Electromagnetism
NCERT Exercise
State whether the following statements are true or false.
- An electric motor converts mechanical energy into electrical energy.
- An electric generator works on the principle of electromagnetic induction.
- The field at the centre of a long circular coil carrying current will be parallel straight lines.
- A wire with a green insulation is usually the live wire of an electric supply.
Answer: (a) False, (b) True, (c) True, (d) False
Which of the following correctly describes the magnetic field near a long straight wire?
- The field consists of straight lines perpendicular to the wire.
- The field consists of straight lines parallel to the wire.
- The field consists of radial lines originating from the wire.
- The field consists of concentric circles centred on the wire.
Answer: (d) The field consists of concentric circles centred on the wire
The phenomenon of electromagnetic induction is
- The process of charging a body.
- The process of generating magnetic field due to a current passing through a coil.
- Producing induced current in a coil due to relative motion between a magnet and the coil.
- The process of rotating a coil of an electric motor.
Answer: (c) Producing induced current in a coil due to relative motion between a magnet and the coil.
The device used for producing electric current is called a
- Generator
- Galvanometer
- Ammeter
- Motor
Answer: (a) Generator
The essential difference between an AC generator and a DC generator is that
- AC generator has an electromagnet while a DC generator has permanent magnet.
- DC generator will generate a higher voltage.
- AC generator will generate a higher voltage.
- AC generator has slip rings while the DC generator has a commutator.
Answer: (d) AC generator has slip rings while the DC generator has a commutator.
At the time of short circuit, the current in the circuit
- Reduces substantially
- Does not change
- Increases heavily
- Vary continuously
Answer: (c) Increases heavily
List three methods of producing magnetic fields.
Answer: Three methods of producing magnetic fields are as follows:
- By permanent magnet
- By electromagnet
- By current carrying conductors
How does a solenoid behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the help of a bar magnet? Explain.
Answer: A solenoid begins behaving like a magnet when electric current flows through it. We know that any current carrying conductor creates a magnetic field around it and that is what happens in case of solenoid. For determining the different poles of a solenoid, we can use a bar magnet and look for interaction between different poles of two magnets. If the north pole of the bar magnet gets repulsed by a particular pole of the electromagnet (solenoid) then it gets confirmed that the bar magnet was brought near the north pole of the electromagnet.
When is the force experienced by a current–carrying conductor placed in a magnetic field largest?
Answer: From Fleming’s Left Hand Rule, it is clear that when the direction of current and magnetic field are in mutually perpendicular directions, the deflection in conductor is the maximum. This shows that when magnetic field and direction of current are perpendicular to each other, the force experienced by the conductor is the largest.
Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Answer: Here, the electron beam is moving towards the viewer, i.e. out of the plane of the page. This means that the direction of current is towards the page. This shows the direction in which the forefinger is pointing. The thumb is pointing towards right which is same as the direction of deflection. The middle finger is pointing downwards; which shows the direction of the magnetic field.
