Hans Christian Oersted accidentally observed that a compass needle was
deflected when placed near a metallic wire carrying electric current.
Observation:
A current-carrying wire produces a magnetic effect around it.
Significance:
Electricity and magnetism are not separate phenomena; they are closely linked.
Legacy:
This discovery became the foundation of several modern technologies,
including radio, television, and fibre-optic communication systems.
The unit of magnetic field strength called the
oersted is named after Hans Christian Oersted.
Oersted’s experiment established that an electric current can produce
a magnetic field.
II. Magnetic Fields and Magnetic Field Lines
A magnetic field is the region around a magnet where
its magnetic influence or force can be detected.
Magnetic field is a vector quantity, meaning it has both
magnitude and direction.
Compass Needle
A compass needle is a tiny bar magnet. Its north-seeking pole points
towards the geographical north, while its south-seeking pole points
towards the geographical south.
Properties of Magnetic Field Lines
Direction:
Outside a magnet, magnetic field lines emerge from the North pole
and enter the South pole.
Inside the Magnet:
Field lines travel from South to North, forming closed curves.
Strength:
The closer the field lines are to one another, the stronger the
magnetic field.
The magnetic field is strongest near the poles where the lines are
most crowded.
Non-Intersection Rule:
Two magnetic field lines never intersect each other.
If two magnetic field lines crossed, a compass needle placed at that
point would have to point in two different directions simultaneously,
which is impossible.
Magnetic field lines are imaginary lines used to represent the
direction and relative strength of a magnetic field.
III. Magnetic Fields Produced by Current-Carrying Conductors
A. Straight Wire Conductor
The magnetic field around a straight current-carrying conductor consists
of concentric circular field lines centred on the wire.
Factors Affecting Magnetic Field Strength
Current:
The magnetic field becomes stronger when the current through the
conductor increases.
Greater current produces greater deflection of a nearby compass needle.
Distance:
Magnetic field strength decreases as the distance from the wire increases.
The field lines become more widely spaced farther away from the conductor.
Right-Hand Thumb Rule
To determine the direction of the magnetic field around a straight
current-carrying wire:
Point the thumb of your right hand in the direction of current.
The curled fingers indicate the direction of magnetic field lines.
B. Circular Loop
At every point on a circular wire, the magnetic field lines are circular.
Near the centre of the loop, these circular field lines appear almost
as straight, parallel lines.
The magnetic field becomes stronger when the number of turns is increased.
Magnetic field of a coil with n turns = n × magnetic field of one turn
The fields produced by each turn act in the same direction and add
together, making the total magnetic field stronger.
C. Solenoid
A solenoid is a cylinder-shaped coil consisting of
many closely wound circular turns of insulated copper wire.
Magnetic Field Inside a Solenoid
The field lines inside a solenoid are parallel straight lines.
This indicates that the magnetic field inside the solenoid is
uniform.
A uniform magnetic field has the same strength and direction at every point.
Magnetic Field Outside a Solenoid
The magnetic field pattern outside a solenoid resembles that of a bar magnet.
One end of the solenoid behaves as a North pole.
The other end behaves as a South pole.
Electromagnet
When a soft iron core is placed inside a current-carrying solenoid,
the strong magnetic field magnetises the core and forms an
electromagnet.
IV. Force on Conductors Placed in Magnetic Fields
Ampere’s Law
Andre Marie Ampere proposed that if a current-carrying conductor exerts
a force on a magnet, then the magnet must exert an equal and opposite
force on the conductor.
Maximum Force
The force on a current-carrying conductor is maximum when the direction
of current and the direction of the magnetic field are at
90° to each other.
Fleming’s Left-Hand Rule
Stretch the thumb, forefinger, and middle finger of your left hand
mutually perpendicular to one another.
Forefinger: Direction of magnetic field
Middle Finger: Direction of current
Thumb: Direction of motion or force
Direction of Conventional Current
Conventional current is considered to flow in the direction opposite
to the movement of electrons.
If an electron moves from left to right, the conventional current is
considered to flow from right to left because an electron carries
negative charge.
V. Biological Magnetism and Medical Applications
Nerve impulses in the human body are weak electric currents.
These currents produce weak magnetic fields.
Magnetic Fields in the Human Body
The human heart produces a significant magnetic field.
The brain also produces measurable magnetic fields due to electrical
activity in nerve cells.
These magnetic fields can provide useful information about the
functioning of internal organs.
MRI — Magnetic Resonance Imaging
Magnetic Resonance Imaging, or MRI, uses magnetic fields and radio waves
to obtain detailed images of internal body structures for medical diagnosis.
VI. Domestic Electric Circuits and Safety
Power Supply in India
Standard domestic supply: 220 V, 50 Hz
Wires Used in Domestic Circuits
Wire
Colour
Function
Live Wire
Red
Carries the potential of approximately 220 V.
Neutral Wire
Black
Completes the circuit. The potential difference between live
and neutral wires is 220 V.
Earth Wire
Green
Connected to a metal plate buried deep in the ground and
provides a safe path for leakage current.
Circuit Ratings
5 Ampere Circuit
Used for light appliances such as:
Bulbs
Fans
Small electrical devices
15 Ampere Circuit
Used for heavy appliances such as:
Geysers
Air conditioners
Other high-power appliances
Short-Circuiting
Short-circuiting occurs when the live and neutral wires come into
direct contact, usually due to damaged insulation.
The resistance suddenly becomes very low.
A very large current flows through the circuit.
This may cause overheating, sparks, fire, or damage to appliances.
Overloading
Overloading occurs when too many appliances are connected to a single
socket or when the supply voltage suddenly increases.
The current drawn becomes greater than the safe limit.
Excessive current causes heating of the wires.
It may damage electrical appliances or cause fire.
Electric Fuse
An electric fuse is a safety device that melts when excessive current
flows through it, breaking the circuit and protecting electrical equipment.
The fuse works because of the heating effect of electric current,
also known as Joule heating.
Earthing
Earthing protects users from electric shock by providing a
low-resistance path for leakage current from the metallic body of
an appliance directly to the ground.
Quick Revision
Oersted’s Discovery
Electric current produces a magnetic field.
Field Lines Outside Magnet
North pole to South pole.
Field Lines Inside Magnet
South pole to North pole.
Straight Wire Field
Concentric circles.
Right-Hand Thumb Rule
Thumb gives current direction; curled fingers give magnetic field direction.