PHY101 Assignment #9

1. When you rub two transparencies with tissue and hold them close together, they stand apart. Give two reasons that the force causing this cannot be gravity.

The first reason is that the transparencies repel each other due to the electron charges. The second reason is that this force depends on whether the transparencies are rubbed together, meaning that if they werent, the two transparencies with tissue will not stand apart.

2. Two small electrically charged objects are placed 6 centimeters (cm) apart, where they exert a force of 8 N on each other. How far apart must they be placed in order to exert a force of 1 N on each other? Assuming that the charges on the two objects are equal, what is the charge?

Since the force between two small electrically charged objects is one eighth of the charged objects that are placed 6cm apart, the calculation distance and the charges between them is shown below.

Hence, the objects must be placed 16.92cm apart from each other to exert a force of 1N. Also, the charges on the two objects are 1.79x10-6c.

3. A 1.5 volt battery is short-circuited by a 2meter long wire having a resistance of 0.02 ohm per meter. How large is the current flowing through the wire (before the wire or the battery burn out)?

Hence, the current flowing through the wire is 37.5amp.

I = VR=1.50.4

= 37.5amp

d = 8 6=16.97cm= 0.1697m

F = kq1q2d 2

1= (8.99109 )q2

0.16972q2 = 3.201012q =1.79106c

4. How does the electric force between two hydrogen nuclei placed a certain distance apart compare with the force between two helium nuclei placed twice as far apart?

Hydrogen nuclei has a charge of +1 and the helium nuclei has a charge of +2. According to Coulombs law, force is proportional to the charge of the particles by the distance between them squared. If the charge for helium nuclei of q1 and q2 are each doubled that of the hydrogen nuclei, where the two helium nuclei are placed twice as far apart, the force between two helium nuclei will remain the same, as the effect of increase in charge will be cancelled out by the increase in distance.

5. About how many atoms thick is a sheet of paper?

An atom is about 10-10m/atom across. A typical textbook is around 5cm thick with around 500 pages. Hence, a piece of paper from the textbook is around 10-4m thick.

Hence, a typical sheet of paper is about 106 atoms thick.

104m1010m =10

6atoms