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1. 1
PHY-102
Electricity and Magnetism
Lecture 1
Introduction

2. 2
-Forces of Nature:
There are four FUNDAMENTAL forces of nature:
1. Gravity, (mass)
2. Strong (Nuclear),
3. Weak (quark),
4. Electromagnetism (charges)

3. 3
- Where does charge come from?
Atom:
Nucleus (small, massive,
positive charge)
Electron cloud (large, very
low density, negative
charge)
An overly simplistic model
for several reasons.

4. 4
- Where comes electricity? Charge
Electron
Proton
Neutron
Proton charge +e,
electron charge -e
where e = 1.60210-19
C
m(e)=

5. 5
Origin of Charge:
-Charge is an intrinsic property of matter

6. 6
-Static Electricity:
Objects can be charged by rubbing,
When a glass rod is rubbed with silk, electrons are
transferred from the glass to the silk.

7. 7
Electric Charges: Positive and
Negative in Material
(Macroscopy)
Charge comes in two types,
like charges repel and
opposite charges attract

8. 8
-Conservation of Electric
Charge
Electric charge is conserved – the arithmetic
sum of the total charge cannot change in
any interaction.

9. 9
Forces Between Charges
 We observe that:
Like charges repel each other
Opposite charges attract each other

10. 10
Electric Force and Coulomb’s Law
We can measure the force of attraction or repulsion between charges,
call them q1 and q2 (we will use the symbol q or Q for charge).
When we do that, we find that the force is proportional to the each of the
charges, is inversely proportional to the distance between them, and is
directed along the line between them (along r).
In symbols, the magnitude of the force is where k is some
constant of proportionality.
This force law was first studied by Coulomb in 1785, and is called
Coulomb’s Law. The constant k = 8.98755109
N m2
/C2
is the Coulomb
constant.
2
2
1
r
q
q
k
F 

11. 11
Electric Force and Coulomb’s Law
 Note also that the mass is an intrinsic property of matter.
Likewise, charge is also an intrinsic property. We only know it
exists, and can learn its properties, because of the force it
exerts.
 Because it makes other equations easier to write, Coulomb’s
constant is actually written

where e0 = 8.8510-12
C2
/N-m2
is called the permittivity constant.
2
2
1
0
4
1
r
q
q
F


Full form of Coulomb’s Law

12. 12
Let’s Calculate the Exact Location
 Force is attractive toward both negative charges, hence
could balance.
 Need a coordinate system, so choose total distance as L,
and position of + charge from -q charge as x.
 Force is sum of the two force vectors, and has to be zero,
so
 A lot of things cancel, including Q, so our answer does not
depend on knowing the + charge value. We end up with
 Solving for x, , so slightly less than half-
way between.
2
2
1
)
(
2
x
x
L


2
2
)
(
2
2





x
x
L
x
x
L
L
L
x 412
.
0
2
1




13. 13
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:
2
2
1
0
4
1
r
q
q
F

