Magnetic Effect of Electric Current
NCERT Exemplar Problem
Long Answer Questions
Question 29: Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are diffferent from commercial motors?
Answer: Working of Electric Motor:-
Electrical energy is converted into mechanical energy by using an electric motor. Electric motor works on the basis of rule suggested by Marie Ampere and Fleming’s Left Hand Rule.
In an electric motor, a rectangular coil is suspended between the two poles of a magnetic field. The electric supply to the coil is connected with a commutator. Commutator is a device which reverses the direction of flow of electric current through a circuit.
When electric current is supplied to the coil of electric motor, it gets deflected because of magnetic field. As it reaches the half way, the split ring which acts as commutator reverses the direction of flow of electric current. Reversal of direction of current reverses the direction of forces acting on the coil. The change in direction of force pushes the coil; and it moves another half turn. Thus, the coil completes one rotation around the axle. Continuation of this process keeps the motor in rotation.
In commercial motor, electromagnet; instead of permanent magnet; and armature is used. Armature is a soft iron core with large number of conducting wire turns over it. Large number of turns of conducting wire enhances the magnetic field produced by armature.
Question 30: Explain the phenomenon of electromagnetic induction. Describe an experiment to show that a current is set up in a closed loop when an external magnetic field passing through the loop increases or decreases.
Answer: Electromagnetic Induction: When a conductor is set to move inside a magnetic field or a magnetic field is set to be changing around a conductor, electric current is induced in the conductor. This is just opposite to the exertion of force by a current carrying conductor inside a magnetic field. In other words, when a conductor is brought in relative motion vis-à-vis a magnetic field, a potential difference is induced in it. This is known as electromagnetic induction.
Activity:- Demonstrating electromagnetic induction
Materials Required: For this, take a galvanometer, coil, bar magnet and some wires.
- The coil is inserted over a hollow tube of cardboard.
- With the help of wires, the two ends of the coil are attached to the galvanometer.
- The north pole of the bar magnet is moved towards the end ‘B’ of the coil.
- It is observed that the galvanometer needle shows deflection to right.
- When the magnet is moved away from the coil, the galvanometer needle shows deflection towards left.
- When the magnet is in static position, no deflection is seen in galvanometer needle.
- Induction of electric current in the coil is the cause of deflection in galvanometer needle.
- If the magnet is kept stationary and coil is moved; then also the galvanometer needle shows deflection.
It can be concluded that when the coil and the bar magnet are in relative motion, a current is induced in the coil.
Question 31: Describe the working of an AC generator with the help of a labeled circuit diagram. What changes must be made in the arrangement to convert it to a DC generator?
The structure of electric generator is similar to that of an electric motor. In case of an electric generator a rectangular armature is placed within the magnetic field of a permanent magnet. The armature is attached to wire and is positioned in way that it can move around an axle. When the armature moves within the magnetic field an electric current is induced.
The direction of induced current changes, when the armature crosses the halfway mark of its rotation. Thus, the direction of current changes once in every rotation. Due to this, the electric generator usually produces alternate current, i.e. AC.
To convert an AC generator into a DC generator, a split ring commutator is used. This helps in producing direct current.
Question 32: Draw an appropriate schematic diagram showing common domestic circuits and discuss the importance of fuse. Why is it that a burnt out fuse should be replaced by another fuse of identical rating?
Answer: Importance of Fuse: The electric fuse is an important device in household wiring and also in many electrical appliances. By melting, the fuse wire breaks the circuit and thus helps in saving the wiring or the appliance from damage.
A fuse wire works because of its lower melting point which is possible because of its respective rating. If a fuse with larger rating is used with an appliance, the fuse wire shall not melt and hence would fail to serve the required purpose. Due to this, a fuse with defined rating should not be replaced by one with a larger rating.