Electrical circuit| complete notes| class 12|NEB|

REVIEW AND SUMMMARY:

• Kirchhoff's  first law states that the algebraic sum of currents at a junctions of an electric circuit is zero.
• Kirchhoff's  second law states that the closed loops of an electric circuits, the algebraic sum of emfs is equal to the algebraic sum of products of currents and the resistances in the various parts of the loops.
• Wheatstone bridge is an electrical circuits which is used for the accurate measurements of resistance of a conductor.
• meter bridge is an electrical device used for measuring unknown resistance. its working principle is the balanced condition of Wheatstone bridge.
• POX BOX is a compact form of Wheatstone bridge .
• potentiometer is a electrical device used to measure the emfs and the internal resistance of a cell, to compare the emfs between two cells and the potential differences between two points in an electrical circuit.it also be employed to measure the current and resistance in a circuit accurately.
• principle of potentiometer states that the potentials differences across any portions of the potentiometer wire is directly proportional tp the length of that portion provided the current is uniform.
• superconductors are the materials which have a zero electrical resistance.
• perfect conductors is an electrical conductors with no resistivity.
• shunt is a small resistance which is connected parallel to the galvanometer.

USEFUL INFORMATIONS FOR MULTIPLE CHOICE QUESTIONS

• Meter bridge is based on the null deflections methods.
• meter bridge cannot be used to measure very small unknown resistance.
• potentiometer is more accurate than voltmeter.
• potentiometer wire is made of an alloy like eureka or manganin.
• more is the length of the potentiometer wire more is its sensitivity.
• Kirchhoff's 1st law and 2nd law are also unknown as junctions rule and loops rule.
• meter bridge is more sensitive between the length of 40 cm and 60 cm for null deflection.
• the reciprocal of the potential gradients is the measurement of the sensitivity of potentiometer.
• potentiometer is equivalent to the voltmeter of infinite resistance and ideal ammeter has zero resistance.
• more is the range of the ammeter lesser is the resistance.

Kirchhoff's  laws

it states that the algebraic sum of currents at a junction of an electric circuit is zero.

hence the sum of currents flowing towards that junctions is equal to the sum of currents flowing out of the junctions. this law called Kirchhoff's current law and also called Kirchhoff's current law and also called Kirchhoff's junctions law.it follows the principle of conservation of charge. seconds law states that in a closed loop of an electric circuit, the algebraic sum of emfs is equal to the algebraic sum of potentials difference in the various parts of the loops.

Wheatstone bridge

it is an electrical circuits use for the measurements of accurate measurements of resistance of conductors.it is an arrangements of four resistors in which three of them are known and one is unknown. P Q R are the three known resistor of X is a resistors of unknown resistance. A cell E connected across two junctions A and C , and a galvanometer G between the junctions B and D . adjusting P and Q in known value, R varies that the current through the galvanometer is zero. the galvanometer shows the following conditions;

P/Q=X/R

potentiometer 

A potentiometer is a device which is used to measure the internal resistances, and comparing the potential difference between the two cells .it can also be able to employed to measure the current and resistance in a circuit accurately. The principle of potentiometer is the potential differences across any portions of the potentiometer wire is directly proportional to the length of that provided the uniform current. this is the main principle of potentiometer.

CONVERSION OF GALVANOMETER

An ammeter is an instruments used to measure the currents passing through a circuit. it is connected in a series circuit. since connecting an ammeter in a circuit should not change the currents in it, the resistance of an  ammeter should be very small ,ideally zero. such instruments can be constructed by connecting a low resistance in parallel to the galvanometer. this low resistance is called shunt which passed by the most of the circuit currents through it, An ammeter constructed using a shunt.

the importance of shunt are as follows;

• convert a galvanometer into ammeter .
• increase the current in a circuit.
• increase the range of galvanometer.

conversions of galvanometer into a  Ammeter;

A volt meter is an electrical device used to measure potential difference in a circut.it is connected across into two points between which the potential differences is to be measure by connecting a voltmeter in a circuits, it must not changes the currents and hence the potentials differences in the circuits so a voltmeter should have a very high resistance , ideally infinite resistance. such as the resistance made by connecting a very high resistance in series with a galvanometer. the high resistance is called multiplier ,R A voltmeter obtained from connecting the multiples R 

R=V/IG-G

THIS  equations gives the values of resistances R which is connected in series to the galvanometer .this connection converts the galvanometer into a voltmeter of range 0-v volts  the effective resistance of the voltmeter Rv=R+G. since R is high resistance of the voltmeter Rv is high and it will not draw much currents from the circuits.

Numerical; 3*5=15

1. the resistance of the coil of a pivoted coil galvanometer is 10.69 ohms and a current of 0.2224amp cause it to deflect full scale. we want to convert this galvanometer to an ammeter reading 2oA full scale. the only shunt available has a resistance  of 0.0250ohms. what is the resistance R must be connected in series with the coil.
2. A galvanometer  with scale divided into  100 equal divisions has a currents sensitivity of 1o division per milliampere and a voltage sensitivity of 2 divisions per millivolts .how can the instruments be adapted to serve as a ammeter reading to 1 A?
3. in a potentiometer arrangements , a cell of emf 1.34v gives a balance point at 35cm length of the wire. If the cell is replaced by another cells  and the balanced point shifts to 60 cm ,what is the emfs of the second cells.