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Restoring force and Newton’s law

We know that whenever a body is deformed a restoring force originates and tries to restore the shape of the body and according to newtons third law of motion whenever a force acts on a body an equal and opposite force acts on the other body . So is the restoring force an opposite reacton to the deforming force and if it is not then why do we consider its direction equal and opposite to the deforming force while solving problems .

(Asked Akshat )

Answer: Restoring force comes into play only when a deforming force is exerted. When no external force acts on a body, the molecules are in equilibrium. No net force acts on it. If an external force is applied on it, it will try to  increase or decrease the intermolecular distance giving rise to a state of inequilibrium to teh molecules and they will tend to go back to their original positions to retain their state of equilibrium. This gives rise to the RESTORING FORCE.

Restoring force the the internal force that comes into play whenever an external force tries to change the inter-molecular distance. The restoring force at any instant (within limits) is equal in magnitude and opposite in direction to the deforming force

What are the similarities and differences in magnetic and electric field?

Similarities and differences in magnetic and electric field (Dilpreet posted this question)

Similarities Differences
 Both electric and magnetic field are conservative forcesBoth obey inverse square law

Both are non contact forces (Forces can be exerted without contact)

Both are attractive as well as repulsive (Like poles repel, like charges repel; unlike poles attract, unlike charges attract)

 Electric field is produced by a charge whether at rest or in motionBut magnetic field is produced only by a moving charge

 

The total magnetic flux through any closed surface is always zero, but the total electric flux through any closed surface is equal to the net charge enclosed by the surface multiplied by the reciprocal of absolute permitivity

Electric field lines are discontinuous as they have a starting point (+ charge) and an ending point (- charge); But magnetic field lines are continuous, they always form closed loops

 

Gravitation – Questionbank with answer for CBSE Class 9 SA1

The following questions are for a quick revision of the chapter “Gravitation” for CBSE Class 9. There are many other questions which can be asked from the portion allotted for SA1, but will mainly based on these concepts. The questions are based on the bare essential concepts; the minimum level of learning expected from the children

  1. State the universal law of gravitation.
    Universal law of gravitation states that every object in the universe attracts every other object with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them
  2. Define G. Write its value with proper units of SI
    G is the universal constant of gravitation. It is defined as the gravitational force of attraction between two objects of mass 1 kg each separated by a distance of 1m. Its value is 6.67 x 10-11 Nm2kg-2
  3. What is the difference between gravity and gravitation?
    The force of gravitation exerted by earth (or any other planet) on the objects on its surface is called gravity. Every object attracts every other object due to the force of gravitation. So, Gravity is a special case of Gravitation
  4. Define acceleration due to gravity.
  5. Why the moon is not “falling” to earth?
  6. Derive an expression for the acceleration due to gravity.
  7. Write the differences between g and G.
  8. calculate the value of g on the surface of earth.
  9. Discuss the variation of g with height, depth and latitude.
  10. Distinguish mass and weight.

Electric Potential

Lalit asks:

“Please explain electric potential in some other
words, other than every book,every author says..I
want something different..”

Answer:

If you know the meaning of the word POTENTIAL, and already studied gravitational potential, then understanding Electric Potential would be quite easy.

We know the law of conservation of energy which states that “Energy can neither be created nor be destroyed; it can be transformed from one form to another or be transferred from one body to another”

If we imagine a positive point charge kept at a point, and if we want to bring another positive point charge closer to it, we have to perform some work. This work done is stored as the potential energy of the system. The work done in doing so depends on (i) the magnitude of the original charge and (ii) the distance at which the new charge is brought.

The Electric Potential at a point in an Electric Field is a measure of the work required to bring a unit positive charge taken from infinity to the point against the electrostatic force and brought without any acceleration.

Please refer to the following links for detailed information

Electric Potential and Potential Difference

Why Electric Current is a Scalar Quantity?

Vaishali asks:

“Why is current a scalar quantity even after having both magnitude as well as direction?”

 

Answer:

For a physical quantity to be termed a vector quantity, having magnitude and direction is not enough. The quantity should obey the laws of vector addition too. Like the triangle law or the parallelogram law. As we know, if two currents meet at a junction, the total current of the resultant current will be the algebraic sum of the two current and not the vector sum.

 

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

Is ohm’s law universally applicable?

Amritha asked:

“Is ohm’s law universally applicable?”

Answer:

The Ohm’s law states that “The current through a conductor is directly proportional to the potential difference applied across its ends provided the physical conditions such as temperature remains the same

From the statement it is implied that it is not a universal law, and is applicable to conductors. It is not valid for semiconductors and electronic devices.

The substances which obey Ohm’s law are said to be ohmic and those which do not obey Ohm’s law are “Non Ohmic

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