# 12th Class Physics Chapter 2 Current Electricity Short Question Answers

## 12th Class Physics Chapter 2 Current Electricity Short Question Answers Below

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i)The drift velocity of free electrons increases with the increase of potential difference.This is because by increasing the value of applied potential difference the value of electric field increases which causes the increase in the value of drift velocity of the electrons.

ii) When the length of the wire is decreased then the value of resistance of wire will also decrease.Thus the value of flowing current will increase it means that the value of drift velocity will also increase.Similarly the resistance of wire will decrease with the decrease of temperature.Thus the value of the current will increase due to which the drift velocity will also increase.

First band for Brown = 1

Second band for Green = 5

Third band for Red = 00

This means that given resistance is

R = 1500 Ω

Fourth band is gold which shows tolerance = ±5%

Thus the actual of resistance R = 1500±5%

For Figure B:-

First band for Yellow = 4

Second band for white = 9

Third band for orange = 000

since the value of yellow is 3 therefore it has three zeros (000)

Resistance = 49000

Tolerance value in fourth band of slicer = ±10%

Thus the actual value of resistance R = 49000 ±10%

Tolerance:- Tolerance means the possible variation from the marked value.For example a 1000Ω resistance with a tolerance of ±10% will have an actual resistance anywhere between 900Ω and 1100Ω.

Increase in resistance with temperature:- We know that the resistance offered by a conductor to the electric current is due to the collidions of free electrons with the atoms present in the latices of conductor.With the rise of temperature these atoms vibrate with larger amplitude and the chances of their collision with free electrons increases.It shows that with rise of temperature the collisions of free electrons with the atoms become more frequent and hence the resistance increases.The following relation expresses the change of resistance with temperature.

R_{t} = R_{o} (1+ ἆt)

Where R_{o} is the resistance of conductor at OᵒC and R_{t} is the resistance at t ᵒC and ἆ is the temp.Co – efficient of resistance t is the rise in temperature.

R

_{t}= R

_{o}(1+ ἆt)

Where ἆ is the temperature co – efficient and t is the rise of temperature.It means that the temperature of a lighted bulb does not remain constant so its filament does not obey Ohm’s law.Thus the main difficulty in testing whether the filament of lighted bulb obeys Ohm’s law is the change in its temperature with the flow of current and electrical resistance.

IR = E – Ir

Or V

_{t}= E – Ir …………….. (1)

Here E= emf of battery

r = internal resistance of battery

Ir = P.D across internal resistance

Equation (1) shows that when 1 is increase the factor Ir becomes large and V

_{t}becomes small.Thus terminal potential difference of a battery decreases when the current drown from it is increase.

E.M.F:-The emf of a battery is the potential difference between its terminals when no current is being drown from at to an external circuit i,e.when the circuit is open.

Terminal Potential Difference:-The voltage across the terminals of a battery when the current is being dream from it to an external circuit is called terminal potential difference.In other word’s it means that f a battery is not giving current to any external circuit the P.D between its known as emf (E) and if it is giving current to some external circuit the terminal potential difference V is given by

V= E – Ir

Where Ir potential drop across the internal resistance r’of battery.Thus whenever current is drown form a battery it terminal potential and emf become different.

The terminal P.D of battery (source ) is always less than emf.

drop takes place.As a result the actual e.m.f.of the cell decreases.That is why voltmeter cannot read exact value of a cell.However e.m.f of a cell can be exactly measured by a potentiate meter as it draws no current from the circuit.

V α I

So for high value of potential difference a long wire of uniform diameter is required.On the other hand a long wire provides a wide range of potential difference which can be used to balance the P.D.across a circuit.

H= I

^{2}R

_{t}

Magnetic Effect:-

The magnetic field around a moving charge was first found by prof.H Oersted in 1819.He showed that an electric current in a wire deflects a nearby compass needle.It means that an electric current in a wire produces a magnetic field around it.

The SI unit of power is Joule/Seconds or watt.

Three equations:- Three relations for the power dissipation are given below.

i) P = VI

ii) P = I

^{2}R

iii) P = V

^{2}/R

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