A Wheatstone bridge arrangement is shown as below: 
Using Kirchoff’s second law to the loop ABDA, we get 
Applying Kirchoff’s law to loop BCDB, we get
When the bridge is balanced, Ig = 0
Then, the equations can be written as,
... (1) 
... (2)
On dividing equation (1) by (2), we get
, which is the balanced condition of a Wheatstone bridge.
Describe briefly, with the help of a circuit diagram, how a potentiometer is used to determine the internal resistance of a cell.
Potentiometer can be used to measure the internal resistance of the cell.
A cell of emf E is connected across the resistance box through key K1.
When key K1 is opened galvanometer shows deflection at the balancing length l1.
So, E = k
If both keys are closed, then balancing point is obtained at length l2 (l2 < l1).
So, V = k
Now, using the relation, 
Therefore, we have
Graph showing the variation of current versus voltage for a material GaAs is shown in the figure. Identify the region of
i) negative resistance
ii) where Ohm’s law is obeyed.
i) The region of negative resistance is DE because, the slope is negative for this part of curve.
ii) BC is the part of the curve where Ohm’s law is obeyed because here, current is varying linearly with with the voltage. This gives us direct proportionality between current and voltage.
Explain the term ‘drift velocity’ of electrons in a conductor. Hence obtain the expression for the current through a conductor in terms of ‘drift velocity’.
The velocity gained by the accelerating electrons in uniform electric field inside the conductor is drift velocity. The average velocity, acquired by free electrons along the length of a metallic conductor, due to existing electric field is called drift velocity.
Let ‘n’ be the number density of free electrons in a conductor of length ‘l’ and area of cross-section ‘A’.
Total charge in the conductor, Q = Ne
= (nAl)e
Time taken, t is given by, 
Therefore, the current flowing across the conductor is given by, 
That is, 
, which is the amount of current flowing through a conductor in terms of drift velocity.
Define the current sensitivity of a galvanometer. Write its S.I. unit. Figure shows two circuits each having a galvanometer and a battery of 3 V. When the galvanometers in each arrangement do not show any deflection, obtain the ratio R1 / R2. 
Current sensitivity is defined as the ratio of deflection produced in the galvanometer to the current flowing through it.
Mathematically it is given as, 
SI unit is radian per ampere.
No current flows through the galvanometer for balanced Wheatstone bridge.
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