## 2nd PUC Physics Current Electricity Chapter Summary:

In 2nd PUC Physics Current Electricity Chapter you will learn about the laws controlling the nature of electric current and the contribution of prominent scientists like Kirchhoff and Ohm towards Physics.

## 2nd PUC Physics Current Electricity - List of Topics

 Introduction to electric current Electric currents Electric currents in a conductor Current density Ohm’s law Statement and explanation Electrical resistivity and conductivity Derivation of the relation ȷ= σE Limitations of Ohm’s law Resistivity of various materials Temperature dependence of resistivity relaxation time and mobility Colour code of carbon resistors Electrical energy and power Internal resistance of a cell Combination of resistors in series Combination of resistors in parallel Cells, emf, internal resistance Equivalent EMF and equivalent internal resistance (a) in series and (b) in parallel combination Balancing condition Kirchhoff’s rules Junction rule Loop rule Wheatstone bridge Principle and applications Meter bridge Potentiometer Numericals Concept based problems

## Current:

Current through a given area of a conductor is the net charge passing per unit time through the area.

## Emf:

Emf is called as electromotive force, but it is not a force. It is the voltage difference between the two terminals of a source in open circuit.

## Ohm’s law:

The electric current I flowing through a substance is proportional to the voltage V across its ends, i.e., or V = RI.
where R is called the resistance of the substance.
The unit of resistance is ohm. ## Resistance (R):

Resistance (R) of a conductor depends on its length “l” and constant cross-sectional area “A” through the relation, Where ρ, called resistivity is a property of the material and depends on temperature and pressure.

## Electrical resistivity:

Electrical resistivity of substances varies over a very wide range.
• Metals have low resistivity, in the range of 10–8 Ω m to 10–6 Ω m.
• Insulators like glass and rubber have 1022 to 1024 time’s greater resistivity.
• Semiconductors like Si and Ge lie roughly in the middle range of resistivity on a logarithmic scale.

## Current density:

Current density (j) gives the amount of charge flowing per second per unit area normal to the flow , where n is the number density (number per unit volume) of charge carriers each of charge q. is the drift velocity of the charge carriers.
For electrons q = e ,
if j is normal to a cross-sectional area A and is constant over the area, the magnitude of the current I through the area is, Using E = V/l, and Ohm’s law,
one obtains, • The proportionality between the force eE on the electrons in a metal due to the external field E and the drift velocity Vd (not acceleration) can be understood, if we assume that the electrons suffer collisions with ions in the metal, which deflect them randomly. If such collisions occur on an average at a time interval , where a is the acceleration of the electron.
This gives, In the temperature range in which resistivity increases linearly with temperature, the temperature coefficient of resistivity α is defined as the fractional increase in resistivity per unit increase in temperature.
Ohm’s law is obeyed by many substances, but it is not a fundamental law of nature.
It fails if:
(a) V depends on I non-linearly.
(b) The relation between V and I depends on the sign of V for the same absolute value of V.
(c) The relation between V and I is non-unique.
• An example of (a) is when ρ increases with I (even if temperature is kept fixed). A rectifier combines features (a) and (b). GaAs shows the feature (c).
When a source of emf ε is connected to an external resistance R, the voltage Vext  across R is given by, where r is the internal resistance of the source.
(a) Total resistance R of n resistors connected in series is given by, (b) Total resistance R of n resistors connected in parallel is given by, ## Kirchhoff’s Rules:

(a) Junction Rule: At any junction of circuit elements, the sum of currents entering the junction must equal the sum of currents leaving it.
(b) Loop Rule: The algebraic sum of changes in potential around any closed loop must be zero.

## Wheatstone bridge:

The Wheatstone bridge is an arrangement of four resistances –R1, R2, R3, R4 .
The null-point condition is given by, using which the value of one resistance can be determined, knowing the other three resistances.

## The potentiometer:

The potentiometer is a device to compare potential differences. Since the method involves a condition of no current flow, the device can be used to measure potential difference; internal resistance of a cell and compare emf’s of two sources.