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Current Electricity

So far our study of electrical phenomenon has been confined to the study of charges at rest, or electrostatics. We now consider a situation involving electric charge in motion. We use the term ‘electric current’ to describe the rate of flow of charges through some region of space.

Electric Current
Electric charges in motion form an electric current. In a metal, the electrons in the outermost orbits of atoms are loosely bound and are free to move about in the whole metal block. These are called the free electrons. They are responsible for electrical conductivity. They are also known as conduction electrons.

The strength of an electric current through a conductor is the rate of flow of charges across a section of the conductor. If ΔQ is the amount of charge that passes through the cross-sectional area of the conductor in an interval of time Δt, then current I is written as follows:
Current I = Description: 91740.png

SI unit of current is ampere (A):
Description: 91747.png

One ampere of current is said to flow across any cross-sections of the wire if 6.24 × 1018 electrons flow per second across that cross-section. (This is also equal to the flow of 1 coulomb per second of charge.)

Electric Circuits

A continuous conducting path between the two terminals of a battery is called an electric circuit.
There are two types of electrical circuits. They are


Description: Description: 97457.png
Ohm’s Law
The current through a metallic conductor is proportional to the potential difference applied between its ends, provided the temperature remains constant. If a potential difference V is applied to a conductor and a current I passes through it, then according to Ohm’s law:
Description: 91767.png
V = IR, where R is a constant for the given conductor and is called its resistance.
Electrical Resistance
Obstruction offered to the flow of current by the material of a wire is called its electrical resistance. The SI unit for electrical resistance is ohm (Ω).
The reciprocal of the electrical resistance Description: 91845.png is called conductance. The SI unit of conductance is denoted as mho or siemen.

Factors that Affect Resistance

  1. Area of cross-section of the conductor (A)
    Resistance is inversely proportional to the area of cross-section (A): R ∝ Description: 91857.png.
  2. Length of the conductor (L)
    Resistance is proportional to length R ∝ L.
  3. Temperature (T)
    Resistance increases with temperature R ∝ T.

It is the measure of material’s ability to oppose current:
Description: 91873.png 
Description: 91881.png 
where ρ is a constant for a given material at a given temperature. It is called the resistivity of the material. The SI unit of resistivity is ohm metre. It is also sometimes called ‘the specific resistance’.

Electromotive Force
For an electric circuit to have a continuous current, the circuit must contain a source of electric energy. Such an element is called a source of electromotive force (emf). A cell or a battery is a source of emf. The emf of a cell is defined as its ability to drive a current through the circuit.
Grouping of Cells
Electrical devices can operate only when a required current is supplied at a designed voltage. A single cell may not serve this purpose. Hence, cells are grouped to form a battery which provides both the required voltage and current.

Description: Description: 97466.png

Series and Parallel Combination of Resistance

A conducting material of a particular resistance meant for use in a circuit is called a resistor. Resistors can be connected in series or parallel to get the desired resistance.
Series Combination If the resistors are connected end-to-end as shown in the figure so that a certain current passing through one side also passes through all the others, then they are said to be in series.
This circuit is also known as constant current circuit. The equivalent resistance or effective resistance of the circuit is given by Req = R1 + R2 + R3.
Description: 99949.png
Parallel Combination If the resistors are connected such that one end of all the resistors is brought to terminal A and the other ends brought to terminal B such that the same potential difference gets applied to each of them, then they are said to be in parallel.
This circuit is known as constant voltage circuit. The equivalent resistance in parallel combination is given by Description: 91894.png.
Description: 102440.png
Electrical Power
Electrical power is the rate at which electrical energy is generated or consumed. It is denoted by P.
Electrical power P in watts = Description: 91901.png
If V is the potential difference applied across a resistor of resistance R, and if I is the current flowing through it, then P = VI.
Unit of Power The SI unit of power is watt (W). 1 watt = 1 joule per second.
A larger unit of power for commercial purposes is a kilowatt (kW). 1 kW = 1000 W.
Kilowatt Hour One kilowatt hour is the energy consumed when 1000 watts of electric power is used for 1 hour: 1 unit = 1 kWh = 1000 W × 1 hour.

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