Summary
Key terms
Electromagnetism: The term electromagnetism is defined as the production of a magnetic field by current flowing in a conductor.
Solenoid: Coiling a current–carrying conductor around a core material that can be easily magnetized, such as iron, can form an electromagnet. The magnetic field will be concentrated in the core. This arrangement is called a solenoid.
Ampere: A unit of electric current in the meter–kilogram–second system. It is the steady current that when flowing in straight parallel wires of infinite length and negligible cross section, separated by a distance of one meter in free space, produces a force between the wires of 2 × 10-7 newtons per meter of length.
Magnetic Levitation: A method of supporting and transporting objects and vehicles by using a magnetic field to counterbalance the gravitational pull.
Magnetic Resonance Imaging: Magnetic resonance imaging (MRI) is a technique that uses a magnetic field and radio waves to create detailed images of the organs and tissues within your body.
Key concepts
- Electromagnetism: It is the physics of the electromagnetic field; it encompassing all of space, composed of
electric field and the magnetic field. The term "electromagnetism" comes from the fact that the electric and magnetic fields are closely inter wined and under many circumstances, it is impossible to consider them separately. We use electromagnets to generate electricity, store memory on our computers, etc. The magnetic field at a point due to electric current depends on:
- Strength of the current.
- Distance from the current carrying conductor.
- Direction of the current.
- Magnitude of the magnetic field produced by a straight conductor carrying current: The magnitude of magnetic field (or strength of magnetic field) B produced by an infinitely long conductor in vacuum at a distance r from it is given by: B= (μ0I/2πr) Where, B = Magnetic field; μ0 = permeability of vacuum; I = current flowing in the conductor r = Distance from the conductor where the magnetic field is measured.
- Direction of Magnetic field: The direction of the magnetic field can be found using any of the following laws:
- Oersted's right hand rule: The direction of the thumb points to the direction of current , when a wire is folded with the right hand, the direction in which the fingers of the right hand are folded represents the direction of the magnetic field.
- The Maxwell's Screw Rules: The Maxwell Screw Rules / Maxwell's Corkscrew Rule, states that, If a right handed screw is being turned so that it bores its way in the direction of the current in the wire then the direction of rotation gives the direction of the magnetic field.
- Biot–Savart Law: The Biot–Savart Law relates magnetic fields to the currents which are their sources. In a similar manner, Coulomb's law relates electric fields to the point charges which are their sources. Finding the magnetic field resulting from a current distribution involves the vector product, and when the distance from the current to the field point is continuously changing. The magnetic field 'B' due to the entire length of the conductor is obtained by summing the contributions of small current elements like Idl.
-
Calculating Magnetic flux: Biot–Savart law is used to
calculate the magnetic field around a current loop.
Possible cases Magnetic Field Magnetic field at the center of current carrying circular coil of n turns and radius 'a' , each carrying current(I) in the same direction is given by , 
Magnetic Field on the axis of a circular current carrying coil is given by , 
In the above case 'a' is the radius of the coil and r is the distance from the center of the coil to the point where the field is measured. When point P is at the center of the coil and if r = 0 then equation becomes , 
In case 1, When point P is on the axial line far away from the center of coil. In that case, r >> a so that a2+ r2 = r2. Therefore , 
In case 2, If the field is directed along the axis of the coil and falls off as the cube of the distance from the coil. When point P is on axial line at a distance equal to radius of coil. In that case, r = a so that we have , 
- Ampere's Law: Ampere's law states that for any closed loop path, the total sum of the products of all length elements and the magnetic field in the direction of the length element
is equal to the product of permeability and the electric current enclosed
in the loop. Ampere's law can be used to calculate 'B' for
various current carrying conductor configurations. The Biot Savart law can be derived
from Ampere's law.
- Force between two parallel conductors:
When two current carrying conductors are placed next to
each other, we notice that each induces a force on the other.
Each conductor produces a magnetic field around itself
(Biot–Savart law) and the second experiences a force that is
given by the Lorentz force. If, on the other hand, currents in two
parallel conductors are opposite in direction, the force between them
is repulsive. The force of attraction per unit length between the two parallel
conductors is given by
F =
- Solenoid: A solenoid is a coil
wound into a tightly packed helix. Solenoid (in physics) refers to a long,
thin loop of wire, which when wrapped around a piece of metal, produces a magnetic field when an electricity is passed through it. Solenoids are important because they
can create controlled magnetic fields and can be used as electromagnets. The term
solenoid refers specifically to a coil designed to produce a uniform magnetic
field in a volume of space (where some experiment might be carried out).
BL = μ0 I N
∴B =μ0 I N / L ;
where N is the number of turns encircled by the rectangular loop and I is the current flowing in the wire of the solenoid. One of the important applications of magnetic field produced by solenoids is in magnetic resonance imaging (MRI), which is used in medical diagnostics to get a detailed analysis of the internal parts of the body.
key formulae
- Magnetic field at the centre of current carrying circular coil of 'n' turns =
- Magnetic field on the axis of a circular current carrying coil =
- Ampere's law:
- Force between two parallel conductors:
- Magnetic field due to a solenoid:
