HW: electric charges and coulomb's law

. A positively charged glass rod is brought close to a suspended metal needle. What can you say about the charge on the needle given that the needle is (a) attracted (b) repelled?

Assuming that no charge gets transferred between rod and needle, there are 2 scenarios by which the needle could get attracted to the glass rod. The obvious case would be if the needle possesses a net charge that is opposite in kind to that of the glass rod. The second case would be if the needle is electrically neutral but gets charged by induction in the presence of the glass rod. This, too, would cause an attraction. The only scenario inwhich the needle would be repelled by the glass rod would be when it possessed a net charge of the same type as the rod.

14. A newspaper reports that a new elementary particle with a charge of 9.00 x 10-19 C has been discovered. What is your response?

Given the fact that the charge of a single electron is 1.602 x 10-19 C, this new elementary particle would consist of approximately 5.6 fundamental charges. This would violate the property that charge is quantized, i.e., that it must exist in whole number multiples of the fundamental unit of charge.

Plastic wrap becomes electrically charged when pulled from its box. As a result, it is attracted to objects such as food containers. Does the wrap stick better to plastic containers or to metal containers? Explain.

It sticks better to plastic containers. Plastic is a good insulator, so when the charged plastic wrap makes contact with it, the charges will remain relatively fixed in place, maintaining electrical attraction. When charged plastic wrap comes in contact with metal, on the other hand, the charged plastic wrap will discharge through the metal since metal is a good conductor, causing it to lose its electrical properties

Why do clothes often cling together after tumbling in a clothes dryer?

Neutral clothes rub together and get charged by friction. The more electrophilic material will gain electrons while the least electrophilic material will lose electrons. This causes clothing to have opposite charges. Since opposite charges attract, clothes come together.

9. Strictly speaking, when an object acquires a positive charge via electron transfer, what happens to its mass? What about when it acquires a negative charge?

Objects become positively charged by losing electrons. Thus the mass of the object would decrease very slightly. Objects become negatively charged by gaining electrons, thus increasing mass slightly.

17. A typical lightning flash delivers about 25 C of negative charge from cloud to ground. How many electrons are involved?

Q/E (25C)/(1.6E-19)=1.6x10^20

1. How is the Coulomb (electrostatic) force similar to the gravitational force? How is it different? (2)

Similarities: both forces directly proportion to some property of matter (mass vs charge) and inversely proportional to separation distance Differences: Gravitational force is always attractive whereas electrostatic force can be both attractive and repulsive; Gravitational force is many orders of magnitude weaker than electrostatic force.

3. When an electroscope is charged by conduction with a plastic rod that has been rubbed with cat fur, will the metal leaves converge or diverge when the cat fur is brought near the probe? Explain.

The leaves will diverge because they will both be charged similarly, either positively or negatively. A positively charged plastic rod will result in the leaves becoming positively charged as electrons migrate from the leaves to the plastic rod. A negatively charged plastic rod will result in the leaves becomingnegatively charged as electrons will transfer from the rod to the leaves.

15. A free neutron is unstable and soon decays to other particles, one of them a proton. Must there be others? If so, what electric properties must they have?

Yes, there must be another particle created with an equivalent amount of negative charge in order for the law of conservation of charge not to be violated.

13. Why do electrostatic demonstrations tend to fail on humid days?

for charges to be static they should be in an "insulating environment" or the will stay static. any conducting medium that allows charges to move will mess up the experiment. humidity means you have water molecule hanging everywhere well water is poor conductor but it does ionize certain extent and capable of transferring charge and messing experiment.

10. How does the magnitude of the electrostatic force between a pair of charged particles change when the particles are moved half as far apart? Twice as far apart?

half as far apart = (1/2)^2 = decrease by (1/4) twice as far apart= decrease by (2^2)=(4)

18. Protons and neutrons are made from combinations of the two most common quarks, the u quark (charge +2/3 e) and the d quark (charge - 1/3 e). How could three of these quarks combine to make (a) a proton and (b) a neutron?

proton = 2u+1d neutron = 1u+2d

6. Why will neutral dust particles be attracted to a DVD that has been wiped with a dry cloth?

the neutral dust particle will be attracted to a dvd becuase after the dvd has been wiped with dry cloth it has been charged. the neutral dust causes induction force towards the dvd. the dvd is charged by friction force. the neutral dust is polarized and causes to be attracted to the charged dvd.

16. A light, uncharged metallic sphere suspended from a thread is attracted to a charged rubber rod. After it touches the rod, the sphere is repelled by the rod. Explain the cause of both attraction and repulsion on a microscopic level.

the sphere is repelled by rod because they both have the same charges. the rod's charge is transferred to the sphere.

At some auto toll booths, a thin metal wire protrudes from the road, making contact with cars before they reach the toll collector. What is the purpose of this wire?

to drain off the static electricity charge built up by car rolling down the road, so the toll collector doesnt get a shock every time someone hands over some money.

4. If you touch the probe of a charged electroscope with your finger, the leaves will fall. Explain why this happens on a microscopic level when the leaves are initially charged (1) positively and (2) negatively

when you touch the probe of charged electroscope with your finger you REMOVE ANY EXCESS CHARGES to make it neutral. Leaves are initially charged (positive) and (negative) the leaves are dicerge so when you touch the probe to remove any excess charge (+) amd (_) the leaves will fall.