Electric Charge

Electrostatics lesson for high school physics students

1. 1
Electric ChargeElectric Charge
Physics 30
Topic 2-1
D.G. MacCarthy

2. 2
Atoms & ElectricityAtoms & Electricity
•All matter is made of atoms, which contain
electrons, protons, and neutrons.
•Objects normally contain equal numbers of
electrons and protons: such objects are called
neutral.
•When an object has an imbalance in the
number of electrons and protons, it is
electrically charged.
•Neutrons are not involved in electric
interactions.

3. 3
Electric ChargeElectric Charge
•An object with more electrons than protons is
said to carry a negative charge.
•An object with more protons than electrons is
said to carry a positive charge.
•The imbalance between p
+
and e
–
in a charged
object is proportionally very small…
•The difference may be only 10
12
particles out
of a total of 10
25
; i.e. an imbalance of 1 per 10
trillion!

4. 4
Electric ChargeElectric Charge
•Like energy and momentum, electric charge is
a conserved quantity:
Charge cannot be created or destroyed,
but it can be transferred between objects.
•Charged objects exert electric forces on each
other:
Opposite charges attract;
Like charges repel.

5. 5
Units for Electric ChargeUnits for Electric Charge
•The symbol for electric charge is usually a “q”.
•The elementary charge unit (e) is equal to the
charge of a single proton or electron:
q
proton
= +1e
q
electron = –1e
•The elementary charge unit is not the standard SI
unit.
•The SI system uses the Coulomb (C) as the
standard unit of charge.

6. 6
Units for Electric ChargeUnits for Electric Charge
•A Coulomb is defined as the number of electrons
passing through a current of 1 Amp each second:
C = A·s.
•The conversion between C and e was determined
by Robert A. Millikan:
1 e = 1.60×10
–19
C.
•It is much easier to measure charge flowing in a
current than to count protons and electrons
individually!

7. 7
PolarizationPolarization
•In some cases, neutral objects may be
electrically active through a phenomenon
known as polarization.
•When a charge (either + or –) is brought
near a neutral object, the electrons and
protons are pushed in opposite directions.

8. 8
PolarizationPolarization
•The two edges of the object acquire
opposite charges. The object is polarized.
•The near side is attracted and the far side
is repelled.
•The attraction is stronger because it is
closer.

9. 9
PolarizationPolarization
•The net force is attractive.
•If a – charge is used instead of a + one, the
net force is still attractive.
•If they can be polarized, neutral objects
will attract to both + and – charges.

10. 10
PolarizationPolarization
•Some molecules (like H
2O) have a
permanent polarization: one end is + and the
other is –.
•When a charge is nearby, the molecule will
spin so that opposite charges are close
together, and like charges are far away.

11. 11
PolarizationPolarization
•The molecule will be attracted to the charge
because the attractive force is stronger than
the repulsive one.

12. 12
PolarizationPolarization
•If you charge a balloon (by
rubbing it on your hair) it
will stick to the wall.
•The balloon (–) polarizes
the wall by repelling
electrons.

13. 13
Electric ChargeElectric Charge
•There are three common ways for a neutral
object to become charged.
1.Friction
2.Conduction
3.Induction

14. 14
Charging by FrictionCharging by Friction
•Charging by Friction occurs when two
different materials rub against each other,
causing a transfer of electrons.
•The material with the greater affinity for
electrons becomes negative, and the other
material becomes positive.
•E.g. Clothes in a dryer.
•E.g. Rub a balloon on your hair.

15. 15
Charging by FrictionCharging by Friction
Rubber
Plastic
Ebonite
Cotton
Fur
Wool
Glass
Acetate
Negative
Positive
The electrostatic
series shows which
material will be +
and which will be –
when different
materials are
rubbed together.

16. 16
Charging by ConductionCharging by Conduction
•Charging by Conduction occurs when a
neutral object is placed in contact with an
already-charged object.
•If the object is –, electric repulsion will push
some of the excess electrons from the charged
to the neutral object.

17. 17
Charging by ConductionCharging by Conduction
•If the objects are spheres of the same size, and
are made out of conductive material, they will
have the equal amounts of charge after
contact.
•The charge is located on the surface (not the
interior) because the electrons repel each
other.

18. 18
Charging by ConductionCharging by Conduction
•In solids, protons are usually not mobile; only
electrons are able to move.
•If the object is +, electric attraction will pull
some electrons from the neutral object to the
charged one.

19. 19
Charging by InductionCharging by Induction
•Charging by Induction uses a similar
mechanism to polarization.
•A charged object (“A”) is brought close to
(but not touching) the object (“B”) to be
charged. This object is in connected to a
more distant object (“C”) such as Earth.

20. 20
Charging by InductionCharging by Induction
•“A” produces electric force, causing electrons
to flow between “B” and “C”.
•When “B” and “C” are separated, “B” has a
charge of the opposite sign compared to “A”.
•“C” has a charge of the same sign as “A”.