## 2nd PUC PHYSICS CURRENT ELECTRICITY

## 2nd PUC Physics Current Electricity Chapter Summary:

## 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

The unit of resistance is

**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**to^{–8}Ω m**10**.^{–6}Ω m - Insulators like glass and rubber have
**10**to^{22}**10**time’s greater resistivity.^{24 } - 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

For electrons

**the drift velocity**of the charge carriers.For electrons

**q =**

if^{–}e ,**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**V**(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 ,_{d }

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)

(b) The relation between

(c) The relation between

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

where

**ε**is connected to an external resistance**R**, the voltage**V**across_{ext }**R**is given by,where

**r**is the internal resistance of the source.
(a) Total resistance

(b) 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)

(b

**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

The null-point condition is given by,

using which the value of one resistance can be determined, knowing the other three resistances.

**Wheatstone bridge**is an arrangement of four resistances –**R**_{1}, R_{2}, R_{3}, R_{4 .}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.