Physics112 HW1

Due Friday, 11 September 2015

U2-CI01. A 100% copper penny has a mass of 3.1g and thus contains about 2.941022 copper atoms. Each neutral copper atom has 29 electrons. The penny starts out electrically neutral. If the penny subsequently acquires a charge of +2 μC, determine the fraction of electrons that have been removed from the penny.

U2-CI02. A spherical metal (conducting) shell acquires a charge of -2 μC, which means that it has a lot of extra electrons.

a) If this shell is far away from any other charges and metals, where do these extra electrons reside? What can you say about their distribution?

Now a second metal shell (ungrounded) that is electrically neutral is brought in contact with the first shell, and stays in contact with it.

b) Where do the electrons reside now? What can you say about their distribution?

U2-CI03. A spherical metal shell acquires a charge of +5 μC. A second metal shell that is electrically neutral is brought close to the first one but does not touch it.

a) Where would one find a net charge on each shell? Draw a picture showing where they are, and explain your reasoning behind where these charges are.

b) The second sphere is then grounded briefly (ground it, then break ground). Where would one now find a net charge on each shell? Again, draw a picture and explain your reasoning.

c) The first shell is removed. Where are the charges on the second shell now?

d) Repeat b)-c) if the second shell were instead an insulator.

U2-CI04. Start with an uncharged spherical conducting shell, a conducting ball A with charge +3μC, and a conducting ball B with charge +5μC, all widely separated from each other. First, insert A into the shell without touching the shell. While A is in there, ground the shell momentarily. Then remove A from the shell, still without touching the shell. Now insert B into the shell and touch the inside of the shell with B. Then remove B. What are the final charges on A? On B? On the shell?

U2-CI05. A piece of amber rubbed with fur attracts electrically neutral bits of paper off the table. Why? Explain.

U2-CI06. Two metal spheres, both initially electrically neutral, are connected together with a rod. A piece of plastic with a lot of excess electrons is rubbed onto the right sphere, wiping some electrons onto it. What is the resulting net charge distribution of both spheres if the connecting rod is

a)a conductor?

b)an insulator?

How would your answers change if the piece of plastic were just brought near the right sphere without touching it?