Evans SPH 4U1

Physics Grade 12

Unit 6: Electricity

Note 1: Electric Charge

Reference: Chapter 7.1 http://www.sciencemadesimple.com/static.html

Law of Electric Charges:

·         Opposite charges attract each other

·         Similar charges repel each other

·         Charged objects attract some neutral objects

To fully explain how objects can attract neutral objects scientists came up with a simple Model of the Structure of Matter.

·         All matter is composed of atoms.  Atoms are made up of positively charged protons, neutral neutrons & negatively charged electrons.  When the # of electrons in an atom is equal to the # of protons, the atom is electrically neutral. 

·         All electric charges in solids are due to an excess or a deficit of electrons

·         Conductors, Insulators can be solids, liquids or gases.

·         In conductors electrons can move easily from one atom to another (i.e metals) but in insulators electrons are not free to move (i.e. plastic, cork)

Charging By Friction Electrostatic Series

·         When 2 substances are rubbed together the one higher in the table (page 321) becomes negatively charged while the other becomes positively charged.

Induced Charge Separation

·         Distribution of charge resulting from a change in distribution of electrons in an object.

·         Through induced charge objects can attract neutral objects (remember when the pith ball was attracted to the Van de Graaff generator since the pith ball was polarized).

·         Strength of attraction or repulsion (electric force) depends on the distance between the objects.

 see your diagram from class notes 

Charging By Contact

·         If a negatively charged object touches a neutral object some of the excess electrons move over to the neutral object making it negative and they will now repel each other.

·         An object charged by contact has the same charge as the charging object

  see your diagram from class notes 

Charging By Induction

·         An object charged by induction has a charge opposite to that of the charging object.

   see your diagram from class notes 

Law of Conservation of Charge

·         The total charge in an isolated system is conserved (difference between the amounts of positive and negative charge).

Some fun questions you can read on your own and test your knowledge:

1.      Vehicles carrying inflammable materials have chains or metallic ropes touching the ground during motion. Why?

The vehicle moving in the air has its body charged due to friction with the air. The metallic ropes are used to ground the charge, which builds up on the body of the vehicle. 

2.      A negatively charged ebonite rod attracts a suspended ball of straw. Can we infer the ball is positively charged?

No, we cannot infer the ball is positively charged.  The ball could also be neutral since the rod could repel the electrons to the opposite side of the neutral ball and the closest side of the ball to the rod would then be negative. The neutral ball would then be attracted to the negatively charged rod.

3.      Ordinary rubber is an insulator. But special conducting rubber is used for making aircraft tires. Why?

The aircraft moving in the air accumulates charge on its body due to friction with the air. To make the charge transfer to the earth the tires are made of conducting rubber.

4.      What is the origin of frictional force between the charges?

The frictional force between the charges arises due to the fact that when we rub two bodies’ electrons from one are transferred to the other. This results in one body gaining electrons (acquiring a negative charge) and the other body loosing electrons (acquiring a positive charge). Thus there results an attractive force between them, which is electrical in nature.

5.      A bird perches on a high voltage power lines and nothing happens. A man standing on the ground touches the same wire and it could be fatal. Why?It should be remembered that the current passing through the body is dangerous and not the potential on the body. When the bird sits on the high voltage wire no current passes through it but when the person touches the wire a conducting path is set up