Railway Science (Physics-Electric Current and its Effects) (Part-III)

Solenoid is a coil of wire that acts as a magnet when current passses through it. The number of turns is called the loop of solenoid. More loops have stronger magnetic fields. The magnetic field inside a solenoid is proportional to both the applied current and the number of turns per unit length.

Faraday’s second law of electromagnetic induction - The magnitude of the induced emf is equal to the rate of change of magnetic flux associated with the coil. Factors affecting EMF - Magnetic Strength, Number of Turns, Cross Sectional Area, Associated magnetic flux with coil. Current is the flow of electrons through a closed circuit. SI Unit - Ampere (A). Factor affecting strength of Current - Resistance, Potential difference across the ends.

Current is induced in a conductor only when there is a change in the magnetic field associated with it. If we rotate the bar magnet along the axis of the solenoid, there is no change in the magnetic field, and therefore, no induced current.

Ohm’s law - Voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions and temperature remain constant. V = IR, where V is the voltage across the conductor, I is the current flowing through the conductor and R is the resistance provided by the conductor to the flow of current.

The magnitude of the magnetic field produced at a given point is directly proportional to the magnitude of the current passing through the conductor and inversely proportional to the distance from the wire.

When length l is doubled, while Area A and resistivity ρ remains the same. Hence, the new resistance becomes two times the initial resistance, Resistance of a wire R = . 𝑝𝑙/𝐴. SI unit - ohm (Ω). Factor affecting Resistance - Length of the conductor, material, cross section area of the conductor.

A short circuit is a low-resistance connection between the two conductors supplying electrical power to any circuit. Causes: Vermin or pests chewing through wires, Loose connections, Old or damaged outlets, switches, lights.

The magnetic field is a region around a magnetic material or a moving electric charge within which the force of magnetism acts. Magnetic lines Properties: It seeks the path of least resistance between opposite magnetic poles, Never intersect one another, All have the same strength, and Density decreases when they move from an area of higher permeability to lower permeability.

Resistivity can be  defined as the resistance of a conducting material per unit length with unit area of cross-section of the material of the conductor. Formula → R = ρ𝑙/Π𝑟.
Resistivity (ρ, rho) depends only on the temperature and nature of the conductor and not on the shape and size or density of the material. Its unit is ohm-meter (Ω.m).

The direction of a magnetic field at a point is determined by placing a compass needle. Maxwell's Right-Hand Thumb Rule - If the thumb of the right hand represents the direction of the current flow, the rest of the curled fingers determine the direction of the magnetic field around it.