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Potentiometer & Its Applications | Physics Class 12 - NEET PDF Download

What is Potentiometer?

Potentiometer working can be explained when the potentiometer is understood. It is defined as a three-terminal resistor having either sliding or rotating contact that forms an adjustable voltage divider. In order to use the potentiometer as a rheostat or variable resistor, it should have only two terminals with one end and the wiper.

Following are the terms used to describe types of potentiometers:

1. Slider pot or slide pot: This can be adjusted by sliding the wiper right or left with a finger or thumb.

2. Thumb wheel pot or thumb pot: This can be adjusted infrequently with the help of small thumb wheel which is a small rotating potentiometer.

3. Trimmer pot or trim pot: This can be adjusted once for fine-tuning of an electric signal.

Necessity of Potentiometer

  • Practically voltmeter has a finite resistance (ideally it should be ∞). In other words, it draws certain current from the circuit. To overcome this problem a potentiometer  is employed because at the instant of measurment , it draws no current from the circuit.

Working Principle of Potentiometer 

  • Any unknown potential difference is balanced on a known potential difference which is uniformly distributed over the entire length of a potentiometer wire.
    This process is termed as zero deflection or null deflection method.

Note :
(i) Potentiometer wire : Made up of alloys of manganin, constantan, eureka.
(ii) Special properties of these alloys are high specific resistance, negligible temperature co-efficient of resistance (α). This results in invariability of resistance of potentiometer wire over a long period.

Circuits of Potentiometer

Potentiometer & Its Applications | Physics Class 12 - NEET

  • Primary circuit contains source of constant voltage & rheostat or Resistance Box.
  • Secondary circuit contains battery & galvanometer.

➢ Potential gradient (x) (V/m)

  • Potential difference corresponding to unit length of potentiometer wire is called potential gradient.
  • Rate of growth/fall of potential per unit length of potentiometer wire is equal to potential gradient.
  • Let r = 0 and R= 0 then VAB = E (max. in the ideal case) then x = E/L
    Unit and dimensions : (V/m ; MLT-3A-1)
  • Always VAB < E ; (∵ r + R1 ≠ 0)
    x = VAB/L
  • Now VAB = I RP (RP = resistance of potentiometer wire)
  • Let ρ = Resistance per unit length of potentiometer wire
    Potentiometer & Its Applications | Physics Class 12 - NEET
    current in primary circuit I
    Potentiometer & Its Applications | Physics Class 12 - NEET
  • If cross-sectional radius is uniform ⇒ x is uniform over the entire length of potentiometer wire.
  • If I constant, then
    Potentiometer & Its Applications | Physics Class 12 - NEET
  •  'x' depends on → ρ, r , σ  etc.

➢ Factors affecting 'x' 

  • If VAB = constant and L = constant then for any change → x remains unchanged.
  • If there is no information about VAB then always take VAB as constant so (x ∝ 1/L)
  • If VAB and L are constant :
  • For any change like radius of wire, substance of wire (σ) there is no change in x.
  • Any change in the secondary circuit results in no change in x because x is an element of primary circuit.

Note:
Potentiometer & Its Applications | Physics Class 12 - NEET
xmax or xmin on the basis of range of rheostat or resistance box (R.B.)
Potentiometer & Its Applications | Physics Class 12 - NEET
Potentiometer & Its Applications | Physics Class 12 - NEET
Potentiometer & Its Applications | Physics Class 12 - NEET


Applications of Potentiometer

Comparison of EMF's of two cells using potentiometer

  • Consider the circuit arrangement of potentiometer given below used for comparison of emfs of two cells
    Potentiometer & Its Applications | Physics Class 12 - NEET
  • Positive terminals of two cells of emfs E1 and E2 (whose emf are to be compared) are connected to the terminals A and negative terminals are connected to jockey through a two way key K2 and a galvanometer
  • Now first key K1 is closed to establish a potential difference between the terminals A and B then by closing key K2 introduce cell of EMF E1 in the circuit and null point junction J1 is determined with the help of jockey.If the null point on wire is at length

    l1 from A then
    E1 = Kl1
    Where K → Potential gradient along the length of wire

  • Similarly cell having emf E2 is introduced in the circuit and again null point J2 is determined. If length of this null point from
    A is l2 then
    E2 = Kl2
    Therefore
    E1/E2 = l1/l2
    This simple relation allows us to find the ratio of E1/E2

  • if the EMF of one cell is known then the EMF of other cell can be known easily

Determination of internal resistance of the cell

  • Potentiometer can also be used to determine the internal resistance of a cell
    Potentiometer & Its Applications | Physics Class 12 - NEET
  • For this a cell whose internal resistance is to be determined is connected to terminal A of the potentiometer across a resistance box through a key K2
  • First close the key K1 and obtain the null point. Let l1 be the length of this null point from terminal A then
    E = Kl1
  • When key K2 is closed ,the cell sends current through resistance Box (R). If E2 is the terminal
    Potential difference and null point is obtained at length l2(AJ2) then
    V = Kl2
    Thus
    E/V=l1/l2
    But E = I(R+ r) and V = IR
    This gives
    E/V = (r+R)/R
    So (r + R)/R = l1/l2
    giving
    r = R(l1/l2-1)
  • Using above equation we can find internal resistance of any given cell
The document Potentiometer & Its Applications | Physics Class 12 - NEET is a part of the NEET Course Physics Class 12.
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FAQs on Potentiometer & Its Applications - Physics Class 12 - NEET

1. What is a potentiometer?
Ans. A potentiometer is a type of variable resistor that consists of a resistive element with a sliding contact, which allows the resistance value to be adjusted by moving the contact along the resistive element.
2. What are the applications of a potentiometer?
Ans. Potentiometers have various applications, including volume control in audio devices, control of brightness in lamps or display screens, measurement of angular displacement in robotics, and calibration of analog instruments.
3. How does a potentiometer work?
Ans. A potentiometer works by varying the resistance along a resistive element. When the sliding contact is moved, the effective length of the resistive element changes, which in turn changes the resistance value and alters the output voltage or current.
4. Can a potentiometer be used as a voltage divider?
Ans. Yes, a potentiometer can be used as a voltage divider. By connecting the input voltage across the resistive element and taking the output voltage from the sliding contact, the potentiometer can divide the input voltage into desired proportions.
5. What are the advantages of using a potentiometer?
Ans. Some advantages of using a potentiometer include its simplicity, cost-effectiveness, and ability to provide a smooth and continuous variation of resistance. Additionally, potentiometers can be easily integrated into electronic circuits and offer precise control over various parameters.
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