Magnetic effect of current - Class 10
Magnet: Magnet is an object that attracts objects made of iron, cobalt and nickle. Magnet comes to rest in North – South direction, when suspended freely.
Use of Magnets: Magnets are used
·
in refrigerators.
·
in radio and stereo speakers.
·
in audio and video cassette players.
·
in children’s toys and;
·
on hard discs and floppies of computers.
Properties of Magnet
·
A free suspended magnet always points towards the north and
south direction.
·
The pole of a magnet which points toward north direction is
called north pole or north-seeking.
·
The pole of a magnet which points toward south direction is
called south pole or south seeking.
·
Like poles of magnets repel each other while unlike poles of
magnets attract each other.
Hans Christian Oersted (1777-1851)
Oersted showed
that electricity and magnetism are related to each other. His research later used in radio, television etc. The unit of magnetic field
strength is name Oersted in his honour.
Oersted Experiment
On passing
the current through
the copper wire XY in the circuit,
the compass needle which is placed near the conductor gets deflected.
If we reverse the direction
of current, the compass needle deflects in reverse direction. If we stop the flow of current, the needle comes
at rest.
Hence, it concludes that electricity and magnetism are linked to each other. It shows that whenever the current will flow through the conductor, then magnetic field around. it will developer
Magnetic field: The area around a magnet where a magnetic force is experienced is called the magnetic field. It is a quantity that has both direction and magnitude, (i.e., Vector quantity).
Magnetic field and field lines: The influence of force surrounding a magnet is called magnetic field. In the magnetic field, the force exerted by a magnet can be detected using a compass or any other magnet. The magnetic field is represented by magnetic field lines.
Direction of field line: Outside the
magnet, the direction of magnetic field line is taken from North pole to South
Pole. Inside the magnet, the direction of magnetic field line is taken from
South pole to North pole.
Strength of magnetic
field: The
closeness of field lines shows the relative strength of magnetic field, i.e.
closer lines show stronger magnetic field and vice – versa. Crowded field lines
near the poles of magnet show more strength.
·
The magnitude of magnetic field increases with increase in
electric current and decreases with decrease in electric current.
·
The magnitude of magnetic field produced by electric current
decreases with increase in distance and vice – versa. The size of concentric
circles of magnetic field lines increases with distance from the conductor,
which shows that magnetic field decreases with distance.
·
Magnetic field lines are always parallel to each other.
·
No two field lines cross each other.
It is taken by convention that magnetic field
lines emerge from North pole and merge at the South pole. Inside the magnet,
their direction is from South pole to North pole. Therefore magnetic field
lines are closed curves.
Magnetic field lines due to current a current carrying straight
conductor
A current carrying straight conductor has
magnetic field in the form of concentric circles, around it. Magnetic field of
current carrying straight conductor can be shown by magnetic field lines.
The direction of magnetic field through a
current carrying conductor depends upon the direction of flow electric current.
Let a current carrying conductor be suspended
vertically and the electric current is flowing from south to north. In this
case, the direction of magnetic field will be anticlockwise. If the current is
flowing from north to south, the direction of magnetic field will be clockwise.
current through a straight conductor can be
depicted by using the Right Hand Thumb Rule. It is also known as Maxwell’s
Corkscrew Rule.
Maxwell’s Corkscrew rule: As per Maxwell’s Corkscrew Rule, if the direction of forward movement of screw shows the direction of the current, then the direction of rotation of screw shows the direction of magnetic field.
Magnetic field lines due to a current through a circular loop
In case of a circular current carrying conductor, the magnetic field lines
would be in the form of iron concentric circles around every part of the
periphery of the conductor. Since, magnetic field lines tend to remain closer
when near to the conductor, so the magnetic field would be stronger near the
periphery of the loop. On the other hand, the magnetic field lines would be
distant from each other when we move towards the centre of the current carrying
loop. Finally, at the centre, the arcs of big circles would appear as a
straight line.
The direction of the magnetic field can be identified using
Right Hand Thumb’s Rule. Let us assume that the current is moving in
anti-clockwise direction in the loop. In that case, the magnetic field would be
in clockwise direction, at the top of the loop. Moreover, it would be in an
anti-clockwise direction at the bottom of the loop.
Clock Face Rule: A current carrying
loop works like a disc magnet. The polarity of this magnet can be easily
understood with the help of Clock Face Rule. If the current is flowing in anti
– clockwise direction, then the face of the loop shows north pole. On the other
hand, if the current is flowing in clockwise direction, then the face of the
loop shows south pole.
Magnetic
field due to a current in a Solenoid: Solenoid is the coil with many
circular turns of insulated copper wire wrapped closely in the shape of a
cylinder. A current carrying solenoid produces similar pattern of magnetic
field as a bar magnet. One end of solenoid behaves as the north pole and another
end behaves as the south pole.
Magnetic field lines are parallel inside the solenoid, similar to a bar magnet,
which shows that magnetic field is same at all points inside the solenoid.
The strength of magnetic field is proportional to the number of turns and magnitude of current.
By producing a strong magnetic field inside the solenoid, magnetic materials can be magnetized.
Magnetic field and
number of turns of coil: Magnitude of magnetic field gets summed up with increase
in the number of turns of coil. If there are ‘n’ turns of coil, magnitude of
magnetic field will be ‘n’ times of magnetic field in case of a single turn of
coil.
The strength of the
magnetic field at the centre of the loop(coil) depends on :
(i) Increasing
cross-section area/The radius of the coil: The strength of
the magnetic field is inversely proportional to the radius of the coil. If the
radius increases, the magnetic strength at the centre decreases
(ii) The number of turns
in the coil : As the number of turns in the coil increase,
the magnetic strength at the centre increases, because the current in each
circular turn is having the same direction, thus, the field due to each turn
adds up.
(iii) The strength of the
current flowing in the coil: As the strength of the
current increases, the strength of three magnetic fields also increases.
(iv) Inserting a soft
iron core: when core is inserted it get magnetized due to magnetic effect
of coil due to which it also form magnetic lines thus magnetic field intensity
increase.
Electromagnet: An electromagnet consists of a long coil of insulated copper wire wrapped on a soft iron. Magnet formed by producing magnetic field inside a solenoid is called electromagnet.
Electromagnets are very widely used in electric and
electromechanical devices, including:
- Motors and generators.
- Electric bells and buzzers.
- Loudspeakers and headphones.
- Magnetic recording and data
storage equipment: tape recorders, VCRs, hard disks.
- MRI machines.
Force on a current carrying conductor in a magnetic field: A current
carrying conductor exerts a force when a magnet is placed in its vicinity.
Similarly, a magnet also exerts equal and opposite force on the current
carrying conductor. This was suggested by Marie Ampere, a French Physicist and
considered as founder of science of electromagnetism.
The direction of force over the conductor gets reversed with the
change in direction of flow of electric current. It is observed that the
magnitude of force is highest when the direction of current is at right angles
to the magnetic field.
If we stretch the thumb, middle finger and the index finger of the left hand mutually Perpendicular to each other in such a way that Index Finger represents the direction of the magnetic field (B), Middle Finger represents the direction of the current (I) than Thumb points towards the direction of force (F) act on the conductor placed in magnetic field.
Electric motor: A device that converts electrical energy to mechanical
energy. It is of two types : AC and DC Motor.
Principle of Electric Motor: When a rectangular coil is placed in a
magnetic field and a current is passed through it, force acts on the coil,
which rotates it continuously. With the rotation of the coil, the shaft
attached to it also rotates.
Construction: It consists of the following parts :
·
Armature: It is a rectangular coil (ABCD) made up of laminated
copper wire which is suspended between the two poles of a magnetic field.
The electric supply to the coil is connected with a commutator.
·
Commutator or Split – ring: Commutator is a device which
reverses the direction of flow of electric current through a circuit. It is two
halves of the same metallic ring.
·
Magnet: Magnetic field is supplied bv a permanent magnet NS.
·
Sliding contacts or Carbon Brushes B1,B2 which are fixed made up
of graphite.
· Battery: These are consists of few cells.
Working
- Now, when the electric current is passed through the
rectangular coil ABCD. We notice that the current between BC and AD arm
are parallel to the magnetic field, whereas the current between AB and CD
is perpendicular to the magnetic field. Therefore the magnetic field will
only act upon the AB and CD arms.
- From Fleming’s left-hand rule,
in the AB arm, the direction of force is downwards and the magnetic field
is from north to south. Similarly, in the CD arm, the direction of force
is upward.
- Therefore, the forces in the AB
and CD arm are in opposite directions, this will result in the rotation of
the rectangular coil ABCD.
- After half rotation, the ring
S1 will come in contact with the brush B2 and ring S2 in contact with the
B1, this will lead to a change in direction of the current.
- Since the direction of the
current is changed, the direction of forces in the arm AB and CD will also
change, hence the coil continues to rotate in the same direction.
In commercial motor, electromagnet instead of permanent magnet
and armature is used. Armature is a soft iron core with large number of
conducting wire turns over it. Large number of turns of conducting wire
enhances the magnetic field produced by armature.
Uses of motors :
·
Used in electric fans.
·
Used for pumping water.
·
Used in various toys.
Electricity production as a result of magnetism (induced current) is called Electromagnetic
Induction.
When a conductor is set to move inside a magnetic field or a magnetic field is
set to be changing around a conductor, electric current is induced in the
conductor. This is just opposite to the exertion of force by a current carrying
conductor inside a magnetic field. In other words, when a conductor is brought
in relative motion vis – a – vis a magnetic field, a potential difference is
induced in it. This is known as electromagnetic induction.
Fleming’s Right-Hand Rule:
The mutually perpendicular directions also point to an important fact that when the magnetic field and movement of conductor are perpendicular, the magnitude of induced current would be maximum.
Electromagnetic induction is used in the conversion of kinetic energy into electrical energy.
Electric generators are of two types: AC generator and a DC generator.
Principle of electric generator: Electric motor works on the basis of
electromagnetic induction.
The Electric AC Generator Consists of the Following Components
- A rectangular wireframe or a
rectangular coil, which is connected to the brushes.
- Two strong magnets can be any
kind like it may be horseshoe magnets, a bar
magnet,
etc.
- The ends of the coils are
connected to the rings as shown in the diagram. The edges of the rings are
further connected to brushes.
- To detect the electricity a
galvanometer is used.
Working
of An AC Electric Generator
·
A rectangular coil is placed between the two
magnets. Let us assume that we are rotating the coil in a clockwise direction
with the help of axles connected to the rings.
·
On rotating the coil
in a clockwise direction AB arm will move upwards and the CD arm will move
downwards. We can apply Fleming’s Right-hand rule to the AB arm and the current
flows from A to B. Similarly, we use Fleming’s Left-hand rule to CD arm and we
note that current flows from C to D. Hence the current will flow from B2 to B1.
To convert an A.C generator into a D.C generator, a split ring
commutator is used. This helps in producing direct current.
Electrical generator is used to convert mechanical energy into electrical
energy.
DC generator
Construction of DC generator :
A simple DC generator consists of
a coil of insulated copper wire. The coil is placed between the two poles of a
strong horseshoe magnet. In actual practice, a large number of turns of the
insulated copper wire are wound on a soft iron core.
The two
ends of the coil are connected to the two halves of a split ring (R1,
R2 called commutator). Two carbon brushes press against the
two half–rings lightly. The current is taken out through the brushes B1 and
B2.
Working Principle of DC generator
Let the
coil ABCD be initially in the horizontal position, and be rotated
anticlockwise. When the coil is rotated anticlockwise, the arm AB moves
downwards and the arm CD moves upwards.
The coil during this motion cuts the magnetic lines of force and an induced current is produced in the coil. According to Fleming’s right hand rule, during the downward motion of the arm AB, the induced current flows from B to A in the arm AB, and from D to C in the arm CD. The current so produced is taken out through the two half–split rings, and the carbon brushes.
After half the rotation (rotation through 180º), the arms of the coil interchanged their positions; the arm AB comes to right and the arm CD to the left. Then the arm CD starts moving downwards, and the arm AB upwards. During this half–rotation, the induced current flows from C to D in the arm CD, and from A to B in the arm AB.
The two half–split rings (R1 and R2) rotate with the coil and touch the two carbon brushed (B1, B2) one by one. As a result, each carbon brush continues to have the same polarity, (+ or –). The brush B2 always remains positive (+) terminal, and the brush B1 remains negative (–) terminal. The current so produced is called direct current (DC).
DC generator differ from an AC generator
The basic
design of AC and DC generators is similar. The two generators differ only in
the design of slip rings at the ends of the coil wire. An AC generator use two
full rings called slip rings, one at each end of the coil wire, while an DC
generator has two half–rings (called split rings) of a commutator.
A.C
and D.C Current
A.C – Alternate Current: Current in which direction is changed
periodically is called Alternate Current. In India, most of the power stations
generate alternate current. The direction of current changes after every 1/100
second in India, i.e. the frequency of A.C in India
is 50 Hz. A.C is transmitted upto a long distance without much loss of energy
is advantage of A.C over D.C.
D.C – Direct Current: Current that flows in one direction only is called Direct
current. Electrochemical cells produce direct current.
Advantages of A.C over D.C
·
Cost of generatior of A.C is much less than that of D.C.
·
A.C can be easily converted to D.C.
·
A.C can be controlled by the use of choke which involves less
loss of power whereas, D.C can be controlled using resistances which involves
high energy loss.
·
AC can be transmitted over long distances without much loss of
energy.
· AC machines are stout and durable and do not need much maintenance.
Disadvantages of AC
·
AC cannot be used for the electrolysis process or showing
electromagnetism as it reverses its polarity.
·
AC is more dangerous than DC.
|
Alternating Current |
Direct Current |
|
AC is safe to transfer longer
distance even between two cities, and maintain the electric power. |
DC cannot travel for a very long
distance. It loses electric power. |
|
The rotating magnets cause the
change in direction of electric flow. |
The steady magnetism makes DC flow
in a single direction. |
|
The frequency of AC is dependent
upon the country. But, generally, the frequency is 50 Hz or 60 Hz. |
DC has no frequency of zero
frequency. |
|
In AC the flow of current changes
its direction backwards periodically. |
It flows in a single direction
steadily. |
|
Electrons in AC keep changing its
directions – backward and forward |
Electrons only move in one
direction – that is forward. |
The 3 wires are as follows
·
Live wire – (Red insulated, Positive)
·
Neutral wire – (Black insulated, Negative)
·
Earth wire – (Green insulated) for safety measure to ensure that
any leakage of current to a metallic body does not give any serious shock to a
user.
It happens due to
·
damage pf insulation in power lines.
·
a fault in an electrical appliance.
A sudden large amount of current flows through the wire, which causes
overheating of wire and may cause fire also.
·
Fuse is always connected to live wire.
·
Fuse is always connected in series to the electric circuit.
·
Fuse is always connected to the beginning of an electric
circuit.
·
Fuse works on the heating effect.
·
)Electric Fuse current rating defines the maximum value of safe
current allowed to flow through fuse with out melting it
· Tin-lead alloy is general used to make fuse wire
Comments
Post a Comment