Transcript electric charge

Physics
Electricity and Magnetism
HEAT AND MAGNETISM
Static Electric Charge
• All matter is made of atoms.
• All atoms contain particles which possess electric charge.
protons possess positive charge
electrons possess negative charge
• The unit of measurement of electric charge is the Coulomb.
• If a body has equal quantities of positive and negative charges, it is said
to be electrically neutral.
• If a material allows electrons to move through it easily, it is a conductor.
All metals are conductors because they have free electrons (that is,
electrons which are not bound to a particular atom).
• A material through which electrons cannot easily flow is an insulator.
Forces between charges
similar charges repel
opposite charges attract
Also, a positively or negatively charged object will weakly
attract an object which is neutral. This occurs because the charged
object has the effect of "rearranging" the charges inside the originally
neutral object.
When two different insulators are rubbed together, electrons
can be transferred from one insulator to the other. The body which
has gained electrons has a negative charge and the one which lost
electrons has a positive charge of equal magnitude.
This process is called charging by friction.
A body can also be charged by simply placing it near a
charged body. This process is called charging by induction. This is
the process of "rearranging" the charges mentioned above.
The diagram below represents the situation when a piece of
metal is charged by induction.
The situation is different when an insulator is charged by
induction, as shown by the following diagram.
Electric charges which remain still are called static
charges.The only charges which can move freely through metals
are negative charges carried by electrons.
A flow of electric charges is called an electric current
Electric Fields
Coulomb’s Law
The force between two charges is directεly proportional to their
magnitudes and inversely proportional to the square of the distance
between them.
F α Q 1Q 2
F α 1/r²
This law has been verified by experiment.
The constant of proportionality depends on the medium.
F = (1/πε)(Q1Q2/r2)
The constant ε is called the permittivity of the medium.
If the medium is a vacuum the constant is written eo. The
units of e are N-1C2m-2, (this is usually written as Farads per metre,
Fm-1).
Electric Field Strength (E)
The electric field strength at a point is the force acting on a
unit positive charge placed at that point.