Chapter 5: Electricity and Magnetism

Electricity and Magnetism

What is Electricity?

Electricity describes all the phenomena caused by positive and negative charges.

Negatively charged bodies contain more electrons than protons.

Positive charged bodies contain more protons than electrons.

What is Electricity?

The charge of one electron or proton has a value of: 1.602x10-19 C (C: Coulomb)

What is Electricity?

1 C = 6.25x1018 electrons or protons

What is Electricity?

-

+

+

+Equal charges: repel each other

Opposite charges: attract each other

Conductors and Insulators

Since most objects are electrically neutral, charging an object consists in creating an imbalance in the electrical charge of that object.

Materials can be divided in three groups, according to their reaction to the process of “charging”:ConductorsSemiconductors Insulators

Conductors and InsulatorsConductors : permit the free flow of electrical charges.

Usually metals (weak attraction of valence electrons on the part of the atomic nucleus).

When charged, its electrons move in one general direction, pushing one another and making the charge move along the circuit.

Electrolytic solutions are also conductors (ions).

Conductors and Insulators

Insulators: impede the free flow of electrical charges.

Usually non-metals (strong attraction of valence electrons on the part of the atomic nucleus).

When charged, its electrons are strongly held by the atomic nuclei, so charges do not move.

Wood, glass, plastic, ceramics, paper, rubber, silk and air are also insulators.

Static Electricity

Describes all the phenomena related to electrical charges at rest.

Metal-leaf electroscope: apparatus used to detect static electricity in an object (see Fig 5.11 on page 145 Textbook)

Static Electricity

Charging by friction:Two neutral bodies rubbed against each other.Some electrons are pulled from one of them.Resulting: two opposite charged bodies

(attraction!)

Static Electricity

Charging by conduction:

Two solid bodies in contact with each other.The charge is transferred from the charged

object to the second object.Resulting: two equally charged bodies, each

with less charge than initially

Static ElectricityCharging by induction:

One charged body (A) approaches a neutral one (B). If B is a conductor: accumulation of equal amount of charges of

opposite sign to those of A in the region of B closest to A. B remains after A is withdrawn.

If B is an insulator: accumulation of equal amount of charges of opposite sign to those of A in the region of B closest to A. B will discharge as soon as A is withdrawn.