always connected in series in circuits	It is always connected in parallel in circuits.

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Q8. What is the principle of a moving coil galvanometer?

Ans.

The deflection produced in current coil suspended freely in a uniform magnetic field is directly proportional to the current flowing in the coil.
   $$ i \propto \theta$$

 $$ i =\,k\, \theta$$

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Q9. What is the smallest value of current that can be measured with a moving coil galvanometer?

Ans.

The smallest current can be measured with a moving coil galvanometer is $10^{-9}$amp.

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Q10. How do you convert a moving coil galvanometer into an ammeter?

Ans.

A moving coil galvanometer can be converted into an ammeter, by connecting a low resistance called Shunt (s) in parallel to the galvanometer.

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Q11. How do you convert a moving coil galvanometer into a voltmeter?

Ans.

 A moving coil galvanometer can be converted into voltmeter, by connecting a high resistance in series with the galvanometer.

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Q12. What is the relation between the permittivity of free space $\varepsilon_{\circ}$, the permeability of free space $\mu_{\circ}$ and the speed oflight in vacuum?

Ans.

The velocity of light in vacuum   $ C\,= \frac{1}{\sqrt{\mu_{\circ} \, \varepsilon_{\circ}}}$

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Q13. A current carrying circular loop lies on a smooth horizontal plane. Can a uniform magnetic field be set up insuch a manner that the loop turns about the vertical axis.

Ans.

No, A uniform magnetic field cannot be set up in such a manner that the loop turns about the vertical axis.The torque acting on the loop is given by 
  $$  \vec{\tau}= \vec{M} \times\vec{B}= i\left(\vec{A}\times \vec{B} \right)$$
Since the area vector is along the vertical, the torque on the loop becomes zero.

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Q14. A current carry circular loop is placed in a uniform external magnetic field. 
If the loop is free to turn, what is itsorientation when it is achieves stable equilibrium?

Ans.

The loop achieves stable equilibrium when torque $\left( \tau\right) = 0$ 
As   $$  \vec{\tau}=i\left(\vec{A}\times \vec{B} \right)$$

$$  0=i\left(\vec{A}\times \vec{B} \right) $$

$$\therefore \left(\vec{A}\times \vec{B} \right)=0$$
Which means area vector$ \vec{A}$ must be parallel to magnetic field$ \vec{B}$

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Q15. A wire loop of irregular shape carrying current is placed in an external magnetic field. 
If the wire is flexiblewhat shape will the loop change to? Why?

Ans.

The shape of wire loop become circular with its plane normal to the field. 
This happens to maximize the flux.

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