Ch. No. 5. All about Electromagnetism
Q1. (A) Choose the correct alternative and rewrite the following.
1. The device used for producing current is called a ___________
a. Voltmeter
b. Ammeter
c. Galvanometer
d. Generator
2. At the time of short circuit, the current in the circuit __________
a. Increases
b. Decreases
c. Remains the same
d. Increases in steps
3. The direction of the magnetic field around a straight conductor carrying current is given by ____________
a. Right hand thumb rule
b. Fleming’s left hand rule
c. Fleming’s right hand rule
d. None of these.
(B) Say true or false. If false, write the correct sentence.
1. Magnetic poles exist in pairs.
Ans. True
2. Magnetic field increases as we go away from a magnet.
Ans. False:- Magnetic field decreases as we go away from a magnet.
3. Magnetic lines of force always cross each other.
Ans. False:- Magnetic lines of force never cross each other.
4. Electric generator is used to generate current.
Ans. True
(C) Find the odd one out.
1. Loudspeaker, microphone, electric motor, bar magnet.
Ans. Bar Magnet: - It is a magnet while the rest are devices working on electromagnetism.
2. Armature coil, brushes, commutator, direct current.
Ans. Direct current: - The rest are parts of a generator.
3. Fuse, insulating material, rubber shoes, generator.
Ans. Generator: - The rest are safety measures in using electricity.
4. Voltmeter, ammeter, galvanometer, thermometer.
Ans. Thermometer: - The rest are dives working on electromagnetism.
Q2. Define
1. Magnetic field: - The region around magnet, in which the force of attraction and repulsion exists, is called as ‘magnetic field’.
2. Solenoid: - A coil of many turns of insulated copper wire wrapped in the shape of cylinder is called a solenoid.
3. Electric motor: - A device which converts electrical energy into mechanical energy is called an electric motor.
Q3. State the rules
1. Fleming’s right hand rule
Ans.
i. Stretch the thumb, forefinger and middle finger of the right hand so that they are perpendicular to each other.
ii. If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of the motion of conductor, then the middle finger will show the direction of induced current.
2. Right hand thumb rule
Ans.
Imagine that you are holding a current carrying straight conductor in your right hand such that the thumb points towards the direction current, then the curled fingers around the conductor will give the direction of the magnetic field. This is known as right hand rule.
Q4. Distinguish between electric motor and electric generator.
| Electric motor | Electric generator |
| 1. Electric motor converts electrical energy into mechanical energy. 2. It uses electricity. 3. It is based on the principle that current carrying conductor placed in a magnetic field experiences a force. | 1. Electric generator converts mechanical energy into electrical energy. 2. It generates electricity. 3. It is based on the principle of electromagnetic induction. |
Q5. State the characteristics of magnetic lines of force.
Ans. The characteristics of magnetic lines of force are:
1. Magnetic lines of force are closed continuous curves. They start from North Pole and ends on South Pole.
2. The tangent at any point on the magnetic lines of force gives the direction of the magnetic field at that point.
3. No two magnetic lines of force can intersect each other.
4. Magnetic lines of force are crowded where the magnetic field is strong and far from each other where the field is weak.
Q6. Give scientific reasons.
1. Wires carrying electricity should not be touched bare footed.
Ans.
i. Our body is a good conductor of electricity.
ii. If we touch the wire bare footed, a large current may pass through our body.
iii. Due to this, we may receive severe electric shock and sometimes even death.
iv. Therefore, wires, carrying electricity should not be touched bare footed.
2. We should not use many electrical appliances simultaneously.
Ans.
i. Many of the electrical appliances that we use at home have a high power rating. E.g. grinder, A.C, Oven, washing-machine, etc.
ii. When we use these electrical appliances simultaneously, it causes overloading, that is flow of large amount of current in the circuit, occurs.
iii. This causes fire.
iv. Hence, we should not use many electrical appliances simultaneously.
3. Alloys like alnico or nipermag are used in industry.
Ans.
i. Alloys like alnico or nipermag are very hard and are used in the production of permanent magnets.
ii. Permanent magnets of these alloys are used in microphones, loudspeaker, ammeters, voltmeters, etc.
iii. Alnico is an alloy of iron containing aluminium, nickel and cobalt. Nipermag is an alloy of iron containing nickel, aluminium and titanium.
iv. Hence, permanent magnets prepared from these alloys are most suitable for use in industries.
4. A magnetic crane is used to load and transport scrap iron.
Ans.
i. It is not feasible to create a permanent magnet of a big size required to load and transport scrap iron.
ii. It is also not possible to store such a big magnet.
iii. Hence, we create a powerful electromagnet by passing a current through a big iron disc.
iv. Therefore, a magnetic crane is used to load and transport scrap iron.
Q7. Explain the construction and working of
1. Electric motor.
Ans.
Principle: - An electric motor converts electrical energy into mechanical energy.
Construction: - The main parts of an electric motor are
i. Armature coil: - It consists of a large number of turns of insulated copper wire wound around a rectangular iron core (ABCD)
ii. Strong magnet: - The armature coil is placed between the poles of a strong magnet to create a strong magnetic field.
iii. Split ring commutator:- A metallic ring divided into two halves (R1 and R2). The ends of the armature are connected to the two rings. This commutator helps to reverse the direction of the current in the armature coil.
iv. Brushes: - Two carbon brushes B1 and B2, used to press upon the commutator.
v. Battery: - it supplies the current (D.C.) to the armature coil.
Working: -
i. When current is passed through the coil ABCD, arms AB and CD experience force in the downward and upward direction respectively.
ii. These equal and opposite forces rotate the coil in clockwise direction until the coil is vertical.
iii. At this position, the contact between commutator and brushed breaks, the supply to the coil is cut off and no force acts on the coil.
iv. But the coil goes on rotating due to inertia until the commutator again comes in contact with the brushed B1 and B2.
v. Again the current starts passing through the coil and the arm AB rotates through 900, 1800, 2700 and 3600.
vi. Now, the force acting on arm AB and CD is downward and upward respectively which moves the coil again in clockwise direction.
2. Electric generator.
Ans. An electric generator is a device used to convert mechanical energy into electrical energy. It works mainly on the principle of electromagnetic induction. There are two types of electrical generators – A.C. generator and D.C. generator.
Construction:- The main components of an AC generator are: -
i. Armature coil: - A large number of turns of insulated copper wire wound on iron core in rectangular shape forms an armature coil ABCD.
ii. Strong magnet: - the armature coil is placed in between two poles of a strong magnet which provides a strong magnetic field.
iii. Split rings: - The two ends of the armature coil are connected to two brass split rings R1 and R2 which rotate along with the armature coil.
iv. Brushes: - Two carbon brushes B1 and B2 are used to press the split rings.
Working: -
1. When the armature coil ABCD rotates in the magnetic field provided by the strong magnets, it cuts the magnetic lines of forces.
2. Thus, the changing magnetic field produces induced current in the coil.
3. The current flows out through the brush B1 in one direction in the first half of the revolution and through the brush B2 in the next half revolution in the reverse direction, this process is repeated.
Therefore, the induced current produced is of alternating nature.
