ELECTROSTATICS Fact Sheet 2
(1) CHARGING BY RUBBING
If two objects are rubbed together, a transfer of electrons from one object to the other may take place. (Rubbing simply increases the area brought into intimate contact - which helps with electron transfer.) Each object now has an imbalance of electrons and protons, and will have become charged. The object with an excess of electrons will now be negatively charged and the other object with a deficiency of electrons will now be positively charged. Note that only the electrons are transferred, never the protons. Why is this?
Different substances have a different affinity (attraction) for electrons. The following list shows substances in order of increasing affinity for electrons :
Aluminium Strong electron donators
Copper or brass
Fur
Perspex
Wool
Glass
Cotton
Silk
Rubber
Sulphur
Ebonite
Polystyrene Strong electron acceptors
Contact between any two substances in the list will result in electron transfer. A substance higher up the list will become positively charged by giving up electrons to a substance lower in the list which becomes negatively charged by accepting electrons. Can you deduce that..
Rubbing a perspex rod with silk will cause the perspex rod to become positively charged and the silk to become equally negatively charged.
Rubbing an ebonite rod with fur will cause the ebonite rod to become negatively charged and the fur to become positively charged.
(2) CHARGING BY CONTACT
Note: Your teacher may demonstrate the “electroscope” at this point.
A device used for detecting an electric charge is called an electroscope (or electrometer if it has a scale associated with it). Your teacher will demonstrate and explain how this instrument works to detect an electric charge.
An electroscope can be electrically charged by touching the cap with a charged rod. The electroscope will acquire the same sign of charge as the rod used to charge it. Discuss the transfer of electrons that must have occurred to charge the electroscope..
An uncharged object may become electrically charged through simple contact with another charged object. This results from the transfer of electrons from one object to the other.
continued next page.. Charging by induction
(3) CHARGING BY INDUCTION
Follow the sequence of diagrams below to see how an object can become charged by induction. This process only works for conductors since charged electrons are not free to move over insulators.
We can also charge a single conductor by induction if the conductor is earthed at the appropriate stage. Again, follow the sequence of diagrams to see how this can be achieved.
Note that if we charge a conductor by induction, then an induced charge is opposite in sign to its inducing charge. An inducing rod does not lose any of its charge – the same rod could again be used to induce a charge on many insulated conductors.
Charging by induction is not restricted to conductors. Instead of the movement of free electrons, the presence of an inducing charge near to an insulator can cause a change in the charge distribution in the atoms and molecules that make up the material. We state that the atoms or molecules become electrically polarized to form electric “dipoles”. This explains why a charged rod attracts light pieces of uncharged paper. On becoming polarized, the alignment of polarized atoms or molecules in an insulating material results in an induced surface charge. The materials are attracted because of unbalanced forces on the induced surface charges. The attractive force of nearer opposite charges is greater than the repulsive force between the further apart similar charges.
Note that on reaching the charged rod, electron transfer may occur which leaves the material with a similar repulsive charge which causes the material to be repelled away.
Many molecules, water for example, are electrically polarized in their normal states. The distribution of electric charge is not perfectly even – there is a little more negative charge on one side of the molecule than on the other. Hence we can see why a thin stream of water can be diverted when a charged rod is brought near to the stream.
DISTRIBUTION OF CHARGE ON A CONDUCTOR
The fact that electrons can flow readily within a conductor is responsible for several effects. When electrons are placed on a body, they will be subjected to mutually repulsive forces which will set them in motion away from each other until they can separate no further.
Note that..
(1) Charge is equally distributed over the surface of a spherical conductor.
(2) Charge occurs only on the outside of a hollow charged conductor, unless a charged object is placed inside.
(3) Charge concentrates in regions of greatest curvature. Thus we observe that charge accumulates (collects) at points.
Your teacher will discuss the operation of the following:
· The lightning conductor
· The Van De Graaff generator (demonstration)
CONDUCTORS AND INSULATORS
The difference between a conductor and an insulator lies in the mobility of the electrons. In a metal, outer electrons are not anchored to the nuclei of particular atoms, but are free to roam about through the material. Enormous numbers of free “conductance” electrons can be made to drift almost unimpeded within the specimen. Such materials are good conductors of electricity (and also of heat).
In insulators, the electrons are tightly bound and remain with particular atoms. The electrons are not free to wander about to other atoms in the material. Any added electrons are anchored to the atoms touched to the charging device. Insulators are poor conductors of electricity (and are generally poor conductors of heat also).
Some materials, such as germanium or silicon, are good insulators in their pure crystalline form but increase tremendously in conductivity when even one atom in ten million is replaced with an impurity that adds or removes an electron from the crystal structure. Such materials are called “semiconductors”- they have a conductivity between conductors and insulators. Thin layers of semiconducting materials sandwiched together make up transistors which are used essentially as switches in many electronic devices. Incident light can also produce free electrons in semiconductors, increasing their conductivity. Electric eyes, solar cells and photocopy machines use semiconductors that are sensitive to light.
QUESTIONS TO ANSWER (on separate paper please)
1. Which force (gravitational or electric) repels as well as attracts?
2. Is an electron in a hydrogen atom the same as an electron in a uranium atom?
3. How does a negative ion differ from a positive ion?
4. What does it mean to say that charge is conserved?
5. Describe the difference between a good conductor and a good insulator.
6. What are the three main methods of charging objects?
7. What is the function of a lightning rod? Include a diagram.
8. Explain how an object that is electrically neutral can be attracted to an object that is charged.
9. Three different plastic rods were charged by rubbing. On bringing each rod up to the others, rod A repelled rod B, and rod A attracted rod C. Which of the following could be the arrangement of charges on the rods?
10. Why do you think the linoleum floors of operating theatres are impregnated with carbon?
11. How would you charge an electroscope by induction using a positively charged rod? State the sign of the charge left on the electroscope.
12. Describe one practical use which is made of the discharging action at a sharp point.
Towards Nanotechnology Designed by Doug Medwell Feb. ‘06
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