Magnetism

Ch. No. 5. All about ElectromagnetismQ1. (A) Choose the correct alternative and rewrite the following.•The device used for producing current is called a ___________

1.Voltmeter2.Ammeter3.Galvanometer4.Generator

•At the time of short circuit, the current in the circuit __________1.Increases2.Decreases3.Remains the same4.Increases in steps

•The direction of the magnetic field around a straight conductor carrying current is given by ____________

1.Right hand thumb rule2.Fleming’s left hand rule3.Fleming’s right hand rule4.None of these.

(B) Say true or false. If false, write the correct sentence. •Magnetic poles exist in pairs. Ans. True•Magnetic field increases as we go away from a magnet. Ans. False:- Magnetic field decreases as we go away from a magnet. •Magnetic lines of force always cross each other. Ans. False:- Magnetic lines of force never cross each other. •Electric generator is used to generate current. Ans. True

(C) Find the odd one out. •Loudspeaker, microphone, electric motor, bar magnet. Ans. Bar Magnet: - It is a magnet while the rest are devices working on electromagnetism. •Armature coil, brushes, commutator, direct current. Ans. Direct current: - The rest are parts of a generator. •Fuse, insulating material, rubber shoes, generator. Ans. Generator: - The rest are safety measures in using electricity. •Voltmeter, ammeter, galvanometer, thermometer. Ans. Thermometer: - The rest are dives working on electromagnetism.

Q2. Define•Magnetic field: - The region around magnet, in which the force of attraction and repulsion exists, is called as ‘magnetic field’. •Electric motor: - A device which converts electrical energy into mechanical energy is called an electric motor. •Solenoid: - A coil of many turns of insulated copper wire wrapped in the shape of cylinder is called a solenoid.

Q3. State the rules•Fleming’s right hand ruleAns. Stretch the thumb, forefinger and middle finger of the right hand so that they are perpendicular to each other. 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. The induced current is found to be the maximum when the direction of motion of the coil is at right angles to the magnetic field.

•Fleming’s Left hand rule: Ans. Stretch the forefinger, the central finger and the thumb of you left hand mutually perpendicular to each other. If the forefinger shows the direction of the field and the central finger shows the direction of the current, then the thumb will point towards the direction of the motion of the conductor.

•Right hand thumb ruleAns. 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 thumb rule.

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.

Q.4. Distinguish between electric motor and electric generator.

State the characteristics of magnetic lines of force. Ans. The path along which the unit north pole moves in a magnetic field is called magnetic lines of force or magnetic field lines. The characteristics of magnetic lines of force are:Magnetic lines of force are closed continuous curves. They start from North Pole and ends on South Pole. The tangent at any point on the magnetic lines of force gives the direction of the magnetic field at that point. No two magnetic lines of force can intersect each other. 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.•Wires carrying electricity should not be touched bare footed. •Our body is a good conductor of electricity. •If we touch the wire bare footed, a large current may pass through our body. •Due to this, we may receive severe electric shock and sometimes even death. •Therefore, wires, carrying electricity should not be touched bare footed.

•We should not use many electrical appliances simultaneously. •Many of the electrical appliances that we use at home have a high power rating. E.g. grinder, A.C, Oven, washing-machine, etc. •When we use these electrical appliances simultaneously, it causes overloading, that is flow of large amount of current in the circuit, occurs. •This causes fire. Hence, we should not use many electrical appliances simultaneously.

•Alloys like alnico or nipermag are used in industry. •Alloys like alnico or nipermag are very hard and are used in the production of permanent magnets. •Permanent magnets of these alloys are used in microphones, loudspeaker, ammeters, voltmeters, etc. •Alnico is an alloy of iron containing aluminium, nickel and cobalt. Nipermag is an alloy of iron containing nickel, aluminium and titanium. •Hence, permanent magnets prepared from these alloys are most suitable for use in industries.

•A magnetic crane is used to load and transport scrap iron. •It is not feasible to create a permanent magnet of a big size required to load and transport scrap iron. •It is also not possible to store such a big magnet. •Hence, we create a powerful electromagnet by passing a current through a big iron disc. Therefore, a magnetic crane is used to load and transport scrap iron.

. Explain the construction, working and uses of Electric motor. •Electric motor.Ans. A device which converts electrical energy into mechanical energy is called an electric motor. Principle: - Electric motor works on the principle that a current carrying conductor placed in a magnetic field experiences a force.

Construction:

ELECTRIC MOTOR•Armature coil: - A large number of turns of insulated copper wire wound on iron core in rectangular shape forms an armature coil ABCD as shown in the above figure. •Strong magnet: -The armature coil is placed in between two pole pieces (N and S) of a strong magnet. This provides a strong magnetic field. •Split ring commutator: - It consists of two halves (R1 and

R2) of a metallic ring. The ends of the armature coil are

connected to these rings. Commutator reverses the direction of current in the armature coil. •Brushes: - Two carbon brushes B1 and B2, used to press the

commutator. •Battery: - The battery supplies the current (D.C.) to the armature coil.

Working: - •When current is passed through the coil ABCD, arms AB and CD experience force. According to Fleming’s left hand rule the force experienced by arm AB is in the downward direction and arm CD in the upward direction. •Both these forces are equal and opposite. This force rotates the coil in clockwise direction until the coil is vertical. •At this position, the contact between commutator and brushes break. So the supply to the coil is cut off. Thus no force acts on the coil. •But the coil does not stop due to inertia. It goes on rotating until the commutator again comes in contact with the brushes B1 and B2. •Again the current starts passing through coil and the arm AB rotates through 900, 1800, 2700 and 3600. Now the force acting on arm AB is downward and CD is upward. Again this force moves the coil in clockwise direction. •Thus, the coil rotates with the help of electrical energy. The coil of DC motor continues to rotate in the same direction.

Uses of DC motor:•These motors are used in domestic appliances like mixers, blenders, refrigerators and washing machines. •These motors are used in electric fans, hair dryers, record players, tape recorders and blowers. •These motors are used in electric cars, rolling mills, electric cranes, electric lifts and electric trains.

Electric generator. An electric device which converts mechanical energy into electric energy is called an electric generator. Principle: Electric generator works on the principle of electromagnetic induction. When the coil of electric generator rotates in a magnetic field, the magnetic field induces a current in this coil. This induced current then flows into the circuit connected to the coil.

Types of electric generators:•AC GeneratorDC Generator

•AC Generator: A generator which converts mechanical energy into electric energy in the form of alternating current is called AC Generator.

Construction:

Construction: The main components of AC generator are as shown in figure.1. Armature 2. Strong magnets 3. Slip rings and 4. Brushes. Armature, strong magnets and brushes are the same as used for electric motors.

Slip rings: The two ends of the armature coil are connected to two brass slip rings R1 and R2. These rings rotate along with the armature coil.

Working:a.When the armature coil ABCD rotates in the magnetic field provided by the strong magnets, it cuts the magnetic lines of forces.b.Thus the changing magnetic field produces induced current in the coil. The direction of induced current is determined by the Flemings right hand rule.c.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.d.Therefore, the induced current produced is of alternating nature. Such a current is called as alternating current.

•DC Generator: A generator which converts mechanical energy into electrical energy in the form of direct current is called as DC generator.

Construction:

Construction: The main components of D.C. generator are: 1. Armature coil 2. Strong magnets 3.

Split rings or commutator 4. Brushes 5. Bulb.

The components such as armature coil, strong magnet, brushes are the same which are used for AC generators. Slip rings or commutator is the same as used in electric motor.

Working of DC generator:

1.When the coil of DC generator rotates in the magnetic field, potential difference is produced in the coil. This gives rise to the flow of current. This is shown by glowing of the bulb.2.In D.C. generator, the flow of current in the circuit is in the same direction as long as the coil rotates in the magnetic field.3.This is because one brush is always in contact with the arm of the armature moving up and other brush is in contact with the arm of the armature moving downward in the magnetic field.

Observe the figure. If the current in the coil A is changed, will some current be induced in the coil B ? Explain.

•If the current in coil A is changed, then some current will be induced in coil B.•As the current in coil A changes, the magnetic field related to it also change.•Due to the changing magnetic field, current is induced in coil B.