Download on Electromagnetism

WHAT IS ELECTROMAGNETISM?
ELECTROMAGNETISM IS NOTHING BUT A
MAGNETIC FIELD PRODUCED BY AN
ELECTRIC CURRENT.
HANS OERSTED OBSERVED IN HIS EXPERIMENTS THAT
WHEN A CURRENT IS PASSED THROUGH A
CONDUCTING WIRE, A MAGNETIC FIELD IS
PRODUCED AROUND IT.
WHAT HAPPENS IN A STRAIGHT CONDUCTOR THROUGH WHICH CURRENT IS
PASSED? HOW IS THE MAGNETIC FIELD FORMED?
ONE OF THE RULES BY WHICH THE DIRECTION OF MAGNETIC FIELD IN A STRAIGHT
CONDUCTOR IS GIVEN BY FLEMING’S RIGHT HAND THUMB RULE.
IT STATES THAT IF WE HOLD A CURRENT CARRYING CONDUCTOR IN RIGHT
HAND SUCH THAT THE THUMB POINTS IN THE DIRECTION OF CURRENT,
THEN THE DIRECTION OF FINGERS ENCIRCLING THE CONDUCTOR GIVES
THE DIRECTION OF MAGNETIC LINES OF FORCE AROUND IT.
MAGNETIC FIELD DUE TO CURRENT IN A LOOP
THE MAGNETIC LINES BECOME DENSER IF
 THE STRENGTH OF CURRENT IS INCREASED
 THE NUMBER OF TURNS IN THE LOOP ARE INCREASED
WHAT IS A SOLENOID?
IT IS AN INSULATED COPPER WIRE WOUND OVER A PLASTIC TUBE WHICH BEHAVES
LIKE A MAGNET WHEN CURRENT IS PASSED THROUGH IT.
CURRENT CARRYING CONDUCTOR IN A MAGNETIC FIELD
The force F α current I
The force F α strength of magnetic field
The force F α length of the conductor
 when no current flows in the conductor, no force acts on the conductor and there
is no movement
 when current is passed through the conductor, a force acts on the conductor.
 on reversing the direction of current, the force acts in the opposite direction.
BEFORE WE GO INTO THE WORKING OF D.C.MOTOR, WE SHOULD KNOW FLEMING’S
LEFT HAND RULE.
WHY SHOULD WE KNOW?
ONLY THEN WE CAN FIND WHICH DIRECTION THE CONDUCTOR MOVES UP
PARTS OF A D.C.MOTOR
 CORE: Soft iron laminated cylinder
 COIL : Insulated copper wire of many turns
 COMMUTATOR: Reverses the direction of the coil every half turn.
 BRUSHES: To supply a continuous current to the coil through the commutator
 ELECTRO MAGNET: To provide a strong uniform magnetic field
 A D.C.SOURCE (BATTERY): To provide current to the coil
PRINCIPLE OF A D.C.MOTOR
https://www.youtube.com/watch?v=LAtPHANEfQo
REFER TO THIS LINK WHILE READING THIS
NOTES
PRINCIPLE OF DC MOTOR: WHEN AN ELECTRIC CURRENT IS PASSED
THROUGH A CONDUCTOR PLACED NORMALLY IN A MAGNETIC FIELD, A
FORCE ACTS ON THE CONDUCTOR WHICH MAKES IT TO MOVE. THE
DIRECTION IS GIVEN BY FLEMING’S LEFT HAND RULE
WORKING OF A D.C.MOTOR
 THE PLANE ABCD IS HORIZONTAL AND SPLIT RING S1 TOUCHES BRUSH B1,
SPLIT RING S2 TOUCHES BRUSH B2
 CURRENT FLOWS IN DIRECTION ABCD
 CURRENT IN ARMS AB AND CD IS PERPENDICULAR TO THE MAGNETIC
FIELD. SO EACH ARM EXPERIENCES A FORCE WHICH PRODUCES A COUPLE
 AS PER FLEMING’S LEFT HAND RULE, AB MOVES DOWNWARDS, AND CD
MOVES UPWARDS. (VERY IMPORTANT)
 COIL CD MOVES UPWARDS,
 90 DEGREES- ---- PERPENDICULAR TO THE PLANE OF PAPER. SO NO
CONTACT BETWEEN BRUSH AND SPLIT RING. SO CURRENT FLOWS
 180 DEGREES ---- ROTATES DUE TO MOMENTUM. SO SPLIT RING S1
TOUCHES BRUSH B2, SPLIT RING S2 TOUCHES BRUSH B1. NOW FORCE IS
MAXIMUM WHEN THE COIL TURNS 180 DEGREES
 270 DEGREES ---- AGAIN NO CONTACT LIKE IN 90 DEGREES. BUT DUE TO
MOMENTUM IT ROTATES
 360 DEGREES ---- CONTINUES TO MOVE IN THE CLOCKWISE DIRECTION
IF A MAGNETIC FIELD IS PRODUCED BY AN ELECTRIC CURRENT, THEN IT
SHOULD BE POSSIBLE TO PRODUCE AN ELECTRIC CURRENT BY THE MAGNETIC
FIELD.
WHEN THERE IS A CHANGE IN THE NUMBER OF MAGNETIC
FIELD LINES LINKED WITH THE CONDUCTOR, AN EMF IS
DEVELOPED BETWEEN THE ENDS OF THE CONDUCTOR. THIS
CONTINUES TILL THE CHANGE IS HAPPENING. THIS
PHENOMENON IS CALLED ELECTROMAGNETIC INDUCTION.
To explain in a more simple manner, electric current can be produced in a wire
by simply moving a magnet in or out of a coiled part of wire. Voltage is
induced only as long there is relative motion between the coil and the
magnet.
WHEN AN ALTERNATING CURRENT IS PASSED IN THE PRIMARY
COIL,WOUND ON A SOFT IRON CORE, AN INDUCED EMF IS
PRODUCED IN THE SECONDARY COIL, WOUND ON A SOFT IRON
CORE.
Do you see that one coil is
connected to a AC source and
the other coil connected to a
galvanometer?
We call the first coil(connected
to power) as primary coil. And
the second coil( connected to
galvanometer) as secondary
coil.
When switch is ON in the
primary, current flows in the
primary coil and a magnetic
field is created. When the
switch is continuously switched ON and OFF, a varying magnetic field is
created in the secondary coil due to induction. This creates a voltage in the
secondary coil.
PRINCIPLE OF A TRANSFORMER
TRANSFORMERS WORK ON THE PRINCIPLE OF MUTUAL INDUCTION.
TRANSFORMER IS A DEVICE WHERE THE AMPLITUDE OF AN ALTERNATING EMF
CAN BE EITHER INCREASED OR DECREASED.
HOWEVER THE FREQUENCY IS UNCHANGED (50 Hz)
TRANSFORMERS
STEP UP TRANSFORMERS
1. To change from low to high
voltage
2. Number of turns of sec. coil
is more compared to primary
STEP DOWN TRANSFORMERS
to change from high voltage to low voltage
Number of turns of primary coil is more
compared to primary
Construction of a transformer
TURNS RATIO: NUMBER OF TURNS IN SECONDARY COIL
NUMBER OF TURNS IN PRIMARY COIL
IF THIS RATIO> 1, THEN IT IS A STEP UP TRNASFORMER
IF THIS RATIO< 1, THEN IT IS A STEP DOWN TRNASFORMER
IT CONVERTS MECHANICAL ENERGY TO ELECTRICAL ENERGY USING THE PRINCIPLE
OF ELECTROMAGNETIC INDUCTION
PRINCIPLE : A COIL IS ROTATED IN A MAGNETIC FIELD. WHEN IT IS ROTATING, THE
COIL CUTS THE MAGNETIC FIELD. THIS INDUCES AN EMF. THUS A GENERATOR
ACTS AS A SOURCE OF CURRENT.
CONSTRUCTION OF GENERATOR
PARTS OF A GENERATOR
 FIELD MAGNET: IT PRODUCES A RADIAL MAGNETIC FIELD
 ARMATURE
: IT IS A SOFT IRON CORE ON WHICH A COIL ABCD
HAVING A LARGE NUMBER OF TURNS OF INSULATED
COPPER WIRE
 SLIP RINGS
:TWO COAXIAL METALLIC RINGS WHICH ROTATE WITH
THE COIL
 BRUSHES
: MADE OF CARBON. DO NOT ROTATE WITH THE COIL
AND SLIP RINGS
Working of an ac. Generator:
See this link:
https://www.youtube.com/watch?v=HLNugJwBRow
EXPLANATION OF WORKING OF AN AC GENERATOR
 BASICS TO REMEMBER
 Let the plane of coil be perpendicular to magnetic field. Maximum
magnetic lines are cut.
 As the coil rotates, fewer lines are cut.
 when the plane of coil is parallel, no lines are cut
 At 0 degrees: Coil is rotated clockwise. Magnetic flux decreases and
therefore emf is induced and a current is produced.
 90 degrees coil rotation: plane of coil becomes parallel. No magnetic lines
are cut. Magnetic flux linked with the coil is zero. Emf induced is maximum
because rate of change of magnetic flux is maximum.
 180 degrees coil rotation: coil again becomes normal(perpendicular).
Magnetic lines passing through the coil becomes maximum. Rate of change
of magnetic flux is zero. No emf is induced in the coil.
 270 degrees coil rotation: magnetic flux linked with the coil decreases and
emf again increases to maximum value, but in opposite direction.
 360 degrees coil rotation: induced emf reduces to zero. And magnetic flux
linked with the coil becomes maximum.