Electric Current

Potential Difference and Resistance
Ohm's Law
Grouping of Cells
Kirchhoff's Law

Electric Current





Electric current is the rate of charge flow a given point in an electric circuit, measured in Coulombs/second which is named Amperes.
In most DC electric circuits, it can be assumed that the resistance to current flow is a constant so that the current in the circuit is related to voltage and resistance by Ohm's law. The standard abbreviations for the units are 1 A = 1C/s.












Electric Charge






The unit of electric charge is the Coulomb (abbreviated C). Ordinary matter is made up of atoms which have positively charged nuclei and negatively charged electrons surrounding them. Charge is quantized as a multiple of the electron or proton charge:

Potential Difference














In current electricity, dry cells or secondary cells or generators are employed to create a potential difference in order to cause an electric flow in closed circuits just as a water pump is used to create pressure difference in order to drive water in water pipes.

The unit of potential difference is volt . The volt is defined as that potential difference between two points of a condutor carrying a current of one ampere when the power dissipated between these points is equal to one watt .



Resistance

The electrical resistance of a circuit component or device is defined as the ratio of the voltage applied to the electric current whichflows through it:







where,
R = resistance (ohm)
V = voltage (V)
I = current (A)



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Ohm's Law


It is the most fundamental law of electricity and was given by George Simon Ohm 1828.

Statement : The current which flows in a conductor is proportional to the potential difference which causes its flow provided the temperature of the conductor is constant.

V = IR



where constant R is the resistance of the conductor. In this Ohm's law relation, V is in volt, I is in amperes and R is in ohms.

Resistors In Series

The series circuit is one in which the same current flows in all the components of the circuit. If resistors R1, R2, R3 .... are connected in series, the equivalent resistance of the combination is the sum of the resistance so connected.

R = R1 + R2 + R3



Resistors In Parallel


A parallel circuit of resistor is one in which the same voltage is applied across all the components .

If resistors R1, R2, R3 .... are connected in parallel, then reciprocal of the equivalent resistance is the sum of the reciprocals of the resistance of seperate components.
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Resistivity



The resistance of a conductor is found to be directly proportional to its length and inversly proportional
to its cross-sectional area at constant temperature.






where    is a constant called the resistivity or specific resistance of the material of the wire. L is hte length in metres and A is the area of cross-sectional in m2.
The unit of resistivity is ohm-metre.


Grouping of Cells


There are generally three types of groupings of the cells:

(i) Cells in Series .
(ii) Cells in Parallel .
(iii) Cells in Mixed Grouping .

Kirchhoff's Law

To study complicated circuits containing more than one source of emf, Kirchhoff put forward the following two laws in 1842.
Before going to the Kirchhoff’s laws, let us first define two terms. A node in a network is a point where three or more conductors are joined. A loop is any closed conducting path.


(i) First Law or Current Law


It states, “In any electrical network, the algebraic sum of currents meeting at a point (or junction) is zero, .” The total current flowing towards a node (junction) is equal to the total current flowing away from that node, i.e. the algebraic sum of the currents meeting at a node is zero. The first law is simply a statement of the conservation of charge .







(ii) Second law or Mesh law or Voltage law


It states, “In a closed circuit, the algebraic sum of the products of the current and the resistance in each of the conductors in any closed path (or mesh) in a network plus sum of emfs in that path is equal to zero.”

Colour Coding For Carbon Resistance

Resistors are colour coded in order to find their resistance easily.

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Wheatstone's Bridge

For measurement of a resistance, a network made up of four resistance arms P,Q,R and S is arranged
as shown. Arms AB and BC having resistances P and Q respectively are known as ratio arms .

A galvanometer G is connected across B and D. A battery is connected across A and C. When the values of resistances P, Q, R and S are such that no current flows through the galvanometer G the bridge is said to be balanced . In that case B and D are at the same potential and we have the condition






Usually S is an unknown resistance and P, Q and R are known.
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