The circuit shown in the figure below contains a battery B , a rheostat Rh and identical lamps P and Q . What will happen to the brightness of the lamps if the resistance through the rheostat is increased? Give reasons.

When the resistance of the rheostat is zero, the current through Q is zero since Q posses some resistance so current will take the least resistive path so Q will be open circuit. The circuit is then essentially a battery in series with lamp P . Thus, only P will glow. When the resistance of the rheostat is very large, almost no current flows through it. So, the current will flow through Q and then P . And the current are almost equal. The circuit is essentially a battery in series with lamps Q and P . Making both the lamp glow with same brightness.

Find the current i in the circuit given below: Given, forward resistance of the diode is r f = 1 Ω , R = 2 Ω and V = 10 v o l t s

Here, in the given circuit D 2 will be in reverse bias so it will act as open circuit and the diodes D 1 and D 3 will be in forward bias. By applying KVL, V = i r f + i r f + i R ⇒ 10 V = i ( 1 Ω + 1 Ω + 2 Ω ) ⇒ i = 10 V 4 Ω = 2 . 5 A Hence, the current in the circuit is 2 . 5 A .

Using an expression for drift velocity, show that the mobility of free electron is directly proportional to its relaxation time.

The expression for drift velocity of electrons is v d = e E τ m . Here, e is charge on electron, E is electric field, τ is relaxation time and m is mass of electron. In v d the set of term e τ m is mobility of electron i.e. μ . So, μ = e τ m . Hence, mobility of free electron is directly proportional to its relaxation time.

The potential difference across a given wire is increased. What happens to the drift velocity of the free electrons?

The drift velocity is v d = e E τ m , here e is charge of electron, E is electric field, τ is the relaxation time and m is mass of electron. Here, e τ m is constant and E is variable. E = V l , here V is potential applied and l is the length of the conductor. Thus, increasing potential difference across the conductor, will increase the field, this will eventually increase drift velocity. Hence, by increasing potential difference drift velocity of the free electrons will be increased.

State Kirchhoff’s voltage law.

Kirchhoff's voltage law states that in a closed loop within the circuit, the sum of all the voltages dissipated and the source voltage across the component in the closed loop circuit is equal to zero.

Find I in the following circuit : [Assume negligible internal resistance of the cell.]

Rearranging the resistances the given circuit will form a Wheatstone's bridge as shown in the figure. Now, the condition of the Wheatstone bridge is satisfied as P Q = R S where, P , Q , R , S are the sides of the bridge. From the above, the diagram 2 2 = 6 6 , satisfies the condition for the wheat stone bridge. Therefore, no current is followed through the central resistance. So, neglecting the middle resistance. Now, the total resistance of the upper side is 4 Ω as they are connected in series. Similarly, the total resistance of the lower side is 12 Ω . Hence, the equivalent resistance can be given as, 1 R = 1 4 + 1 12 = 3 + 1 12 R = 3 Ω So, the value of the current I = V R I = 3 × 6 = 18 A

Why do we prefer potentiometer to compare e.m.fs of cells than a voltmeter?

The potentiometer is preferred over voltmeter when there is a measurement of emf of the cell because the potentiometer does not draw any current as it is a null device. Whereas the voltmeter draws current from the cell. This is the reason why a potentiometer is preferred over voltmeter in order to get the actual measurement of e.m.f.

E M B I B E

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR PHYSICS>Chapter 3 - Current Electricity>Meghalaya-2020>Q 12

MEDIUM

12th Meghalaya Board

IMPORTANT

Earn 100

A conductor of radius $r$ and length $l$ has resistance $10 Ω$ . The conductor has stretched in such a way that its volume is maintained constant and its length is increased by $10 %$ of its original length. The resistance of the stretched conductor is

50% studentsanswered this correctly

Important Questions on Current Electricity

EASY

12th Meghalaya Board

IMPORTANT

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR PHYSICS>Chapter 3 - Current Electricity>Meghalaya-2020>Q 13

When resistances are connected in series, the effective resistance increases, why

?

MEDIUM

12th Meghalaya Board

IMPORTANT

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR PHYSICS>Chapter 3 - Current Electricity>Meghalaya-2020>Q 14

What are carbon resistors? A carbon resistor is marked in coloured band in sequence Blue, Green, Orange, Gold. What is the resistance and tolerance value of resistors?

MEDIUM

12th Meghalaya Board

IMPORTANT

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR PHYSICS>Chapter 3 - Current Electricity>Meghalaya-2020>Q 15

Write the relation of e.m.f of a cell and its internal resistance. What is the condition under which the potential difference between the terminals of the cell and its e.m.f are equal?

HARD

12th Meghalaya Board

IMPORTANT

EMBIBE CHAPTER WISE PREVIOUS YEAR PAPERS FOR PHYSICS>Chapter 3 - Current Electricity>Meghalaya-2020>Q 16

From the given circuit, find the value of current flowing through the $2 Ω$ register. Assume that $D_{1}$ and $D_{2}$ are two ideal diodes:

Question Image