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Physical Quantity

Prabesh wants to know more on Physical quantities:

Physical Quantity is a quantity which can be measured. eg; length, mass, force, time etc

(The quantities which cannot be measured are abstract. Eg; Love, anger intelligence)

Questions normally not answered by Students in Viva

Here are some questions which are not properly answered by students during Viva.

  1. Name the Physical quantities to be kept constant for Ohm’s law to be true.
  2. State the Principle of a potentiometer. (The students say that potential drop is proportional to length but the constant quantities are not mentioned)
  3. How can we increase the sensitivity of a potentiometer?
  4. Define figure of merit of a galvanometer.
  5. Which has more resistance – a galvanometer or a milliammeter?
  6. How does an LED emit light?
  7. What is the difference between an ordinary diode and an LED?
  8. Define principal axis of a convex lens?
  9. What happens to the focal length of a concave mirror if it is immersed in water?
  10. What are the factors affecting the intrernal resistance of a cell?
  11. What are the difference between primary and secondary cell?
  12. Why can’t we use a dry cell for starting a car?

Hints/Answers

  1. length, area of cross section,temperature
  2. The potential drop across any length of a conductor of uniform cross section and composition carrying a constant current is directly proportional to the length.
  3. increasing the length of potentiometer wire, decreasing the current, decreasing the potential gradient
  4. Current for unit deflection
  5. galvanometer
  6. The energy released during recombination of electrons and holes across the junction is responsible for the release of light by LED
  7. In ordinary diode the energy emitted during recombination of electrons and holes is in the invisible region of the em spectrum but in the case of LED, the energy is in the visible region.
  8. Straight line joining the centres of curvature of the lens..
  9. The focal length of mirror does not change by changing the medium. Their is a pure geometrical relationship between the radius of curvature and focal length and the relation does not include any term depending on refractive index.
  10. the nature of electrolyte, the concentration of electrolyte, temperature, distance between electrodes, area of electrodes
  11. Primary cell cannot be recharged, secondary cells can be recharged; secondary cells have less internal resistance than primary cells.
  12. Due to the high internal resistance of a dry cell, it won’t be able to provide the current sufficient to start the car

Limitations of Dimensional Analysis

Ashmeeta Bhattarai asked:

“What are limitations of principle of homogeneity of dimensional analysis?”

The dimensional analysis has the following limitations

  1. It fails while using it to derive a relation among physical quantities, if there are more than 3 unknown variables on which a given physical quantity depends
  2. It does not tell whether a given Physical quantity is a scalar or a vector.
  3. It does not tell us the value of constants involved
  4. It does not always tell us the exact FORM of a relation
  5. It cannot be used for deriving logarithmic, trigonometric or exponential relations
  6. A dimensionally correct equation may not always be the correct relation. (Because there are more than one physical quantity having the same dimensions)

Know some more? Write them as comments to this post

Principle of Homogenity of Dimensions

Binay Sharma asked:

“Explain the applications of the Principle of homogenity of Dimensions and its importance”

Principle of homogenity of dimensions states that “For an equation to be dimansionally correct, the dimensions of each term on LHS must be equal to the dimensions of each term on RHS.”

Applications

1. To determine the dimensions of an unknown quantity in an equation

2. To check the accuracy of an equation

3. To derive a formula connecting the given (or assumed) physical quantities

4. To convert a physical quantity from one system of unit to other

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