INTRODUCTION — HOW ARE ELECTRICITY AND MAGNETISM RELATED?
Electricity and magnetism are closely connected with each
other. Whenever electric current flows through a conductor, a magnetic field is
produced around it.
This phenomenon is called:
Magnetic Effect of Electric Current
This discovery completely changed the world and led to
inventions such as:
- Electric
motors
- Generators
- Loudspeakers
- Fans
- Televisions
ACTIVITY 12.1 — CURRENT PRODUCES MAGNETIC EFFECT
Procedure
- Take
a straight copper wire connected to a battery.
- Place
a compass needle near the wire.
- Switch
on the current.
Observation
The compass needle gets deflected.
Conclusion
Electric current produces a magnetic field around the
conductor.
OERSTED’S DISCOVERY
In 1820:
Hans Christian Oersted
discovered that a compass needle gets deflected when placed
near a current-carrying wire.
This proved the relationship between:
- Electricity
- Magnetism
12.1 MAGNETIC FIELD AND FIELD LINES
MAGNETIC FIELD
The region around a magnet where magnetic force can be
experienced is called:
Magnetic Field
Example:
- A
compass needle moves near a magnet because of magnetic field.
MAGNETIC FIELD LINES
Magnetic field lines are imaginary lines representing the
magnetic field around a magnet.
These lines show:
- Direction
of magnetic field
- Strength
of magnetic field
IMPORTANT PROPERTIES OF MAGNETIC FIELD LINES
1. Field lines emerge from North pole and enter South
pole outside magnet.
2. Inside the magnet, field lines move from South pole to
North pole.
Thus:
Magnetic field lines form closed curves.
3. No two field lines intersect each other.
Reason:
At intersection point, field would have two directions, which is impossible.
4. Closer field lines indicate stronger magnetic field.
ACTIVITY 12.2 — IRON FILINGS EXPERIMENT
Procedure
- Place
a bar magnet under white paper.
- Sprinkle
iron filings.
- Tap
paper gently.
Observation
Iron filings arrange themselves in curved patterns around
magnet.
Conclusion
These patterns represent magnetic field lines.
MAGNETIC FIELD DUE TO CURRENT-CARRYING CONDUCTOR
When current flows through a straight conductor:
- Circular
magnetic field lines form around it.
ACTIVITY 12.3 — FIELD AROUND STRAIGHT CONDUCTOR
Procedure
- Pass
a wire through cardboard.
- Sprinkle
iron filings.
- Pass
current through wire.
Observation
Iron filings arrange in concentric circles around wire.
Conclusion
Magnetic field lines around straight conductor are circular.
RIGHT-HAND THUMB RULE
Given by:
James Clerk Maxwell
Rule:
If we hold a straight conductor in right hand such that:
- Thumb
points in direction of current
then:
- Curled
fingers show direction of magnetic field lines.
FACTORS AFFECTING MAGNETIC FIELD
Magnetic field strength increases when:
- Current
increases
- Distance
from conductor decreases
MAGNETIC FIELD DUE TO CIRCULAR LOOP
When a wire is bent into a circular loop:
- Magnetic
field lines around all parts combine at centre.
- Strong
magnetic field forms at centre.
Observation
Larger current:
- Stronger
magnetic field
More turns:
- Stronger
magnetic field
MAGNETIC FIELD DUE TO SOLENOID
SOLENOID
A long cylindrical coil containing many circular turns of
insulated copper wire is called:
Solenoid
MAGNETIC FIELD OF SOLENOID
When current passes through solenoid:
- Magnetic
field resembles bar magnet.
Features:
- One
end behaves as North pole
- Other
end behaves as South pole
- Magnetic
field inside solenoid is strong and uniform
ELECTROMAGNET
A temporary magnet produced using electric current is
called:
Electromagnet
Construction:
- Soft
iron core placed inside solenoid.
Applications:
- Electric
bell
- Cranes
lifting scrap iron
- Relays
- Loudspeakers
ADVANTAGES OF ELECTROMAGNETS
✔ Strength can be changed
✔ Can be switched ON/OFF
✔ Strong magnetic field produced
12.2 FORCE ON A CURRENT-CARRYING CONDUCTOR IN MAGNETIC
FIELD
When a current-carrying conductor is placed in magnetic
field:
- It
experiences a force.
Example:
- Electric
motor
ACTIVITY 12.4
Observation
Current-carrying aluminium rod moves when magnetic field is
applied.
Conclusion
Magnetic field exerts force on current-carrying conductor.
FACTORS AFFECTING FORCE
Force depends on:
- Strength
of magnetic field
- Current
through conductor
- Length
of conductor
- Angle
between conductor and field
Maximum force occurs when:
- Conductor
is perpendicular to magnetic field.
FLEMING’S LEFT-HAND RULE
Used to find direction of force.
Rule:
Stretch:
- Thumb
- Forefinger
- Middle
finger
of left hand mutually perpendicular.
Then:
- Forefinger
→ Magnetic field
- Middle
finger → Current
- Thumb
→ Force/Motion
ELECTRIC MOTOR
Definition
A device that converts electrical energy into mechanical
energy is called:
Electric Motor
Applications:
- Fans
- Mixers
- Washing
machines
- Water
pumps
- Electric
vehicles
PRINCIPLE OF MOTOR
A current-carrying conductor placed in magnetic field
experiences force.
CONSTRUCTION OF SIMPLE MOTOR
Main parts:
- Armature
coil
- Permanent
magnet
- Split-ring
commutator
- Brushes
- Battery
WORKING OF MOTOR
- Current
passes through coil.
- Magnetic
field exerts force.
- Coil
rotates.
- Split
ring reverses current direction every half rotation.
- Continuous
rotation occurs.
SPLIT-RING COMMUTATOR
Function:
- Reverses
current direction
- Maintains
continuous rotation
COMMERCIAL MOTORS
Commercial motors use:
- Electromagnets
- Soft
iron cores
- Large
number of turns
to produce strong torque.
12.3 ELECTROMAGNETIC INDUCTION
MICHAEL FARADAY
Discovered:
Electromagnetic Induction
ELECTROMAGNETIC INDUCTION
The process of producing electric current by changing
magnetic field is called:
Electromagnetic Induction
ACTIVITY 12.7
Observation
Galvanometer shows deflection when magnet moves inside coil.
Conclusion
Changing magnetic field induces electric current.
FACTORS AFFECTING INDUCED CURRENT
Induced current increases when:
- Magnet
moves faster
- Number
of coil turns increases
- Stronger
magnet used
FLEMING’S RIGHT-HAND RULE
Used to determine direction of induced current.
Rule:
Stretch right-hand thumb, forefinger and middle finger
mutually perpendicular.
Then:
- Thumb
→ Motion of conductor
- Forefinger
→ Magnetic field
- Middle
finger → Induced current
ELECTRIC GENERATOR
Definition
A device that converts mechanical energy into electrical
energy is called:
Electric Generator
PRINCIPLE
Works on:
Electromagnetic Induction
TYPES OF GENERATORS
|
Generator Type |
Output |
|
AC Generator |
Alternating current |
|
DC Generator |
Direct current |
AC GENERATOR
Uses:
- Slip
rings
Current direction changes periodically.
DC GENERATOR
Uses:
- Split-ring
commutator
Current flows in one direction.
DOMESTIC ELECTRIC CIRCUITS
Electricity supplied to homes:
- Alternating
current (AC)
In India:
- 220
V
- Frequency
= 50 Hz
LIVE WIRE
Usually:
Red coloured
Carries current from source.
NEUTRAL WIRE
Usually:
Black coloured
Completes circuit.
EARTH WIRE
Usually:
Green coloured
Protects from electric shock.
EARTHING
Connecting metal body of appliance to earth is called:
Earthing
Purpose:
- Prevents
electric shock
ELECTRIC FUSE
A safety device protecting circuits from:
- Overloading
- Short
circuit
OVERLOADING
Occurs when:
- Excess
current flows
- Too
many appliances connected
Effects:
- Heating
- Fire
hazard
SHORT CIRCUIT
Occurs when:
- Live
wire and neutral wire touch directly
Result:
- Very
large current flows
MINIATURE CIRCUIT BREAKER (MCB)
Modern replacement for fuse.
Advantages:
✔ Automatically switches OFF current
✔ Reusable
✔ Safer than fuse wire
IMPORTANT DIFFERENCES
AC vs DC
|
AC |
DC |
|
Direction changes |
One direction |
|
Used in homes |
Used in batteries |
|
Frequency present |
No frequency |
Motor vs Generator
|
Motor |
Generator |
|
Electrical → Mechanical |
Mechanical → Electrical |
|
Uses left-hand rule |
Uses right-hand rule |
