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How many bulbs can be connected in parallel

English: Incandescent light bulbs.

I need to know how many 15W bulbs can be connected in parallel If I use an extension cord in the 5A socket? If I use the 15 A socket then how many bulbs can I connect in parallel?

Asked Anil


Assuming the voltage to be 220 V,

The current drawn by a single bulb is 15/220=0.068 A

So the no. of bulbs which can be connected in parallel = 5/.068=73 (avoiding the fractional part)

If you use 15A, then 220 bulbs can be connected. (follow the same procedure)

Efficiency of Air conditioner

someone designed an air conditioner that takes 500J of thermal energy from a cold room and exhausts 500J of thermal energy into the outside air. The energy is conserved, so the system should work.  Is such a system feasible or are we optimistic about this new design. Why or why not?

asked praveen kumar

Aparelho de ar condicionado

Refraction – Disappearing letters


Asked Kushagr

Body at incline

A free body diagram of a mass on an inclined plane

1. In first case we have the box with weight, say, 10 Newtons on incline. We calculate two components (perpendicular and parallel)of weight. We calculate normal force and force of friction. The box is at rest.

2. In second case we have ball on incline, made of same material as box from first case. Weight of the ball is the same as in first case, components of weight are the same as in first case. Normal force and friction have the same amount as in the case of box. But,unlike the box, ball is moving – it slides down. How’s that possible? If calculated net force is zero?

 Petar asked


It is not true that the ball slides,  but it will roll down.

In the first case, the forces balance each other and there is no motion.

In the second force, the frictional force acting tangentially backwards (up the incline) and the component of weight of the ball acting parallel to the plane and through the centre of the ball constitute a couple and tends to rotate it. Now there is no sliding; it rolls.

(If any further clarification is required please post as comment to this post)

Please refer to http://www.real-world-physics-problems.com/rolling-without-slipping.html for detailed treatment of the Physics of rolling without friction.

Balancing a metre scale on two fingers —

When we place a scale balancing on our two fingers,why can’t we move them simultaneously, without the scale falling?


We can! if the hands are not tilted and if the movement is without a jerk, the scale will not fall no matter how fast the fingers are moved.

This happens because, the centre of mass of the system is not changes because of the happenings within the system without the aid of any external agency.

GPS (Global Positioning System) Accuracy of Measurement

The 24 satellites that orbit the earth and define the Global Positioning System use
precision atomic clocks that are accurate to 1 x 10-9 s, or 1 nanosecond (1ns). A person on the earth, using their mobile phone, can usually “see” 4-12 of the 24 satellites at any one time.

An animation depicting the orbits of GPS satel...
An animation depicting the orbits of GPS satellites in medium Earth orbit. (Photo credit: Wikipedia)

The satellites are moving, relative to the surface of the earth, at 1.4 x 104 km/h.
a- Calculate  for one of these satellites,
Civilian GPS measurements have to be accurate at the level of 5-10m. How accurate must our knowledge of the clocks on the satellites be in order to achieve this level of accuracy in our position measurement?
[HINT: light signals are used by your phone and the satellites to perform the triangulation]
c-If we measure the passage of 1 day on earth using a clock that is identical to the clocks on the GPS satellites, how much time have the clocks on the satellites measured? What about for the passage of 2 days on earth? For this problem, it will be useful to apply the Binomial Expansion again

Mona asked

Light Amplification

If you passed directional light from a device such as a laser through the back of a 2 way mirror at a 45 degree angle and then directed that beam to 3 highly reflective mirrors at 45 degree angles in series so that the beam was directed back to the exact point it emerges from the two way mirror and creates an infinate loop what would happen?  Would the beam change color since the particles would be out of sync because of the time delay?  Would the light and heat of the beam amplify itself if the reflective surfaces were reflecting more than 50% of all the light emitted and more than 50% of the original sources light were transferred through the 2 way mirror?  I included a drawing to simpilify the questions.


Asked Jeff

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