After the comb is rubbed on a wool sweater, both the sweater and the comb are charged. Since comb is charged it can also charge small pieces of paper and since mass of these small pieces of paper is very small, they can be lifted even with as little electrostatic force as the force coming from the charges on a charged comb. Due to a fact that any material wants to stay electrically neutral, comb will try to lose its charge by charging the air surrounding the comb. After a while, comb is electrically neutral and won’t be able to pick up a small piece of paper because there will be no electrostatic force attracting the paper and the comb.

Hint: Paper is lifted because there is electrostatic attraction between the comb and a small piece of paper.

We know that the charges with an opposite sign attract and that charges with the same sign of charge repel. This means that if we bring in a positively charged object near the two strips, the strip that repels is positively charged, whereas the strip that is attracted to this positively charged object is negatively charged. This means that one of the ways to determine which strip is positively charged is to bring in a positively charged object in a proximity of both of the strips and see which one of the two strips repel.

Another way we can do this if we bring in a negatively charged object in proximity of both of the strips and take a look at which one of the strips is attracted to this negatively charged object. This is, in a way, analogous to the first solution.

Hint: Oppositely charged objects attract each other, whereas like-charged objects repel.

We know that the charges with an opposite sign attract and that charges with the same sign of charge repel. This means that if we bring in a positively charged object near the pith ball and it repels, pith ball is positively charged. If pith ball is attracted to the positively charged object that we brought to its proximity, pith ball is negatively charged. If pith ball is neither attracted nor repelled from this positively charged object, pith ball is not charged.

Hint: Oppositely charged objects attract each other, whereas like-charged objects repel. Neutrally charged objects neither repel nor attract each other.

Law of charge conservation states that total charge in the system must remain constant and it’s a consequence of the fact that number of electrons in a system is constant. This means that the only way for wool to become negatively charged is if electrons are transferred from the wool to the rubber rod. This makes the rubber rod negatively charged because it has more negative charge (electrons) than it has positive charge (protons), whereas wool transferred its electrons to the rubber rod, which makes it positively charged because it has less negative charge (electrons) than it has positive charge (protons).

Wool becomes positively charged because it transferred its electrons to the rubber rod. Click for further explanation.

It’s true that apple contains a lot of charged particles, but each atom in the apple is made from the same number of protons (positive charge) and electrons (negative charge). This makes every atom in every apple electroneutral and because of this apple as a whole is electroneutral. Two apples won’t neither repel nor attract when they are brought together because there is no electrostatic force between due to a fact that both apples are electroneutral.

Hint: Every atom in every apple is made from the same number of positive and negative charges.

We are told that metal rod is isolated so that we know that charge won’t flow from the rod to the ground in any way possible and charge, if any, will stay on the rod. When we bring in a a positively charged glass rod to one end of the metal rod, electrons from the metal rod will flow from the metal rod to the glass rod and metal rod will become positively charged. This charge on the metal rod will spread out evenly across it after a short period of time, but there will momentarily be more charge at the end of the rod that we first touched with the glass rod.

Charge on the metal rod will be of the same sign as charge on the glass rod and it will spread out evenly across the metal rod.

You can charge any material with friction, but the main reason why rubber rod stays negatively charged for some time, while the copper rod does not has to do with a person, or any other conductor, holding the copper rod.

When you rub wool across a rubber rod and the rubber rod is negatively charged, rubber rod won’t discharge through your hands or any other conductor that easily because rubber is insulator and it requires high voltage to become conductive.

Copper on the other hand is a conductor and if you hold the copper rod while you’re rubbing it with wool, copper wire does become charged but it quickly discharges through your hands because copper is easily conductive. If you were to hold the copper rod with insulating gloves, you’d make it harder for copper to discharge, but it would still be able to discharge through the air any other conductive material that it touches.

Hint: Copper is conductive and easily discharged. Click for further explanation.

Fluid model of charge can explain the electric current, because we may visualize the electric current the same way we visualize diffusion – as a flow of particles from a place with a higher concentration (higher voltage) to a place with a lower concentration (lower voltage). Fluid model, however, can’t explain why opposite charges attract, whereas like charges repel because there is no reason for the two fluid to repel or attract. Another reason why two-charge model is better is because we have valid proof of existence of both the electron and proton, which represent negative and positive charge in the atom.

Hint: Fluid model can’t explain why charges attract or repel. Click for further explanation.