General Physics 2 slide

4. OBJECTIVES

5. What Is Static Electricity?
•A stationary
electrical charge that
is built up on the
surface of a material

6. Electric Charges: the basis of electricity is charge.
• The charge on an atom
is determined by the
subatomic particles
that make it up.
Proton- has a positive charge and is located in the nucleus.
Neutron- has no charge (is neutral) and is also located in the
nucleus as it fills in the spaces between the protons.
Electron- has a negative charge and is located outside of the
nucleus in an electron cloud around the atom.

7. Particle charges:
• Electrons and protons have the same magnitude of
charge (elementary charge, e).
• Electron (-e): -1.60 x 10-19 C
• Proton (+e): +1.60 x 10-19 C
• This is why electrons are forced to orbit around the
nucleus.
• Electrostatic Forces hold atoms together.
• The Law of Charges- Like charges repel, and unlike
charges attract.
Because atoms have the same
amount of protons and electrons
they are electrically neutral.
(Nitrogen has an atomic number
of 7- 7 protons orbited by 7
electrons).

8. September 5, 2007
The Atom
 We now know that all atoms are made of positive charges in the nucleus, surrounded by a cloud of tiny
electrons.
Proton
Electron
Proton charge +e, electron charge -e
where e = 1.60210-19 C
Neutron
 Atoms are normally neutral,
meaning that they have
exactly the same number of
protons as they do electrons.
 The charges balance, and the
atom has no net charge.
2. Which type of charge
is easiest to remove
from an atom?
A. Proton
B. Electron

9. September 5, 2007
The Atom
Proton charge +e, electron charge -e
where e = 1.60210-19 C
3. If we remove an
electron, what is the
net charge on the
atom?
A. Positive
B. Negative
 In fact, protons are VASTLY more difficult to remove, and for all practical
purposes it NEVER happens except in radioactive materials. In this course,
we will ignore this case. Only electrons can be removed.
If we cannot remove a proton, how do
we ever make something charged
negatively? By adding an “extra”
electron.

14. Rubbing materials does NOT create electric
charges. It just transfers electrons from one
material to the other.
Where do charges come from?

15. How do atoms become “charged?”
• Atoms become charged when they become more
positive or more negative.
• How can this happen?
Remove or add a proton or an electron.
Protons and neutrons are bound together
by the Strong Nuclear Force and it is very
hard to separate them.
Electrons, however, can be more easily removed.

16. When a balloon rubs a piece of wool...
Where do charges come from?
– electrons are pulled from the
wool to the balloon.
The balloon has more electrons than
usual.
+
+
+
+
+
–
–
–
–
–
The balloon: – charged,
The wool: +charged
wool

17. Charge is a fundamental quality like mass.
• Charge is denoted as q.
• Charge has a fundamental unit of a Coulomb (C).
• Charges are usually really really small numbers (10-).
• So what is 1 C?
• An object would have to have 6.25 x 1018 extra electrons to
amount to –1 C of charge.
• A lightning bolt is estimated to carry a charge of 10 C.
• Revisit the charges on an electron and proton.

18. Charges can ONLY be in multiples of e
• Remember:
• -e = an electron = -1.60 x 10-19 C
• +e = a proton = +1.60 x 10-19 C
• An object that has a net charge of 8.0 x 10-19 C has a
net charge of what multiple of e? Hint: How many
electrons would need to be removed to create this
charge?
The net charge would be +5e, 5 electrons were removed

19. Multiples of Charges Chart
1e 1.6 x 10-19
2e 3.2 x 10-19
3e 4.8 x 10-19
4e 6.4 x 10-19
5e 8.0 x 10-19

20. https://phet.colorado.edu/sims/html/balloons-and-static-electricity/latest/balloons-
and-static-electricity_en.html

24. Forces Between Charges

26. Charging by Friction
• Objects made of different types of matter are
rubbed together; this produces a static net charge
on each object because electrons are transferred
during the process.
• Recall: Electrostatic Series
• Matter that has a stronger hold on electrons will gain
electrons and become negative
• Matter that has a weaker hold on electrons will lose
electrons and become positive

27. Recall

28. /Charging by Contact

29. • Charging by contact happens when a charged object touches
a neutral object.
• Electrons will move in attempts to balance the charges
(spread out) – thus will move towards the object that has
the least number of electons
• This happens quickly - can be surprising or painful
• It can even “jump” the gap prior to full contact
• The result is an electric shock!

30. • The charged object acquires the SAME charge as the originally
charged object that touched it; the total charge is shared and
therefore reduced.

33. Charging by Induction
• A charged object can transfer a charge to a
neutral object without touching it.
• Only conductors can be charged in the induction
process
• The presence of the charged object forces the
electrons of neutral object to move.

34. • If the charging object is negative, the electrons repel.
• If the charging object is positive, the electrons attract.
• The result: separation of an induced charge.
• If there is a ground, electrons can move into or out of the object being
charged - the balance is changed and the object is now charged
• *The induced object obtains an OPPOSITE charge to original charged
object.*

36. Summer July 04 PHYS632 E&M 36
Show induction using two conducting spheres and EE

37. September 5, 2007
Conductor
Insulators and Conductors
 Both insulators and conductors can be charged.
 The difference is that
 On an insulator charges are not able to move from
place to place. If you charge an insulator, you are
typically depositing (or removing) charges only from
the surface, and they will stay where you put them.
 On a conductor, charges can freely move. If you try
to place charge on a conductor, it will quickly spread
over the entire conductor.
Insulator

38. September 5, 2007
Insulators and Conductors
4. Which of the following is a good conductor of electricity?
A. A plastic rod.
B. A glass rod.
C. A rock.
D. A wooden stick.
E. A metal rod.

42. Grounding
An object is grounded when it is connected to
the earth through a connecting wire.
What is grounding?
If a charged conductor is grounded, it
will become neutral.

43. Grounding
b How does grounding occur?
+
+
+
+
+
When we touch a metal ball of
positive charge...
electrons flow from the
earth to the metal ball to
neutralize the metal ball.
Metal ball becomes neutral.

44. Similarly, if the metal ball is of
negative charge...
Grounding
How does grounding occur?
–
–
–
–
–
extra electrons flow
from the metal ball to
the earth and the ball
becomes neutral.

45. September 5, 2007
Summary
 Charge is an intrinsic property of matter.
 Charge comes in two opposite senses, positive and negative.
 Mobil charges we will usually deal with are electrons, which can be removed
from an atom to make positive charge, or added to an atom to make
negative charge. A positively charged atom or molecule can also be mobil.
 There is a smallest unit of charge, e, which is e = 1.60210-19 C. Charge
can only come in units of e, so charge is quantized. The unit of charge is
the Coulomb.
 Charge is conserved. Charge can be destroyed only in pairs (+e and –e can
annihilate each other). Otherwise, it can only be moved from place to
place.
 Like charges repel, opposite charges attract.
 The electric force is give by Coulomb’s Law:
 Materials can be either conductors or insulators.
 Conductors and insulators can both be charged by adding charge, but
charge can also be induced.
 Spherical conductors act as if all of the charge on their surface were
concentrated at their centers.
2
2
1
0
4
1
r
q
q
F
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