physics

Pressure, Thrust, Area and Pascal’s Law

“In Pascal’s law we find that pressure does not increase with area. But when we study about pressure, we learn that pressure is inversely proportional to area. How is this possible? Please explain.”

Fahad Imtiaz asked via Speak Pipe

Answer:

Pressure is defined as the thrust (the total force acting normal to a surface) per unit area.

Pascal’s law deals with fluid pressure and the statement goes –

“The pressure exerted anywhere in an enclosed incompressible and non-viscous fluid is transmitted equally and undiminished in all directions through out the fluid, provided the effect of gravity is neglected”

Read the statement carefully.

Here we are not changing the definition or meaning of pressure.

the fact to note that, in an enclosed fluid, the pressure is transmitted equally throughout the fluid. Therefore, if we apply some pressure somewhere in the fluid, the same pressure will be felt  at any other place on the enclosed fluid. This gives us an opportunity to multiply the force. Since the pressure is equal everywhere, if we increase the area the force (thrust) is increased.

P = F/A or F = PA

So, pressure remaining constant, greater the area, greater is the force.

Hope you understand the matter now.

Transformers

A hypothetical ideal transformer considered as a circuit element, consisting of N P turns in the primary winding, and N S in the secondary. Electrical power supplied to the primary circuit is delivered to the load in secondary circuit by means of mutual induction. The time-varying magnetic flux in the core generates an electromagnetic force over each of the windings. The voltage and current relationships are shown below. (Photo credit: Wikipedia)

Please explain what is transformer and tell what is the reason behind the working of transformer that it reduces potenial when current is high and increases potential when current is low?

Explain widely and deeply with concepts.

Answer:

Transformer is a device based on mutual inductance and is used to change the voltage of alternating (or varying) current. There are two types of transformers – stepup and stepdown.

A step-up transformer increases the voltage as the step-down transformer decreases the ac voltage.

When a transformer changes the alternating voltage, it is not adding any power, it only transforms the voltage.

As power is the product of voltage and current, as voltage increases, the current decreases. (and vice versa)

The following links will help you understand transformers in great detail.

• http://www.physlink.com/education/askexperts/ae427.cfm
• http://en.wikipedia.org/wiki/Transformer
• http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/transf.html
• http://www.nuffieldfoundation.org/practical-physics/explaining-how-transformer-works

A doubt from Dimensional Analysis

What about the correctness about the relation,

s_t=u+0.5a(2t-1), both dimensionally or any other

 Prakash Adhikari asked

Answer

The equation refers to the displacement in the t^th second and hence it should have the dimensions of speed; since it is the displacement in one second.

Under this consideration, the equation is dimensionally and otherwise correct.

CBSE Class X Physics Questions selected from Previous Question Papers

1. Name the part of the human eye that helps in changing the focal length of the eye lens.( 1 mark)
2. Write the  value of near point of distinct vision for normal eye (1 mark)
3. What is the cause of dispersion of white light passing through a prism? Which colour of light deviates the (1) most      (2) least? (2 marks)
4. Why does the clear sky appear blue? How would the sky appear in the absence of earth’s atmosphere? (2 marks)
5. The refractive indices of alcohol and turpentine oil with respect to air are 1.36 and 1.47 respectively. Find the refractive index of turpentine oil with respect to alcohol .Which one of these will permit the light to travel faster? (2 marks)
6. Why do we observe difference in colours of the sun during sunrise sunset and noon? (2 marks)
7. Describe the formation of rainbow in the sky with the help of diagram? (2 marks)
8. What is a rainbow? Explain in brief the formation of rainbow? (3 marks)
9. A person cannot see objects farther than 12 m from the eye clearly. Name the , defect of vision he I suffering from and the lens that should be used for correction of this defect. Illustrate with the help of a diagram, how this lens will correct the defective vision. (3 marks)

A point object is placed at a distance of 12 cm from the convex lens on its principal axis. Its image is formed on the other side of the lens at a distance of 18 cm from the lens. Find the focal length of the lens. Is the image magnified? Justify your answer. (3 marks)

11.

1. A compound lens is made of two lenses in contact having powers +12.5 D and _2.5 D. Find the focal length and power of the combination.
2. The magnification produced by a mirror is +1.What does this mean ? (3 marks)

12.    A person is not able to see distinctly the object placed at large distances but is able to read the matter written on a book comfortably. Name the defect of vision he is suffering from? State two causes for this defect? With the help of a ray diagram show how this defect can be corrected    (3marks)

13. Name the type of mirror used in the following:   (a) Solar furnace (b) Side/rear- view mirror of a vehicle. Draw labeled ray diagrams to show the formation of image in each of the above two cases. Which of these mirrors could also form a magnified and virtual image if an object? Illustrate with the help of a ray diagram.(5 marks)

1. Draw a ray diagram in each of the following cases to show the formation of image, when the object is placed:
   1. Between optical centre and principal focus of a convex lens
   2. Between F and 2F of a concave lens
   3. At 2F of a convex lens. What can you say about sign and value of linear magnification ratio in (1) and (2) above (5 marks)
   4. 1. Define principal focus of a concave mirror.
      2. Why do we prefer a convex mirror as a rear view mirror in vehicles?
      3. We wish to obtain an erect image of an object using a convex lens. Draw a ray diagram to show the image formation in this case (5 marks)