Electrostatics

Electrostatics

Deals with electric charges at rest, or static electricity on the surface of an object.

Sub-atomic particles• Effect static electricity charging

• Protons (p+)– in nucleus (center) of an atom– positively charged

• Neutrons (n0)– in nucleus (center) of an atom– neutral (no effect on charge)

• Electrons (e-) – move freely outside of the atom– negatively charged

RULE OF ELECTRIC CHARGE • Electrical Charge – a force that one charge exerts on another

• An amount of static electricity can ignite volatile substances (gasoline)

• A large difference in the charges in the air can trigger a lightning.

• Opposite charges attract each other – protons are attracted to electrons because they have different

electric charges

• Like charges repel – one electron would repel another electron because they have the

same electric charge.

Mobility of charged particles

• Remember, an electron (e-) is free to move but a proton is “locked in” to the nucleus of an atom. This means that when charging occurs, the electrons either move from one physical body to another, OR are rearranged within an existing body. DON’T FORGET THIS!

Migrant Electrons • Electrons are opportunistic migrants – for electrons to make a move from the atoms of

one material to the atoms of another material, there must be an energy source, a motive, and a low-resistance pathway.

– Charged objects (ions) have an unequal number of protons and electrons

Unit of Charge • Coulomb – The charge on a single electron is -1.6 x 10-19 C– The charge on a single proton is +1.6 x 10-19 C – Charge is the difference between number of

electrons and protons

Example 1 – Calculating Charge

• The amount of charge carried by a lightning bolt is estimated at 10 C. What quantity of excess electrons is carried by the lightning bolt?

• 1 electron = 1.6 x 10-19 C• (10 C)(1.6 x 10-19 C) = 6.25 x 1019electrons

Electric Charge is Conserved • When one body loses an electron, the other

body will gain that electron so that there is a net charge on each body, but no electrons are destroyed in the process.

How distance effects strength of attraction – Coulomb’s Law

• The farther apart opposite charges are, the weaker the attraction force between them.

• The closer the charges get, the stronger the force becomes. – F = force– kc = Coulomb’s constant (9x109 Nm2/C2 )– q1 = charge of particle 1– q2 = charge of particle 2– r = distance

Example 2 – Coulomb’s Law• One charge of 2.0 C is 1.5 m away from a – 3.0 C charge. Determine the

force they exert on each other.

• The negative sign just means that one charge is positive, the other is negative, so there is an attractive force between them.

Example 3• Two balloons are charged with an identical quantity and

type of charge of -6.25x109 C. They are held apart at a separation distance of .617 m. Determine the magnitude of the electrical force of repulsion between them.

Example 4

• Two balloons with charges of +3.37 μC and -8.21 μC attract each other with a force of 0.0626 Newton. Determine the separation distance between the two balloons.

Insulators

• Any material that does not conduct electricity • store electrons on their surface.• Ex: Plastics, dried wood, glass, fabric, and

other non-metals

• Metals are CONDUCTORS, and do not store charges (wires in your home, etc.)

METHODS OF CHARGING

• Friction

• Conduction

• Induction

Friction

• When 2 uncharged objects come into contact, electrons are stripped from one object onto the other and they receive opposite charges. Usually happens when objects rub together.

Conduction

• When a charged object touches an uncharged object, some of the charge moves so that both objects have the same charges. The new charge will be weaker than the original one.

Induction

• When a charged object approaches, but does not touch an uncharged object, a migration of electrons creates an “apparent” charge on the object even though the net charge remains zero

Charging by Induction: Charged object drives off like charges, leaves charged object with opposite charge

Electric Field

• Field Lines -Invisible lines surrounding a charge manifesting a force.

• Number of lines per area represents field strength

• As lines diverge, field strength diminishes

• More charges means greater number of field lines

Electric Field

Field lines move away from positive toward negative charges.

Example 5

• Which region experiences greater electric field strength? – region A

• Where is that region located? – near the charge

Electric Dipole

• Two point charges of equal magnitudes and opposite signs.• Near the charge the field lines are radial • All lines that emerge will terminate • Emerge on positive charge and terminate

on negative.

2 Equal Positive Charges

• Field lines do not cross • Repulsion • Like Charges • For Negative charges,

reverse the direction of the field lines.

Unequal ChargesGiven a positive charge that is 2X that of negative, how will the lines terminate? – Only half the lines that

emanate from the positive charge will terminate in negative charge

– Rest of lines terminate at infinity.