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

A galvanometer is a sensitive device which can detect the presence of very small current in a circuit of the order of 100 mA. Other Instruments : Ammeter is used to measure the current in a circuit. The voltmeter is used to measure the potential difference. Ohmmeter is used to measure the electrical resistance.

Ohm’s Law is V = IR, where V = Voltage, I = Current and R = Resistance. For bulbs connected to the same voltage source, the bulb with higher resistance will allow less current to flow through it and thus dissipate less power, resulting in dimmer brightness.

At a given temperature, the Resistance (R) of a conductor depends on its Length (L), Area of cross-section (A) and the Material it is made of. R = ρ × 𝐿/𝐴. Electrical resistivity - A material's intrinsic property that measures how strongly a material opposes the flow of electric current. SI unit - Ohm-meter. Material and Electrical Resistivity at 20°C - Aluminum (2.63 × 10⁻⁸), Copper (1.62 × 10⁻⁸), Mercury (94 × 10⁻⁸), Nickel (6.84 × 10⁻⁸), Silver (1.60 × 10⁻⁸), Tungsten (5.20 × 10⁻⁸).

According to Ohm’s law, the electric current through a conductor is directly proportional to the potential difference across it, provided the physical conditions such as temperature remain unchanged. Mathematically, expressed as V = IR where V = Potential difference, I = Current and R = Resistance.

Resistance - Opposes the flow of Current. SI Unit - Ohm. Formula : R = ρ 𝐿/𝐴, where R is Resistance, is ρ Resistivity, L is length of wire, A is cross sectional area. Resistivity (ρ) - It is a fundamental property, and it shows how strongly the material opposes the electric current. Factors affecting resistance - Material's conductivity, Length, Cross-sectional area, and Temperature.

It is an instrument used to measure the potential difference across two terminals in a circuit. Fuse - It is a safety device that operates to provide over current protection of an electrical circuit.

An electric fuse is a safety device used to protect electrical circuits from excessive current flow. It has a low melting point, hence when current suddenly increases, then fuse melts due to excessive heat damage. Other devices based on the heating effect of currents: electric iron, electric kettle, electric heater, and electric hair dryer.

Nichrome offers a very large resistance. It has a high melting point such that it can be heated till red hot without melting. It is used to make the coils used in water heaters. It is used in heating elements such as hair dryers, toasters, and ovens.

In a series circuit, the current is the same through all components. This is due to Kirchhoff’s current law, which states that the sum of currents entering a junction must equal the sum of currents leaving it. In a series circuit, there’s only one path for the current to take, so it remains the same throughout.

Ohm's Law: The potential difference across the ends of a conductor is precisely proportional to the electric current flowing through it. It is given by formula, V = IR, where V is potential difference, R is resistance and I is current. An ohmic device is one that obeys Ohm's law for all voltages applied to it.