Chapter-13 Magnetic Effects of Electric||Science notes class-10th

Chapter-13 Magnetic Effects of Electric

 

•Magnet is any substance that attracts iron or non-iron like substances.

Property of Magnet

•Every magnet has two poles I.e , North and south.

•like poles repel each other.

•Unlike poles attracts each other.

•A freely suspended bar magnet aligns itself in nearly north-south direction , with its north pole towards north direction.

Magnetic field: The area around a magnetic in which its magnetic force can be experienced (fell).

•Its SI unit is Testa (T)

•Magnetic field has both magnitude and direction.

•Magnetic field can be described with the help of a magnetic compass.

•The needle of a magnetic compass is a freely suspended bar magnet.

Characteristics of field lines

(I)Field lines arise from north pole and end into south pole of the magnet.

(II)Field lines are closed curves.

(III)Field lines are closer in stronger magnetic Field.

(IV)Field lines never intersect each other as for two lines to intersect, three must be two north directions at a point , which is not possible.

(V)Direction of field lines inside a magnet is from south to North.

(VI)The reletive strength of magnetic field is shown by degree of closeness of field lines.

Magnetic Field of a Bar magnet

•H.C oersted was the first person to state that electric current has magnetic field.

          Right Hand Thumb Rule           

Imagine You are holding a current carrying straight Conductor in Your right hand such that the thumb is pointing towards the direction of current.Then the Fingers wrapped around the conductor give the direction of magnetic field.

Magnetic Field Due to current through a straight conductor

•It can be represented by concentric circles at every point on conductor.

•Direction can be given by right hand thumb rule or compass.

•Circle are closer never the Conductor

 

•Magnetic Filed Proportional strength of current.

•Magnetic Field Proportional      1

                                               –––––––––––

                       Distance from Conductor 

Magnetic field Due to current through a circular loop

•It can be represented by concentric circle at every point.

•Circle become larger and larger as we move away.

•Every point on wire carrying current

 would give to magnetic Field appearing as straight line at centre of the loop.

•The direction of magnetic field inside the loop is same.

Factors affecting magnetic field of a circular current carrying conductor

•Magnetic field proportional current passing through the conductor.

•Magnetic field Proportional         1

                                               –––––––––––

                       Distance from Conductor 

•Magnetic Field proportional No of turns in the coil

Magnetic field is additive in nature I.e,

magnetic field of one loop adds up to magnetic field of another loop.This is because the current in each circular turn has same direction.

                 Solenoid                  

A coil of many circular turn of insulated copper wire wrapped closely in a cylindrical form.

•Magnetic field of a solenoid is similar to that of a bar magnet.

•Magnetic field is uniform inside the solenoid and represented by parallel field lines.

•Direction of Magnetic Field

(I)outside the solenoid:North to south

(II)Inside the solenoid:south to North

•Solenoid can be used to magnetise a magnetic material like soft iron.

Electromagnet

1.It is a temporary magnet, so can be easily demagnetised.

2.Strength can be Varied.

3.Polarity can be revered.

4.Generally strong magnet.

Permanent Magnet

1.cannot be easily demagnetised.

2.Strength is fixed.

3.Polarity cannot be reversed.

4.Generally work magnet.

Force on a current carrying conductor in a Magnetic Field

Andre Marie Ampere suggested that the magnet also exerts an equal and opposite force on a current carrying Conductor.

The displacement in the colour in the conductor is the maximum when the direction of current is at right angle to the direction of magnetic Field.

Direction of force is reversed on reversing the direction of current.

            Fleming left hand Rule.           

Imagine you are holding a current carrying straight conductor in Your right hand such that the thumb is pointing towards the direction of current.Then the fingers wrapped around the conductor give the direction of magnetic Field.

              Electric Motor                  

A motor is a device which converts electrical energy into  mechanical energy .Electric motor is used in electric fans,  washing machines refrigerators, mixer and grinder and other appliances.

Principle of a Motor:

An electric motor utilizes that magnetic effect of current.It works on principle that when a rectangular coil is placed in a magnetic Field and current is passed through it a torque acts on the coil which rotates it continuously.when the coil rotates the shaft to it also rotates and electrical energy supplied to the motor is converted into mechanical energy.

Construction of a motor 

1.Armature coil:An electric motor consists of an rectangular coil ABCD of insulated copper wire wound on a soft iron core called armature.

2.Strong Field magnetic: The coil (armature)is placed between two poles of a strong magnet such that arm AB and CD are perpendicular to the direction of the magnetic field.

3.split right type Commutator :It consists of two halves of a metallic ring named as P and Q .The two ends of armature coil are connected to these two halves of ring .The function of commentators is that it reverse the direction of current in armature coil.

4.Brushes:Two carbon brushes X and Y press against the commutator.These brushes act as contact between commutator and terminal battery.

5.Battery :It is connected across the carbon brushes .It supplied current to the armature coil.Current in the coil ABCD enters form the source battery through conducting brush X and Flows back to the battery through brushes Y.

Working of a Motor:

•when current flows through coil, arm AB and CD experiences magnetic force.

•On applying Fleming left hand rule, the force acting on arm AB pushes it downwards and arm CD experiences force in upward direction.

•Both these forces are equal and opposite.Two equal and opposite forces at different position of armature constitute a couple and rotate the coil in anti-clockwise direction.

•At half rotation Q makes contact with brush X and P with Brush Y .Now the current in the coil get reversed and flows along the path DCBA.

•The arm AB of the coil that was earlier pushes down is now pushed up and the arm CD previously pushed up is now pushed down .These two equal and opposite forces constitute a couple, this couple now rotate the coil in clockwise direction.

•The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle .Hence electric energy is converted into mechanical energy.

Commercial Motor Use:

1.An electromagnet in place of permanent magnet.

2.large number of turns of the conducting wire in the coil.

3.A soft iron core on which coil is wound plus the coil , is called the armature.

4.This enhances the Power of the motor.

•Heart and brain in the human body have significant magnetic field.

•MRI(Magnetic Resonance Imaging:

Image of internal organ of body can be obtained using magnetic field of the organ.

Galvanometer:Instrument that can detect the presence of current in a circuit.It also detects the direction of current.

        Electro Magnetic Induction        

When a conductor is placed in a changing magnetic field , some current is induced in it.Such current is called induce current and the phenomenon is called electromagnetic induction.

Activity no-1

(I)Magnet moved into the coil:Momentary deflection in G indicating presence of current.

(II)Magnet kept stationary inside the coil :No Deflection.

(III)Magnet is withdrawn:Momentary deflection in G but in opposite direction of first cause.

Activity No-2

(I)Switched on:Momentary deflection in G.

(II)Steady current :No deflection 

(III) Switched off:Momentary deflection in G but in opposite direction of the first cause.

         Fleming's Right hand Rule         

Hold the thumb, the force finger and the middle finger of right hand at right angles to each other.If the force finger is in the direction of magnetic Field and the thumb points in the direction of motion of conductor, then the direction of included current is indicated by middle finger.

•working principle of electric generator.

 •Used to find direction of induced current.

                 Electric Generator                

The electric generator is a machine for producing electric current or electricity .The electronic generator convert mechanical energy  (or kinetic energy ) into electrical energy.

Principal of electric generator:

(Ac generator) In an electric generator, mechanical energy is used to rotate a conductor in a magnetic field to produce electricity.Generator works on the principle of electromagnetic induction.When a closed is rotated in a uniform magnetic field with it's axis perpendicular to the magnetic field, the magnetic field lines passing through the coil change and an induced EMF is set-up .The principle behind the electric generator is based on Fleming's right hand rule.

 Construction of Generator:

1.Field Magnet:It is strong horse-shoe shaped permanent magnet with concave poles.

2.Armature:ABCD is a rectangular armature coil.It Consists of a large number of turns of insulated copper wire wound on a soft iron cylindrical core.

3.Slip rings:These are two brass rings R¹ and R² rigidly conducted to the two ends of the armature coil.As Coil rotates slip rings also rotates.

4.Brushes:There are two graphite rods B¹ and B² which are kept pressed against the slip rings R¹ and R² .Through these brushes , the current induced in the armature coil is send to the external circuit.

5.Axle:The slip rings are placed on the axle which is made to rotate freely from an external source.

6.Galvanometer:To measure current the outer ends of the brushes are converted to the galvanometer.

 Working of Generator:

•The armature coil ABCD is in horizontal position.

•Now, the coil is rotated clockwise.

•The arm AB moves upwards while the arm CD moves downwards.

•The coil cuts the magnetic lines of force.

•According to Fleming's right hand rule, the induced current flows from A to B in arm AB and C to D in arm CD I.e, Flows along ABCD.

•The induced current flows in the circuit through B² to B¹.

•After half the rotation of the armature, the arm CD moves upwards and AB moves downwards .The induced current Now flows in reverse direction I.e along DCBA .The current Now from B¹ to B².

•Thus the direction of current in the external circuit changes after every rotation.such a current which changes its direction after equal intervals of time is called alternating current.

•This device is called AC generator.

                     D.C Generator                    

DC generator:It is a device which convert mechanical energy into electrical energy.

DC generator has split ring commutator instead of slip rings.

Split ring commutator:It consists of two semi cylinderical brass rings R¹ and R² attached to the two ends of the armature coil.As the armature coil rotates , the two split rings also rotate about the same axis of rotation.

Alternate Current(A.C):The current which reverse its direction periodically.

•In India A.C reverses its direction in every 1/100 second.

Time period =1/100+1/100=1/50

Frequency=1/time period

= 1/¹/50    =› 50Hz

Advantage

•A.C can be transmitted over long distance without much loss of energy.

Disadvantage

•A.C cannot be stored.

Direct current(D.C):The current which does not reverse its direction.

•D.C can be stored.

•loss of energy during transmission over long distance Is high.

•Sources of D.C:Cell , battery , storage cells.

 Domestic Electric circuits

•There are three kinds of wire used:

(I)Live wire (Positive)with red insulation cover.

(II)Neutral wire (negative) with black insulation cover.

(III)Earth wire with green insulation cover.

•The potential difference between live and neutral wire in India is 220v.

•Pole→Main supply →Fuse→Electricity meter→distribution box→To separate circuits.

Earth wire:Protects us from electric shock in case of leakage of current especially in metallic body appliances.It provides a low resistance path for current in case of leakage of current.

Short circuit:when live wire comes in direct contact with neutral wire accidently.

•Resistance of circuit belowes low.

•can result in overloading.

Overloading:when current drawn is more than current carrying capacity of a conductor, it results in overloading.

Causes of overloading:

1.Accidental  hike in voltage supply .

2.Use of more than one appliance in a single socket.

Safety devices

•Electric fuse

•Earth wire

•MCB (miniature circuit breaker)

NCERT Solutions for Class 10 Science Chapter 13 – Magnetic Effects of Electric Current

In-text Questions (Solved)

Page 224

1. Why does a compass needle get deflected when brought near a bar magnet?

   Answer: A compass needle is a small magnet. When brought near a bar magnet, the magnetic field of the bar magnet interacts with the magnetic field of the compass needle. This interaction causes the compass needle to get deflected.

Page 228

1. Draw magnetic field lines around a bar magnet.

   Answer: Magnetic field lines around a bar magnet emerge from the north pole and enter the south pole. The lines are closed curves, and inside the magnet, they go from south to north.

2. List the properties of magnetic field lines.

   Answer:

   • Magnetic field lines emerge from the north pole and merge at the south pole.
   • They never intersect each other.
   • They form closed loops.
   • The closer the field lines, the stronger the magnetic field.

Page 229

1. Why don’t two magnetic field lines intersect each other?

   Answer: If two magnetic field lines intersect, it would mean that at the point of intersection, there would be two directions of the magnetic field, which is impossible. Therefore, magnetic field lines never intersect.

Page 231

1. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.

   Answer: By using the right-hand thumb rule, the magnetic field inside the loop will be directed perpendicular to the plane of the loop, pointing downwards. Outside the loop, the magnetic field lines will circulate around the loop.

Page 233

1. The magnetic field in a given region is uniform. Draw a diagram to represent it.

   Answer: In a uniform magnetic field, magnetic field lines are parallel, equidistant, and directed in the same direction.

Page 236

1. Choose the correct option: The magnetic field inside a long straight solenoid-carrying current

   • (a) is zero
   • (b) decreases as we move towards its end
   • (c) increases as we move towards its end
   • (d) is the same at all points

   Answer: (d) is the same at all points

Exercise Questions (Solved)

1. 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.

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

2. 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

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

3. The device used for producing electric current is called a:

   • (a) generator
   • (b) galvanometer
   • (c) ammeter
   • (d) motor

   Answer: (a) generator

4. A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each:

   • (a) two revolutions
   • (b) one revolution
   • (c) half revolution
   • (d) one-fourth revolution

   Answer: (c) half revolution

5. State the rule to determine the direction of a:

   • (a) Magnetic field produced around a straight conductor
   • (b) Force experienced by a current-carrying straight conductor in a magnetic field
   • (c) Current induced in a coil due to its rotation in a magnetic field

   Answer:

   • (a) Right-hand thumb rule: If you hold the conductor in your right hand with the thumb pointing in the direction of current, the direction in which your fingers curl gives the direction of the magnetic field.
   • (b) Fleming’s Left-Hand Rule: If the forefinger, middle finger, and thumb of your left hand are mutually perpendicular, with the forefinger pointing in the direction of the magnetic field and the middle finger in the direction of current, then the thumb gives the direction of the force.
   • (c) Fleming’s Right-Hand Rule: If the thumb, forefinger, and middle finger of your right hand are perpendicular to each other, with the thumb indicating the direction of motion, the forefinger the magnetic field, and the middle finger the direction of induced current.

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

   Answer: An electric generator works on the principle of electromagnetic induction, where a coil is rotated within a magnetic field to induce an electric current. The brushes in a generator ensure continuous contact between the rotating coil and the external circuit, allowing the current to flow out.

7. When does an electric short circuit occur?

   Answer: An electric short circuit occurs when live and neutral wires come into direct contact with each other, resulting in a very low-resistance path. This causes an excessive current flow, which can lead to overheating and damage.

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

   Answer: The earth wire provides a path for leakage current to flow into the ground, preventing electric shocks. Earthing metallic appliances is necessary to ensure that any leakage current flows into the earth, thereby safeguarding users from potential electric shocks.