Magnetic Effect of Electric Current
NCERT Exemplar Problems
Long Answer Questions
Question 25: Why does a magnetic compass needle pointing North and South in the absence of a nearby magnet get deflected when a bar magnet or a current carrying loop is brought near it. Describe some salient features of magnetic lines of field concept.
Answer: A magnetic compass needle pointing North and South shows deflection when a bar magnet or a current carrying loop is brought near it. This happens because the magnetic fields of the compass needle and the bar magnet (or current carrying loop) interact with each other.
Salient features of magnetic field lines:-
- Magnetic field lines follow the direction from the north pole to the south pole.
- Magnetic field lines always show concentric pattern.
- Magnetic field lines do not cross one another.
- Closer the field lines; stronger is the magnetic field and vice-versa is also true.
- Magnetic field lines are closer near the poles; which shows greater strength of magnetic field near the poles.
Question 26: With the help of a labelled circuit diagram illustrate the pattern of field lines of the magnetic field around a current carrying straight long conducting wire. How is the right hand thumb rule useful to find direction of magnetic field associated with a current carrying conductor?
Answer: The following diagram depicts the pattern and direction of magnetic field lines around a straight current-carrying conductor.
Right Hand Thumb Rule: If a current carrying conductor is held by right hand; keeping the thumb straight and if the direction of electric current is in the direction of thumb, then the direction of wrapping of other fingers will show the direction of magnetic field.
Question 27: Explain with the help of a labelled diagram the distribution of magnetic field due to a current through a circular loop. Why is it that if a current carrying coil has n turns the field produced at any point is n times as large as that produced by a single turn?
Answer: Magnetic Field Due to Circular Loop Current-Carrying Conductor: In case of a circular current carrying conductor, the magnetic field lines would be in the form of concentric circles around every part of the periphery of the conductor. Since, magnetic field lines tend to remain closer when near the conductor, so the magnetic field would be stronger near the periphery of the loop. On the other hand, the magnetic field lines would be distant from each other when we move towards the centre of the current carrying loop. Finally; at the centre, the arcs of big circles would appear as a straight lines.
Magnetic field and number of turns of coil: Magnitude of magnetic field gets summed up with increase in the number of turns of coil. If there are ‘n’ turns of coil, magnitude of magnetic field will be ‘n’ times of magnetic field in case of a single turn of coil.
Question 28: Describe the activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field. How does Fleming’s left-hand rule help us to find the direction of the force acting on the current carrying conductor?
Answer: Activity: To show the effect of magnetic field on current-carrying conductor
Materials Required: For this, we need to take a small aluminium rod, a horse-shoe magnet, battery, plug key, wires and a stand.
- The aluminium rod is suspended horizontally from the stand and tied to two wires at its ends. The wires are attached to rheostat, battery and a plug key to make the circuit.
- The horse-shoe magnet is positioned in a way that the aluminium rod lies between the two poles of the magnet.
- Let us assume that the south pole is above the aluminium rod and the north pole is below it. The plug key is inserted to initiate current supply to the rod.
- It is observed that the aluminium rod deflects towards left.
- When the direction of the current is reversed the aluminium rod deflects towards right.
Based on above observations, it can be said that when a current carrying conductor is placed within a magnetic field; the conductor experiences deflection. Fleming’s Left Hand Rule explains the direction of displacement in this case. Let us assume that the current is moving in anti-clockwise direction in the loop. In that case, the magnetic field would be in clockwise direction; at the top of the loop. Moreover, it would be in anticlockwise direction at the bottom of the loop.