Physics SC136 Finals Notes

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Electric Fields

Properties of Electric Charges

There are two kinds of electric charges; positive and negative. Charges of the same kind repel one another and charges with opposite signs attract one another.

Electric charge is ALWAYS CONSERVED in an isolated system. When two objects are rubbed against each other, charge is not created. The charged state is the result of the transfer of charge from one object to another.

Electric charge= q

---> It is said to be quantized: electric charge exists as discrete "packets" and we can write q= +/- Ne where N is some integer.

electron has the charge -e and proton has the charge which is of equal magnitude but opposite sign of +e.

Charging Objects by Induction

Electrical conductors are materials where electrons can move rather freely eg. metals and tap water

Electrical insulators are materials where electrons are bound to atoms and cannot move freely through the material. eg. glass, rubber, dry wood

-----> When insulators get rubbed, the charge is only in the area rubbed and not spread to the entire material

Semiconductors are materials that are the intermediate between conductors and insulators eg. silicon and germanium

Superconductors are materials that present no resistance to the movement of electrons at all.

To ways to give charge to an object:

1) Charging by induction: even without physical contact, charged object will cause opposite charge of another neutral object to move towards it and the same charge to move further away. The surface of the induced object will then have a certain charge on its surface.

2) Charging by contact: the opposite charge in the neutral object will get transferred to the charged object, leaving behind the other charge.