Introduction
Electric current is a means by which electrical energy is transferred from one place to another for utilisation. Charges in motion constitute an electric current.
Electric Current
A current is any motion of charge from one region to another. The illustration below shows two bodies at different potentials. When these are connected with a wire, free electrons flow from B to A until both bodies attain the same potential, after which the current ceases to flow. Current flows if a potential difference exists throughout a conductor. This branch of physics dealing with charges in motion is called current electricity.
Electromotive Force (EMF) and Voltage
No current flows in a copper wire by itself, just as water in a horizontal tube does not flow. If one end of the tube is connected to a tank with water such that there is a pressure difference between the two ends of the horizontal tube, water flows out of the other end at a steady rate.
Resistance and Resistivity
It is found experimentally that the current I flowing through a conductor is directly proportional to the potential difference V across its ends, provided the physical conditions (temperature, mechanical strain, etc.,) remain constant.
Origin of Resistivity
In electrostatic situations, the electric field is zero everywhere within the conductor, and there is no current. This does not mean that all charges within the conductor are at rest.
Temperature Dependence of Resistivity
The resistivity of a metallic conductor nearly always increases with increasing temperature.
Limitations of Ohm's Law
Ohm's law is not a fundamental law of nature. There are a number of commonly used circuit elements which do not obey this law.
Superconductivity
Some materials, including several metals and alloys, exhibit a phenomenon called superconductivity.
Resistors in Series and in Parallel
When resistors, can be connected in such a way that the same current flows in them, then they are said to be connected in series. The resistors are said to be connected in parallel if the potential difference is the same across each resistor.
Electric Circuits and Kirchhoff's Rules
In many electrical circuits, Ohm's law cannot be applied. This happens when there is more than one source of emf in the circuit or when resistors are connected in a complicated manner. To solve such complex circuits, Gustav Robert Kirchhoff developed two laws based on charge neutrality in a metal.
Measurement of Voltages, Currents and Resistances
These devices measure the voltage and current respectively in a circuit. The basic component of both is the moving coil galvanometer which produces a deflection proportional to the electric current through it.
Measurement of Voltages, Currents and Resistances (Contd...)
This is the simplest form of wheatstone bridge and is specially useful for comparing resistances more accurately.
Summary
Current through a given area of a conductor is the net charge passing per unit time through the area.
Numerical 01
In the case of hydrogen atom, an electron moves in an orbit of radius 5x10-11 m with a speed of 2.2 x 106 ms-1. Calculate the equivalent current. Given: charge on an electron = 1.6 x 10-19C.
Numerical 02
How many electrons flow through the filament of a 120 V, 60 W electric lamp in one second? Given: Electric power is the product of voltage and current.
Numerical 03
What is the number of free electrons in a piece of silver of cross-section 1.0 x 10-4 m2 and length 1m? Atomic weight of silver = 108, density of silver = 105 x 102 kg m-3. Assume that there is one free electron per atom.
Numerical 04
What is the drift velocity of electrons in a copper conductor having a cross-sectional area of 5 x 10-6 m2 if the current is 10A? Assume that there are 8 x 1028 electrons/m3A.
Numerical 05
Calculate the drift velocity of electrons in a silver wire having cross-sectional area of 3.14 x 10-6 m2 and carrying a current of 20 amperes. Given: Avogadro's number = 6.023 x 1023, atomic weight of silver = 108. Density of silver = 10.5 x 103 kg m-3, e = 1.6 x 10-19C.
Numerical 06
A car has a fresh storage battery of EMF 12V, internal resistance 5.0 x 10-2 W. If the starter motor draws a current of 90 A, what is the terminal voltage of the battery when the starter is on?
Numerical 07
Two identical cells of EMF 1.5 V each joined in parallel provide supply to an external circuit consisting of two resistors of 17W each joined in parallel. A very high resistance voltmeter reads the terminal voltage of the cell to be 1.4 V. What is the internal resistance of each cell ? (e=1.6 x 10-19C)
