Chp 13- Magnetic Effects Of Electric Current

Question 1

What is the SI unit of ‘Magnetic field strength’ and who is it named after?

Answer 1

Strength of magnetic field is measured in the terms of physical quantity called magnetic field induction or magnetic flux density. It is the noted by the B n it’s SI unit is tesla
Unit- Oersted (CGS)
Scientist- Hans Christian Oersted

Question 2

Define magnet, poles, magnetic field and magnetic field lines, solenoid, electromagnet

Answer 2

•A solid any shape or size which can attract pieces of material like iron, cobalt, nickel is called magnet
•The place where the attracting power of a bar magnet is maximum is called Pole. Poles r regions /small areas and not point.
•The three dimensional region surrounding a magnet dipole in which the force of the magnet can be detected by another magnent/ electromagnetic objects/ current carrying conductor is magnetic field.
• The imaginary path or a line along which the North Pole moves in the magnetic field and iron filing align themselves is called magnetic field lines.
-A solenoid is a long insulation wire wound in the form of a circular helix where neighboring turns are closely spaced.
-electromagnet- a magnet consisting of a soft iron coe with a coil of insulated wire wound around it is called electromanget. Alloy of nickel, iron and copper are also used.

Question 3

True or false-

Magnetic field is a scalar quantity

Answer 3

False- Magnetic field is a quantity that has both direction and magnitude and thus is a vector quantity

Question 4

define uniform magnetic field

Answer 4

The magnitude as well as the direction of magnetic field at constant such a field is called uniform magnetic field

Question 5

True or false-

The far are the field lines, the strong is the magnetic field

Answer 5

False - The relative strength of a magnetic field as shown by the degree of closeness of the field lines

Question 6

True or false

No two field lines are found to cross each other

Answer 6

True
,Reason - Tangent drawn at any point on the magnetic field gives the direction of the magnetic field. So if the magnetic field lines intersect each other, so it would mean that at the point of the intersection the compass needle point at two different directions which is actually not possible

Question 7

On what factors does the magnetic effect of electric current depend on?

Answer 7

• Current
The magnitude of the magnetic field produced at a given point increases as the current through the wire increases.
• Distance
Magnetic field produced by a given current in the conductor decreases as the distance from it increases
It can be noted that the concentric circle field around a current carrying straight wire becomes larger and larger as the move away from it

Question 8

What does

1) Maxwell’s corkscrew rule indicate
2) flemings left hand rule

Answer 8

Maxwell’s corkscrew rule is also called Right Hand thumb rule And it indicates the direction of the magnetic field
2)direction of force on current carrying conductor.(thumb force, Fore finger: Field, Central fingrer:C urrent)

Question 9

How do magnetic field lines look like in a circular loop?

Answer 9

the direction of magnetic field inside the solenoid id parallel to length of solenoid. at the ends of solenoid,the strength of MF that in the middle of solenoid. A circular loop is made up of many tiny straight wires. And each section of a current carrying wire contribute in the production of magnetic field. Ina circular loop every point on the wire carrying current would give rise to a magnetic field. And these concentric circles representing the magnetic field around it what become larger and larger as we move away from the wire. At the centre of the loop the field lines become straight and perpendicular to the plane of the coil.

Question 10

On what factors does the Magnetic field due to the current in a circular loop depends on

Answer 10

The magnetic field around a straight current carrying wire can we increase the pending it into a circular loop.
Factors: 1) Amount of current flowing
• If the current increases the deflection of the magnetic compass also increases
Directly proportional
2) Radius of the circular wire
• strength of the magnetic field is inversely proportional to the radius of the circular wire
• The deflection of the needle of the magnetic compass is kept at the centre of circular loop with smaller radius is more than that of the circular loop with big radius
3) Number of turns in the circular loop
• If the no. Of turns per unit length in a circular loop is increased then the magnetic field also increases. the more tightly the turns are wound, stronger will be the magnetic field .

Question 11

True. Or false -
If there is a circular coil. having N turns then the magnetic field produced is N times as small asproduced by the single turn

Answer 11

False. LARGE by N times
Reason - This is because the current in it circular turn has the same direction and field due to each turn then just adds up

Question 12

What are the properties of bar magnet

Answer 12

-•Attractive nature-when Iron filing is put near a bar magnet, the magnet attracts the iron filing feeling towards it. The attracting power is maximum near the ends and minimum at the centre.
-•Directive nature- When a magnet is suspended freeely it align itself to north south direction
The pole of the bar magnet pointing towards north direction when suspended freely is called north o ke. The pole of the bar magnet pointing toward south direction when suspended freely is called south pole .
-• Poles-exist in pairs- in bar magnet there are always two poles which are of equal strength and opposite nature. this also means that magnetic mono poles do not exist.
-• Like poles repel unlike poles attract.
-• Inductive nature- Hen certain substances like soft iron, steel , cobalt, nickel are placed near a bar magnet, they acquire magnetisation a called induced magnetisation. The phenomena is called magnetic induction.

Question 13

What are natural and artificial magnets

Answer 13

Natural magnets are the naturally occurring minerals or ores having magnetic power. Due to irregular shape and weak attractive in power, natural magnets like lodestone are nowadays rarely used.

Artificial magnets - nowadays pieces of iron and many other materials of suitable shapes and sizes are made as magnets by passing current through wire wound around them. Such magnets are called artificial magnets. For example – bar magnet, U shaped magnet, magnetic needle. Permanent magnets are usually made up of how to steel, carbon steel, chromium steel, cobalt steel, tungsten steel, Alnico. Etc. Such materials require a strong magnetic field for the magnetisation. But once they get magnetised they have a residual magnetism for a long period of time

Question 14

What are the properties of magnetic lines of force

Answer 14

•-•The direction of the magnetic field is taken to be the direction in which the north pole of the compass needle moves inside it . Magnetic field lines originate from the north pole and terminate at the south pole outside of a magnet. The direction of the magnetic field lines inside the magnet is from south pole to North Pole and outside from north to south.
•-• They form continuous closed curves.
-••The magnetic field lines never intersect each other. If they intersect there will be two directions for the magnetic field which is not possible.
•-• The density of magnetic lines decreases as the distance between the poles increases.
•-• Magnetic strength increases at the dense magnetic field lines.The strength of magnetic field is shown by the degree of closeness of the field lines
•-• The tangent on any point of the magnetic field gives the direction if the magnetic field.

Question 15

How can a magnet attract a piece of iron that is not magnetised?

Answer 15

The magnet causes the piece of iron to become temporary magnetised. It involves inducing opposite Pole. Since opposite poles attract and and magnetised piece of iron is always attracted towards a magnet

Question 16

If you break a magnet into two pieces can we get isolated polls? Give reasons for this and try to explain this on the basis of magnetic field lines

Answer 16

No, we will not get isolated polls bcoz the magnet is always dipole. The magnetic field lines are closed curve that is they not only the surround the magnet but also pass within on the material of the magnet. THUS, when a magnet is broken into two pieces,The end of the broken piece from the where the magnetic field lines emerge out behaves like north pole. Similarly the end of the broken piece from where the magnetic field lines enters into them behave like South Pole. This is how we get two magnets

Question 17

What of ferromagnetic materials and how to demagnetise a magnet

Answer 17

The materials like iron, nickel, cobalt when placed in a magnetic field get strongly magnetised in the direction of the field that is the fear inside them is greatly enhanced. Such materials are called ferromagnetic materials.
The magnetism of a magnet can be removed by heating it to very high temperature or putting the magnet in a current carrying solenoid in such a way that magnetisation of the magnet is opposite to the applied magnetic field

Question 18

What are the uses of magnet

Answer 18

Used in radio N stereo speakers
used in Almira and refrigerator
used on video and audio tape
use on hard disk and floppies of computer
used in different children’s toys
used in magnetic resonance imaging MRI
used to lift heavy iron pieces, electric magnetic separation in metallurgy.

Question 19

You are provided with two similar bars, one is a magnet and the other is a soft iron. How will you distinguish between them without the use of any other magnet or bar?

Answer 19

A magnet when suspended freely will rest only in north-south direction, but the soft iron bar will rest in any direction.

Question 20

Explain the following :

(a) When two pins are hung by their heads from the same pole of a magnet, their pointed ends moves apart.
(b) Several soft iron pins can cling, one below the other, from the pole of a magnet.
(c) The north end of a freely suspended magnetic needle gets attracted towards a piece of soft iron placed a little distance away from the needle.

Answer 20

(a) When two pins are hung by their heads from the same pole of a magnet, they acquire same polarity. Because like poles repel each other, their pointed ends move apart.
(b) Several soft iron pins can cling one below the other from the pole of a magnet because the magnet induces magnetism in an iron nail which gets attracted by the magnet and clings to it. This magnetized nail magnetizes the other nail near it by magnetic induction and attracts it. This process continues until force of attraction on first nail is sufficient to balance the total weight of all nails in chain.
(c) When a piece of soft iron is placed a little distance away from the needle, the needle induces magnetism to the piece of soft iron. Thus, soft iron piece starts behaving like a magnet and it attracts the magnetic needle towards it.

Question 21

what is induced magnetism?and state its laws

Answer 21

the magnetism acquired by magnetic material when it is kept near a magnet is callled induced mamgnetism.when a magnet is brought near a magnetic material, the magnetic pole of the magent induces and oppposite polarity on the near end and simlar polarity at the farther end.
laws-
-induction of poles at ends: during magnetic induction, the opposite poleis induced at near end and like pole is induced at other end
-Induction preceds attraction: when a piece of magnetic material is brought near a magnet , it first becomes a magnet by induction and then it is attracted.
- attraction is the not the sure sign of presence of magnetism but repulsion.

Question 22

Explain why iron filings which are sprinkled on a sheet of cardboard over a bar magnet take up a definite pattern when cardboard is slightly tapped.

Answer 22

The iron filings take up a definite pattern (curved lines). This happens because each piece of iron filing becomes a magnet to the magnetic induction of the magnet. It thus experiences a force in the direction of magnetic field of the bar magnet at that point and aligns itself along curved lines.

Question 23

What is a neutral point? How is the position of neutral point located with the use of a compass needle?

Answer 23

Neutral points are the points where the magnetic field of the magnet is equal in magnitude to the earth’s horizontal magnetic field, but it is in opposite direction. Hence they cancel each other and Thus the net magnetic field at the neutral points is zero.
Since the net magnetic field is zero at neutral points, the compass needle remains unaffected (i.e. it comes to rest pointing in any direction) at these points and hence, they can be detected.

Question 24

State the positions of neutral points when a magnet is placed with its axis in the magnetic meridian and with its north pole (i) pointing towards the geographic north and (ii) pointing towards the geographic south.

Answer 24

i) Neutral points will be in east-west direction.

(ii) Neutral points will be north-south direction.

Question 25

what is the force on a current carrying conductor in a magnetic field

Answer 25

an electric current flowing thorugh the conductor produces a magetic field in the surrounding space and exerts a force on the magnet placed near it.the magnet also exerts an equal and opposite force on the current carrying wire. force of current carrying straight rod is largest when the direction of current is at right angles to the direction of the magnetic field and it is zero when the rod and field a re parallel to each other..
devices that use current carrying conductor and magnetic field include elctric motor, electric generator ,loudspeakers, microphones, and measuring instruments like ammeter, voltmetre,etc.

Question 26

force on a moving charge in a magnetic field

Answer 26

since current is constituted of charges, charges must also experience a force when they are placed in a magnetic field. Force is zero if teh charge is moving in the direction or oppostie to the direction of magnetic field. the magnitude of force is max when velocity is perpendicular to magnetic field. the direction of force is always perpendicular to the plane of velocity and magnetic field. Also, the direcuion of force depends on the sign of charge, i.e it is directed opposite for the negative and positive charges.

Question 27

write a short note on working of electric motor components

Answer 27

An electric motor is used to convert electrical energy into mechanical energy.

A motor works on the principle that when a rectangular coil is placed in a magnetic field and current is passed through it, a torque( two equal, opposite and parallel forces acting on side is coil produce torque) acts on the coil which rotates it continuously when the coil rotates, the shaft attached to it also rotates. In this way the electrical energy supplied to the motor is converted into the mechanical energy of rotation.

Uuses- Drills, Water Pumps, Hard Disc Drives, Washing Machines, Industrial Equipment
commercial motors use : (i) an electromagnet in place of permanent magnet; (
ii) large number of turns of the conducting wire in the current crrying coil; and
(iii) a soft iron core on which the coil is wound.

Question 28

what is

1) power source
2) commutator
3) field magnet
4) armature,armature core and armature coil
5) brushes

Answer 28

◉Power Source: A simple motor usually has a DC power source. It supplies power to the motor armature or field coils.
◉Commutator: It is the rotating interface of the armature coil with a stationary circuit.it reverses the direction of flow of currrent through the circuit.
◉Field Magnet: The magnetic field helps to produce a torque on the rotating armature coil by virtue of Fleming’s left-hand rule.
◉armature: the combination of soft iron core and coil is called armature. soft iron increases the magnetic effects, thereby increasing the power of the motor
◉Armature Core: Holds the armature coil in place and provides mechanical support.
◉Armature Coil: It helps the motor to run.
◉Brushes: It is a device that conducts current between stationary wires and moving parts, most commonly the rotating shaft.

Question 29

magnetism in medicine

Answer 29

An electric current always produces a magnetic field. Even weak ion currents that travel along the nerve cells in our body produce magnetic fields.
When we touch something, our nerves carry an electric impulse to the muscles we need to use. This
impulse produces a temporary magnetic field.
These fields are very weak and are about one-billionth of the earth’s magnetic field.
Two main organs in the human body where the magnetic field produced is significant, are the heart and the brain.
The magnetic field inside the body forms the basis of obtaining the images of different body parts.
This is done using a technique called Magnetic Resonance Imaging (MRI). Analysis of these
images helps in medical diagnosis. Magnetism has, thus, got important uses in medicine.

Question 30

What are the conclusions when a magnet is moved towords the coil or a coil is moved towards the magnet

Answer 30

motion of a magnet with respect to the coil produces an induced potential difference, which sets up an induced electric current in the circuit.

North pole going inside-toward-right North pole coming out -left
South pole going inside-towars left
South moving outside- right.

• Current is induced due to relative motion between coil an magnet.
• Deflection reverses when poles of magnet are reversed
• faster the motion, stronger induced current.

Question 31

Generator principal

Answer 31

The electric generator is an application of electromagnetic induction. The electric generator works on the principle that when a straight conductor is moved in a magnetic field, then current is induced in the conductor. In an electric generator, a rectangular coil (having straight sides) is made to rotate rapidly in the magnetic field between the poles of a horseshoe-type magnet. When the coil rotates, it cuts the magnetic field lines due to which a current is produced in the coil.

Question 32

AC generator and it’s construction

Answer 32

When a coil (conductor) is rotated in a magnetic field, the magnetic flux linked with it Changes and therefore an alternating em.f. is induced in the coil..

Construction: The main parts of a dynamo are-
1. Field magnet: It is a strong horse shoe permanent magnet. An electromagnet run by a DC source can also be used for high power generators.

1. Armature: It is a soft iron core on which a coil ABCD having a large number of turns of insulated copper wire is wound, This armature (or coil) is rotated rapidly in the magnetic field between the poles of the magnet.

Slip rings: The ends of the armature (or the coil) are connected to two coaxial metallic slip rings R1 and R2 which rotate along with the coil.

4. Brushes: Two brushes B₁ and B₂ made of carbon, press against the slip rings R1 n R2 respectively The external circuit (i.e. load) is connected between the other ends of brushes. The brushes B₁ and B₂ do not rotate along with the coil.

Question 33

AC in India

Answer 33

The alternating current (AC) produced in India has a frequency of 50 Hz. That is, the coil is rotated at the rate of 50 revolutions per second. Since in 1 revolution of coil, the current changes its direction 2 times, so in 50 revolutions of coil, the current changes its direction 2 x 50 = 100 times. Thus, the AC supply in India changes its direction 100 times in 1 second. Another way of sad this is that the alternating current produced in India changes its direction every for 1/100 of a second. That is, each terminal of the coil is (+) for the next 1/100 for 1/100 of a sec and negative for 1/100 of a sec

Question 34

what is DC Dynamo

Answer 34

“DC generator” means “Direct Current generator”. That is, a DC generator produces direct current.

Actually, if we replace the slip rings of an A.C. generator by a commutator, then it will become a D.C. generator. The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically.

When the two half rings of commutator are connected to th ends of the generator coil, then one carbon brush is at all ti contact with the coil arm moving down in the magnetic field the other carbon brush always remains in contact with the c moving up in the magnetic field. Due to this, the current
circuit always flows in one direction. So, it is direct current