Class 10 - Physics - Magnetic Effects of Electric Current

Question1.

Which of the following correctly describes the magnetic field near a long straight wire?

(a) The field consists of straight lines perpendicular to the wire.

(b) The field consists of straight lines parallel to the wire.

(c) The field consists of radial lines originating from the wire.

(d) The field consists of concentric circles centred on the wire.

Correct option: - (d) the field consists of concentric circles centred on the wire.

According to right hand thumb rule the magnetic field lines, produced around a straight current - carrying are concentric circles.

Question2.

The phenomenon of electromagnetic induction is

(a) the process of charging a body.

(b) the process of generating magnetic field due to a current passing through a coil.

(c) producing induced current in a coil due to relative motion between a magnet and the coil.

(d) the process of rotating a coil of an electric motor.

Correct option: - (c) producing induced current in a coil due to relative motion between a magnet and the coil.

When a straight coil and a magnet are moved relative to each other, a current is induced in the coil. This phenomenon is known as electromagnetic induction.

Question 3.

The device used for producing electric current is called a

(a) generator.

(b) galvanometer.

(c) ammeter.

(d) motor.

Correct option: - (a) generator.

An electric generator produces electric current. It converts mechanical energy into electricity.

Question 4.

The essential difference between an AC generator and a DC generator is that

(a) AC generator has an electromagnet while a DC generator has permanent magnet.

(b) DC generator will generate a higher voltage.

(c) AC generator will generate a higher voltage.

(d) AC generator has slip rings while the DC generator has a commutator.

Correct option: - (d) AC generator has slip rings while the DC generator has a commutator.

An AC generator has two rings called slip rings. A DC generator has two half rings called commutator.

Question 5.

At the time of short circuit, the current in the circuit

(a) reduces substantially.

(b) does not change.

(c) increases heavily.

(d) vary continuously.

Correct option (c )Increases heavily.

At the time of short circuit, the amount of current in the circuit increases abruptly.

Question 6.

State whether the following statements are true or false.

(a) An electric motor converts mechanical energy into electrical energy.

(b) An electric generator works on the principle of electromagnetic induction.

(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.

(d) A wire with a green insulation is usually the live wire of an electric supply.

• An electric motor converts mechanical energy into electrical energy. False
1. An electric motor converts electrical energy to mechanical energy.
• An electric generator works on the principle of electromagnetic induction. True.
1. A generator is a device that generates electricity by rotating a coil in a magnetic field. It works on the principle of electromagnetic induction.
• The field at the centre of a long circular coil carrying current will be    parallel straight lines. True.
1. A long circular coil is a long solenoid. The magnetic field lines inside the solenoid are parallel.
• A wire with a green insulation is usually the live wire of an electric supply. False.
1. Live wire has red insulation cover, whereas earth wire has green insulation cover in domestic circuits.

Question 7.

List three sources of magnetic fields.

The sources of magnetic fields are following:-

1. Electromagnets
2. Current-carrying conductors
3. Permanent magnets

Question 8.

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.

A solenoid is formed of long coil of circular loops of insulated copper wire.

When a current is allowed to flow through it, magnetic field lines are produced around the solenoid.

The magnetic field produced by it is similar to the magnetic field of a bar magnet.

When the north pole of a bar magnet is brought near the end connected to the negative terminal of the battery, the solenoid repels the bar magnet.

Since like poles repel each other, the end connected to the negative terminal of the battery behaves

as the north pole of the solenoid and the other end behaves as a south pole.

Therefore one end of the solenoid behaves as a north pole and the other end behaves as a south pole.

Question 9.

When is the force experienced by a current–carrying conductor placed in a magnetic field largest?

The force experienced by a current–carrying conductor placed in a magnetic field largest when the direction of magnetic field

and electric current are perpendicular to each other.

Question 10.

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?

The magnetic field inside the chamber is perpendicular to the direction of current and direction of force i.e. either upward or downward. T

he direction of current is given from the front wall to the back wall because negatively charged electrons are moving from back to front wall.

The direction of magnetic force is rightwards. Hence, by using Fleming’s left hand rule, it can be concluded that the direction of magnetic field inside the chamber is downward.

Question 11.

Draw a labelled diagram of an electric motor. Explain its principle and working.

What is the function of a split ring in an electric motor?

Electric motor is a device that converts electrical energy to mechanical energy.

It is used in electric fans, refrigerators, mixers etc.

Principle: -

• A current-carrying conductor, when placed in a magnetic field, experiences a force.
• The direction of the field and that of current is mutually perpendicular to each other. Therefore, the force acting on the conductor will be perpendicular to both and it is given by Fleming’s Left hand rule.

Construction: -

• Rectangular coil ABCD made up of insulated copper wire, this coil is wound over a soft iron core called armature.
• The coil is mounted between the two poles of the magnet.
• A commutator is a device that reverses the direction of flow of current through a circuit.
• The two ends of the coil are soldered to the two ends of commutator whose main function is to reverse the direction of current flowing through the coil.

Working:

• Let the current in the coil ABCD of motor enters from the source through the conducting brush X, flow along ABCD and finally flows back to the battery through the brush Y.
• By applying Fleming’s left hand rule the force acting on arm AB due to magnetic field pushes it downwards.
• But the force acting on arm CD pushes it upwards.
• Both the coil and axle rotate anticlockwise.
• Due to action of split rings P and Q change their contacts with brushes.
• Now, P makes contact with Y and Q with X.
• As a result current begins to flow in coil along DCBA.
• The arms are pushed in opposite direction and coil continues to rotate in same direction.

Question 12.

Name some devices in which electric motors are used.

Devices in which electric motors are used are as flows:-

1. Electric Mixers
2. Washing machines
3. Electric fans
4. Water pumps
5. Air conditioners

Question 13.

A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil,

(ii) withdrawn from inside the coil, (iii) held stationary inside the coil?

• When a bar magnet is pushed into the coil then a current is induced momentarily in the coil due to electromagnetic induction .
• As a result galvanometer shows momentarily deflection.
• When a bar magnet is withdrawn from inside the coil, a current is again induced momentarily in the coil but in opposite direction.
• As a result galvanometer shows momentarily deflection but in opposite direction.
• When a bar magnet is held stationary inside the coil, there will be no current induced in the coil. So there will be no deflection in the galvanometer.

Question 14.

Two circular coils A and B are placed close to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.

When the two circular coils are placed close to each other, when a current in coil A changes, the magnetic field associated with it also changes.

As a result magnetic field around coil B also changes. This change in magnetic field lines around coil B induces an electric current in it.

This is known as electromagnetic induction.

Question 15.

State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current,

(ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and

(iii) current induced in a coil due to its rotation in a magnetic field.

• Maxwell’s right hand rule determines the direction of a magnetic field produced around a straight conductor carrying current.
1. Maxwell’s right hand rule states that if we are holding a current-carrying conductor in our right hand such that the thumb points in the direction of current,
2. then the direction in which fingers encircle, gives the direction of magnetic lines. This rule is also called as Maxwell’s Corkscrew Rule.

• Fleming’s Left hand rule determines the direction of a force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it.
1. Fleming’s left hand rule states if the forefinger, thumb and middle finger of the left hand are stretched mutually perpendicular and the forefinger points in the direction of external magnetic field; middle finger indicates the direction of current, then the thumb points along the direction of force acting on the conductor.

• Fleming’s Right hand rule determines the direction of a current induced in a coil due to its rotation in a magnetic field.
1. Fleming’s Right hand rule it states that the states that if we stretch the thumb, forefinger and the middle finger of right hand at right angles to one another in such a way that the forefinger points in the direction of magnetic field; then the thumb gives the direction of motion of conductor (force), forefinger indicates direction of magnetic field and the middle finger points the direction of induced current.

Question 16.

Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

• An electrical generator is a device that converts mechanical energy to electrical energy.
• The generation of alternating currents (ac) is based on the phenomenon of electromagnetic induction.

Principle:-

• Current is induced in a loop through a change in its orientation or a change in its effective area.
• Induced emf is produced either by changing θ or by changing the area vector.
• Direction of current is given by Fleming’s right hand rule.
• Direction of the current in the circuit changes by the up and down movement of the loops.
• Construction:-
• It consists of a rectangular coil ABCD.
• It consists of 2 poles (north and south) of a magnet in order to have uniform magnetic field.
• There is a coil which is made up of large number of turns and is of rectangular shape also known as armature.
• The armature is connected to 2 slip rings.
• Slip rings helps in electrical contact with the brushes. It does not change current direction.
• The slip rings provide a continuous connection with the wire around the armature.
• These slip rings are attached to carbon brushes.
• The rectangular coil is capable of rotating about an axis which is perpendicular to the magnetic field.
• The axis of rotation is known as axle.
• Working:-
• The generator ABCD is at horizontal position. And if it is rotated in anti-clockwise direction between the two poles of the magnet.
• As the coil rotates in the anticlockwise direction, the side AB of the coil moves cutting down the magnetic lines of force near N-pole of the magnet and the side CD moves up cutting down the lines of force near the S-pole of the magnet.
• As a result current is induced in the sides of AB and CD of the coil.
• By applying Fleming’s left hand rule, the direction of current is in the direction B to A and D to C respectively.
• The induced currents in the two sides of the coil are in the same direction, and as result effective induced current is generated in the direction of BADC.

Question 17.

When does an electric short circuit occur?

Electric shocks occur when the insulation wire gets damaged, and when the live wire and the neutral wire touch each other, the current flowing in the circuit increases.

Also if the resistance of the electric circuit is very low, the current flowing through the circuit increases as a result electric shock can occur.

Question 18.

What is the function of an earth wire? Why is it necessary to earth metallic appliances?

The main function of earthing wire is to transfer any current which is getting leaked from an electrical appliance to the ground.

The metallic body of electrical appliance is connected to the earth wire so that any leakage of electric current is transferred to the ground.

This prevents severe shock to the user. That is why earthing of the electrical appliance is necessary.