# KPK Board 10th Class Physics Ch 6 Electromagnetism Short Questions Answers

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Differentiate between electric and magnetic field.

Ans.

 Electric field Magnetic field Definition: The region around a charge where its effect can be felt. The region around a magnet where its effect can be felt. Cause: Electric field can be caused by both stationary as well as moving charges. Magnetic field can only be caused by moving charge. Nature: Electric field is a conservative field. Magnetic field is also a conservative field. Force (interaction) Unlike charges behavior is “attract each other” whereas like charges behavior is “repel each other” Like poles behavior is “repel each other” whereas unlike poles behavior is “attract each other”. Unit Electric field is measured in Newton per coulomb (N/C) or Volt per meter (V/m) Magnetic field is measured in Gauss or Tesla Poles: Electric monopole is possible (we can have only positive or negative charge on a body) Magnetic monopole is not possible (we have always a dipole i.e. North and South)

Can an electron at rest be set into motion with a magnetic field?

Ans.     No, an electron at rest cannot be set into motion with a magnetic field.

Explanation:    It requires work to set a body in motion which is at rest. Magnetic force acts perpendicular to the motion of charged particles (here it is electron)  due to which it does on work on charge particles. If no work is done by the magnetic field, then there will be no charge in the velocity of charged particle due to magnetic field. Magnetic force can only charge the direction of motion of charged particles. So, an electron at rest cannot be set into motion with a magnetic field.

Which is more likely to show deflection in compass needle, AC current or DC current? Explain

Ans.     Direct Current (DC) is more likely to show deflection in compass needle.

Explanation:    When there is a current in the wire magnetic field produces around it. When the direction of current is reversed, direction of magnetic field will also be reversed. In case of DC current, magnetic field will have only one direction and the compass needle will deflected in the direction of magnetic field. In case of AC, current charges its direction continuously, and hence the magnetic field too, due to which the compass will show almost no deflection.

A constant magnetic field is applied to a current carrying conductor. What angle should the wire make with the field for the force to be (a). maximum, (b) minimum?

Ans.     At 90° the force will be maximum and at 0° the force will be minimum.

Explanation:    The magnetic force on a current carrying wire placed in magnetic field is given by:                                    FB= BIL sin θ

• Maximum force: for the force to be maximum, the angle between magnetic field (B) and current (I) should be 90°.

Mathematically:

FB= BIL sin 90°

As sin 90°1, therefore,  FB(max) = BIL

• Minimum force: For the force to be minimum, the angle between magnetic field (B) and current (I) should be 0°.

Mathematically:

FB= BIL sin 0°

As sin 0° = 0, therefore,  FB(max) = BIL

Why does a compass needle always points to North?

Ans.     There is a small magnet inside the compass which interacts with the earth’s magnetic field due to which a compass needle always points to North.

Explanation:    Earth as a whole is magnet having magnetic field of the order of  T. when the magnet inside the compass interacts with the earth’s magnetic field, it experiences a force unit its axis of rotation is parallel to that of earth’s axis. This force compels the needle to point to North always.

How can magnetic field be used to generate electric current?

Ans.     In case of electromagnetic induction, a magnetic field is used to generate electric current where a changing magnetic field produces an induced causing electric current in closed loop.

Explanation:    in generators, principle of electromagnetic induction is applied. Under which a closed loop is rotated inside a magnetic field, which causes a changing field inside the loop leading to induced current in the loop.

What would happen if we use a slip ring to drive a DC motor?

Ans.     The motor will not rotate and will wiggle when we use a slip ring instead of split ring.

Explanation:    if we use slip ring instead of slip ring in a DC motor, then after half a cycle, the direction of current will reverse.  Then the armature coil of DC motor will start motor in the reverse direction. So, there will be no complete rotation i.e. no work will be done

The primary coil of a transformer is connected to a DC battery. Is there an emf induced in the secondary coil? Why?

Ans.     If the primary coil of a transformer is connected to a DC battery, no emf will be induced in the secondary coil because there will be no mutual induction due to constant current.

Explanation:    A transformer works on the principle of mutual induction. Under this principle a changing current in primary coil will produce an induced emf in the secondary coil. Now if we connect the primary coil to a DC battery (which provides uniform current), no secondary voltage (induced emf ) will be produced.

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