1 Electric Charge Answers

8/10/2019 1 Electric Charge Answers

1/11


2/11

Question 1.1The figure shows five pairs of plates:A, B, and Dare charged plastic platesand Cis an electrically neutral copper plate. The electrostatic forcesbetween the pairs of plates are shown for three of the pairs. For the

remaining two pairs, do the plates repel or attract each other?

Ans.

C and D attract.

B and D attract.


3/11

Question 1.2The figure shows two protons (symbol p) and one electron(symbol e) on an axis. What are the directions of (a) the

electrostatic force on the central proton due to theelectron, (b) the electrostatic force on the central protondue to the other proton, and (c) the net electrostatic forceon the central proton?

Ans. Leftward for all three cases (a), (b) and (c)


4/11

Question 1.3The figure here shows three arrangements of an electron e and twoprotons p. (a) Rank the arrangements according to the magnitude of thenet electrostatic force on the electron due to the protons, largest first.

(b) In situation c, is the angle between the net force on the electron andthe line labelled dless than or more than 45?

Ans. (a) a, c, b; (b) less than


5/11

Question 1.4Sparks are often observed (or heard) on a dry day whenclothes are removed in the dark. Explain.

Ans. Static charge is built up on the clothes as they rubagainst the body. Electrical discharge occurs to groundwhen the charge reaches a certain limit.


6/11

A balloon is negatively charged by rubbing and then clings to the

wall. Does this mean that the wall is positively charged? Why

does the balloon eventually fall?

Ans. No. The balloon induces charge of opposite sign in the wall,

causing it to be attracted. The balloon eventually falls since its

charge slowly diminishes as it leaks to the ground. Some of thecharge could also be lost due to ions of opposite sign in the

surrounding atmosphere which could tend to neutralize the charge.

Question 1.5


7/11

Question 1.6

A large metal sphere insulated from ground is chargedwith an electrostatic generator; a person standing on aninsulating stool holds the sphere while it is being charged.Why is it safe to do this? Why wouldnt it be safe foranother person standing on the floor to touch the sphereafter it has been charged?


8/11

Question 1.6 AnswerThe insulating stool does not allow thecharge to flow to the ground, so there

is no current path. However, a personstanding on the floor could receive anelectrical shock if he is making goodelectrical contact with the ground.

In order to prevent electrical shock, the charged sphere should begrounded before the person holding the sphere leaves the stool.


9/11

Question 1.7

Initially, sphereAhas a charge of -50eand sphere Bhasa charge of +20e. The spheres are made of conductingmaterial and are identical in size. If the spheres thentouch, what is the resulting charge on sphereA ?

Ans. -15e (net charge of -30e is equally shared)


10/11

If you adapt your eyes todarkness for about 15 minutes

and then have a friend chew aPolo Mint, you will see a faintflash of blue light from yourfriend's mouth with each

chomp. (To avoid wear on theteeth, you might crush thecandy with pliers, as in thephotograph.)

Miniproject 1.1 What

Causes this Sparking?


11/11

Question 1.8 AnswerOne part of the fractured Polo Mint has excesselectrons while the other part has excess positiveions. Electrons and ions jump across the gap ofthe fracture and collide with nitrogen molecules inthe air and blue light is produced.

Demo 16-23, 16-24, 16-26

Documents

1 Electric Charge Answers