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# Monthly Archives: September 2011

## A few selected questions fro CBSE Class 10 Physics Summative Assessment 1

Dear students,

If you are preparing for the CBSE Summative Assessment I for Class X, Give the following questions a try.

If you are not able to solve any of it, please post your doubts as comments to this post. The questions can be discussed here with due explanations.

Make it a practice to solve as much Sample papers and Guess papers as you can before writing the Board Exam.

All the best

1. An electric geyser has the ratings 2000W, 220V marked on it. What should be the minimum rating in whole number of a fuse wire, that may be required for safe use with this geyser? (1Marks)
2. Consider a circular loop of wire lying in the plane of a table. Let the current pass through the loop clockwise. Draw the direction of magnetic field inside and outside the loop. (2Marks)
3. A wire is stretched so as to make its length double. What is the effect on its resistivity and why? (2Marks)
4. Define electric power. Write its SI unit. What is the value of 1 kWh in joules. (2Marks)
5. A torch bulb is rated 3.0V, and 600mA. Calculate its (i) power (ii) resistance (iii) energy consumed if this bulb is lighted for 4 hours.  (3Marks)
6. (i) Differentiate between AC and DC giving two points of difference. (ii) What is the frequency of AC and DC in India? (iii) Draw a graph of current – time for AC and DC. (3Marks)
7. Name and state the law which relates the potential difference in a circuit to the magnitude of current that flows through it. Give its mathematical expression and draw the theoretical graph too. (3Marks)
8. What is electromagnetic induction? State Fleming’s Right Hand Rule. A coil of copper wire is connected to a galvanometer. What would happen if a bar magnet is (a) pushed into the coil with north pole entering first? (b) pulled out of the bar magnet? (c) held stationary inside the coil?
9. State Joule’s law of heating. How does the electric fuse prevent fires due to shortcircuit or overloading? A household uses the following electric appliances. (i) Refrigerator of rating 400W for ten hours each day. (ii) two electric fans of rating 80W each for twelve hours each day. (iii) six electric tubes of rating 18W each for six hours each day. Calculate the electricity bill of the household for the month of June if the cost per unit of electric energy is Rs. 3.00. (5Marks)

## Numerical from Motion and Force for CBSE Class 9 SA1

A steam engine of mass 30000 kg pulls 2 wagons each of mass 20000 kg with an acceleration of 0.2 m/s/s. neglecting frictional force calculate (i) the force exerted by the engine (ii) the force experienced by each wagon

(Srija posted)

In this case the engine is pulling the rest of the wagons.

The force exerted by the engine = total mass of the wagons x acceleration = 2 x 20000 x 0.2 = 8000 N

Each wagon experiences a net force = 20000 x 0.2 = 4000 N

## Electric Field of Axons!

A nerve signal is transmitted through a neuron when an excess Na+ inos suddenly enters the axon, along cylindrical part of the neuron. Axons are approximately 10.0 micrometer in diameter, and measurements show that about 5.6×10^11 Na+ ions per meter ( each of charge e+)enter during this process. Although the axon is a long cylinder, the charge doesn’t all enter everywhere at the same time. A plausible model would be a series if point charges moving along the axon. Let us look at a 0.10-mm length of the axon and model it as a point of charge.

(a) If the charge that enters each meter of the axon gets distributed uniformly along it, how many coulombs of charge enter a 0.10-mm length of the axon?
(b) What electric field ( magnitude and direction) does the sudden influx of charge produce at the surface of the body if the axon is 5.00 cm below the skin?
(c) Certain sharks can respond to electric fields as weak as 1.0 microN/C. how far from this segment of axon could a shark be and still detect its electric field?

## Impulse – momentum and packing of glassware

Why are glassware packed with straw? Answer in terms of physics.

This is due to the impulse momentum principle

## The kinematics of a Dolphin Jump

A dolphin jumps from the surface of water and return to the surface of water after reaching the height of  1.8m.Find the initial velocity from 85km/h to 100km/h?

## Motion of a charge particle in an electric field – Numerical Problem

At t=0 a very small object with mass 0.400mg and charge +9.00μC is travelling at 125 m/s in the -x-direction. The charge is moving in a uniform electric field that is in the +y-direction and that has magnitude E= 895 N/C. The gravitational force on the particle can be neglected. How far is the particle from the origin at t= 7.99 ms.

Hint for solving the above problem:

displacement along X direction; x=-uxt=125 x 7.99/1000

Displacement along Y direction, y=0.5 ay t2

ay=qE/m

s=√(x2+y2)

Just substitute the values and get the answer

## Is linear momentum conserved when a ball is thrown vertically up in the sky?

ROUNAK BHATTACHARJEE asked the above question.

The fact is that linear momentum is conserved in all interactions. If you concentrate on the motion of a single body alone, that will not account for an interaction. When thrown above, according to the principle of conservation of momentum, what we can say is that “The total momentum of the ball plus the thrower just before throwing is equal to the total momentum just after throwing”

After that what happens is controlled by the force of gravity acting on it. The force acting on it can change the momentum. Here the interaction is between the ball and the earth. On account of the huge mass of earth, the motion of earth to compensate the motion of ball and to conserve the momentum cannot be detected.

However, the linear momentum is conserved in all events and interactions so far!

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