Nootan Kumar Mittal ISC CLASS 12th PHYSICS Chapter- 6 (DC Current and Measurements)[ Part I]

               Chapter - 6

  DC Circuits and measurements

* Electric Cell.

An electric cell is the source of electrical energy with maintains a continuous flow  of charged in a circuit.

* Electromotive Force in a Cell.

The work done by cell is forcing unit positive - charge to flow in the whole circuit is called the        ' electromotive force' ( emf) of the cell.

* Terminal Potential Difference.

Suppose the EMF of a cell connected in a electric circuit is E and for the flow of charge Dq in the circuit the energy given by the cell is DW. Then.

E= dw/dq.

* Internal Resistance of a Cell.

The resistance offered by the electrolytic of the cell to the flow of current through it is called the 'internal resistance' of the cell.

* Combinations of Cells.

1. In series

 2. In parallel

* Cells,EMF ,Internal resistance & Terminal voltage.

# Cell

1) Cell provides  Electric energy to all circuit elements.

2) Cell do not produce charge.

3) Cell maintains a potential difference which force that charge/electron to move in the circuit.

#.EMF (Electro Motive Force)

Potential difference between two electrons of cell when no current flow. (Open circuit).

Internal resistance is the abstraction offered to part of current due to the material of electrolyte of cell.

*  Factor on which i.r depends.

1) Area of electrode.

2) Distance between Electrode.

3) Concentration of Electrolyte.

4) Temperature.

* Kirchhoff's Laws

1) Kirchhoff's First Law of Junction Rule-

At any Junction where 3 or more wire meets.

                 The sum of current entering the            junction=Sum of current leaving the junction.

Principle of conversion of charge.

i = -ve

Assumed direction is opposite of actual direction.

2). Kirchhoff's Second Law Loop Rule-

" The algebraic sum of charge in potential around a a closed loop is zero"

# How to apply Kirchhoff's  law

1) Draw current from one/ more cell follow  kirchhoff's first law or junction rule for distribution of current.

2) Choosing are loop

3) Direction of loop path

4) Potential drop or Potential gain.

Contractual question

1).

* Wheatstone's  bridge

A simple circuit for measuring an unknown resistance by connecting it so as to form a quadrilateral with three known resistances and applying a voltage between a pair of opposite corners.

Proof -

Metre-Bridge -- Electrical  Instrument.

#) To measure  the value of unknown resistance.

#) It work on Balanced Wheatstone Bridge.

Null point 

Where show no deflection no current.

Conceptual question

1)

* Potentiometer

It measures potential difference more aqrate as compared voltmeter. But voltmeter is used  more often due to its small size & conunience.

Uses 

1) To measure EMF of cell.

2) To compare the EMF of two cells.

3) To determine internal resistance of cell.

* Construction

A  long wire (of few metres) of uniform cross section area.

Potentiometer - null point method

!) To measure EMF of cell.

!) Battery EMF > cell's EMF

Conceptual question.

1)

2)

* Rheostat

It is Electrical device consist of magnin wire cylinder and three terminals.