Important Facts That You Must Know First!
- An atom has electrons [negative charge], protons [positive charge] and neutrons [no charge].
- ONLY electrons [negative charges] move during charging process.
- Like charges repel and unlike charges attract.
- Electrical conductors are materials in which electric charges can move freely. Examples of electrical conductors are metals and water [our daily normal water and not the pure water].
- Electrical insulators are materials in which electric charges cannot move freely. Examples of electrical insulators are rubber, plastic and glass.
- Electric charge CANNOT BE CREATED OR DESTROYED. It is only transferred from one object to another. This is known as charge conservation.
- Electric charging is due to the transfer of electrons from one object to another.
- Discharging is the process of neutralising a charged object by removing excess charges. This is done by addition or removal of electrons.
- An insulator can be discharged by heating it over a flame or using moisture. The heat from the flame creates charged air particles that neutralise the charged object. Water molecules in air can also remove excess charges from an object.
- A conductor can be discharged by earthing. Earthing provides a path for excess electrons to flow away from or for electrons to flow to the conductor.
- When drawing distribution of charges on a solid conductor, you need to remember that the electrons can move freely in the conductor and thus the like charges will repel one another and stay as far away from each other as possible. Thus, the charges are always at the surface of the charged conductor.
- Earth is a very big conductor. It acts as a reservoir of charges. Electrons can flow from the conductor to the Earth and vice versa.
- In a conductor, charges will cluster around the sharp edges. (refer to O level P2 Q7 in 2013)
- Charge polarization is another method of charging by induction for non-conductor. Rub a balloon against your head and put it near tiny pieces of paper and you will see that the tiny pieces will be attracted to the balloon at first. Take a charged rod to replace a balloon, when the charged rod is brought near an insulator (i.e paper), there are no free electrons that can migrate/move throughout the insulating material. So how? Rearrangement of charges take place within atoms and molecules themselves. Atoms don’t move from their relatively fixed positions, but their centres of charge are moved. One side of the atom or molecule is induced into becoming more negative or positive than the opposite side. The atom is said to be electrically polarized. Simply put, when there is an external charged body come near, the electric field of the charged body distort the electron cloud causing the centres of the negative and positive charge to no longer coincide. Refer to Fig.14.1 below for illustration.
Fig.14.1 [ in (a), the centres are coincide while in (b) they don’t due to the distortion created by the presence of an external charged body]- Electric field inside any conductors is always zero as long as there is no electric charge flowing (no current flowing) in the conductors.
Commonly Seen Questions
- A polythene strip is rubbed on wool. Explain, in terms of movement of charges, how the wool becomes positively charged. Even if the question never state about movement of charges, you should know that you need to build your argument from that angle.
When the polythene strip is rubbed on the wool, the atoms at the surfaces are disturbed by friction. Some of the outer electrons are knocked off from the wool and transferred onto the polythene. The wool now has fewer electrons than protons, this results in the wool becoming net positively charged.
Note: The electrons transferred onto the polythene are unable to move freely within the polythene and they remain at the surface where the material has been rubbed. Parts of polythene that were not in contact will remain neutral.
- A positively charged rod is brought near (not touching) a tiny piece of an uncharged aluminium foil. The aluminium foil is seen moving towards the rod. Explain this observation.
When the positively charged rod is brought above the aluminium foil, the free electrons in aluminium foil is attracted and moved closer to the rod as unlike charges attract. This results in the top part (part closer to the charged rod) of the aluminium foil becoming negatively charged and the bottom part becoming positively charged. Since the strength of electrostatic force decreases with distance, the rod will attract the top part more strongly than it repels the bottom part. Thus, there will be a net/overall attractive force between the rod and the aluminium foil.
Note: I hope you know that it is not possible to charge aluminium foil by friction since it is a metal. The separate charges (positive and negative) appear on the aluminium foil is known as induced charges.
- A plastic comb is rubbed on a shirt and became positively charged. The comb is then brought near small pieces of paper. The pieces of paper seem to be attracted to the comb. Explain.
As the comb is approaching the paper, the positively charged comb polarizes atoms in the small pieces of paper and creates a negatively charged surface nearer to the comb. Since the strength of electrostatic force decreases with distance, the comb will attract the top part of the paper more strongly than it repels the bottom part. Thus, there will be a net attractive force between the comb and the pieces of paper.
Note: comb polarizes atoms in the small pieces of paper = the positively charge comb induces negative charges on the top part of the paper.
- Initially the sphere on the right is positively charged while the metal rod and the left metal sphere are neutral. State and explain the charges on both spheres and metal rod after they are in contact in arrangement shown below.
Upon contact, the free electrons in the left sphere and the metal rod will be attracted by the positively charged sphere (right) and move toward it since unlike charges attract. However, based on the conservation of charge, there are fewer negative charges than positive charges when we look at the two spheres and the rod as a single body. Thus, they will be overall/net positively charged. It means that each of the sphere and the rod are all positively charged.
Note: It is difficult to argue from the view point that the left sphere and the rod will have fewer electrons than before which make them net positively charged. However, we can’t be certain of the charge on the right sphere. Although, it does not make sense that the right sphere becomes neutral because then the free electrons will continue to move until there is a balance of charges.
This question can be modified to ask you to predict the likely observation in which the rod is seen to be repelled from the sphere (if the electrostatic force of repulsion is stronger than the weight of the metal rod).
Applications
- Photocopier
- Electrostatic Precipitator (a.k.a removing smoke and dust from chimney gas)
Other questions are just a different arrangement but the way to explain remains unchanged.