resistances – R , R , R , R as shown in the text. The null-point condition is given by using which the value of one resistance can be determined, knowing the other three resistances. . The potentiometer is a device to compare potential differences.
Since the method involves a condition of no current flow, the device can be used to measure potential difference; internal resistance of a cell and compare emf’s of two sources. POINTS TO PONDER . Current is a scalar although we represent current with an arrow. Currents do not obey the law of vector addition.
That current is a scalar also follows from it’s definition. The current I through an area of cross-section is given by the scalar product of two vectors: I = j . ∆ S where j and ∆ S are vectors. .
Refer to V-I curves of a resistor and a diode as drawn in the text. A resistor obeys Ohm’s law while a diode does not. The assertion that V = IR is a statement of Ohm’s law is not true. This equation defines resistance and it may be applied to all conducting devices whether they obey Ohm’s law or not.
The Ohm’s law asserts that the plot of I versus V is linear i.e., R is independent of V . Equation E = ρ j leads to another statement of Ohm’s law, i.e., a conducting material obeys Ohm’s law when the resistivity of the material does not depend on the magnitude and direction of applied electric field. . Homogeneous conductors like silver or semiconductors like pure germanium or germanium containing impurities obey Ohm’s law within some range of electric field values.
If the field becomes too strong, there are departures from Ohm’s law in all cases. . Motion of conduction electrons in electric field E is the sum of (i) motion due to random collisions and (ii) that due to E . The motion Physical Quantity Symbol Dimensions Unit Remark Electric current I [A] A SI base unit Charge Q, q [T A] Voltage,