Contributed by:
Electric charge, Quantization, Coulomb's law, Superposition principle
1.
Electromagnetism
Electromagnetism is one of the fundamental forces
in nature, and the the dominant force in a vast range
of natural and technological phenomena
The electromagnetic force is solely responsible for the
structure of matter, organic, or inorganic
Physics, chemistry, biology, materials science
The operation of most technological devices is based on
electromagnetic forces. From lights, motors, and batteries,
to communication and broadcasting systems, as well as
microelectronic devices.
Engineering
2.
Electromagnetism
Electricity
Electromagnetism Magnetism
Optics
In this course we are going to discuss the
fundamental concepts of electromagnetism:
charge force field potential current
electric magnetic induction alternating waves
circuit field currents
reflection refraction image interference diffraction
Once you master these basic concepts, you will be ready to move forward,
into more advanced subjects in your specific field of interest
3.
System of Units
We will use the SI system – SI International System of Units
Fundamental Quantities
Length meter [m]
Mass kilogram [kg]
Time second [s]
Other Units
Current ampere [A]
Derived Quantities
Force newton 1 N = 1 kg m / s2
Energy joule 1J=1Nm
Charge coulomb 1C=1As
Electric Potential volt 1V=1J/C
Resistance ohm 1=1V/A
5.
Electric Charge
The Transfer of Charge
SILK
Glass Rod
Some materials attract electrons
more than others.
6.
Electric Charge
The Transfer of Charge
SILK
+ -
Glass Rod
As the glass rod is rubbed against silk,
electrons are pulled off the glass onto the silk.
7.
Electric Charge
The Transfer of Charge
SILK
+ -
+ -
Glass Rod
Usually matter is charge neutral, because the number of
electrons and protons are equal. But here the silk has an
excess of electrons and the rod a deficit.
8.
Electric Charge
The Transfer of Charge
+ SILK
+ - - -
+
+ - -
+
Glass Rod
Glass and silk are insulators:
charges stuck on them stay put.
9.
Electric Charge
+ +
Two positively charged rods
repel each other.
10.
Electric Charge
History
600 BC Greeks first discover attractive
properties of amber when rubbed.
1600 AD Electric bodies repel as well as attract
1735 AD du Fay: Two distinct types of electricity
1750 AD Franklin: Positive and Negative Charge
1770 AD Coulomb: “Inverse Square Law”
1890 AD J.J. Thompson: Quantization of
electric charge - “Electron”
11.
Electric Charge
Summary of things we know:
– There is a property of matter called electric charge.
(In the SI system its units are Coulombs.)
– Charges can be negative (like electrons) or
positive (like protons).
– In matter, the positive charges are stuck in place in
the nuclei. Matter is negatively charged when
extra electrons are added, and positively charged
when electrons are removed.
– Like charges repel, unlike charges attract.
– Charges travel in conductors, not in insulators
– Force of attraction or repulsion ~ 1 / r2
12.
Charge is Quantized
q = multiple of an elementary charge e:
e = 1.6 x 10-19 Coulombs
Charge Mass Diameter
electron -e 1 0
proton +e 1836 ~10-15m
neutron 0 1839 ~10-15m
positron +e 1 0
(Protons and neutrons are made up of quarks, whose charge is
quantized in multiples of e/3. Quarks can’t be isolated.)
13.
Coulomb’s Law
q1 q2 F12
r12
r12
kq1 q2 ˆ
F 12 2 r12 Force on 2 due to 1
r12
k = (40)-1 = 9.0 x 109 Nm2/C2
= permitivity of free space
= 8.86 x 10-12 C2/Nm2
Coulomb’s law describes the interaction between bodies due to their charges
14.
Gravitational and Electric Forces
in the Hydrogen Atom
M m = 9.1 10-31 kg
r12 -e
+e M = 1.7 10-27 kg
m r12 = 5.3 10-11 m
Gravitational force Electric Force
15.
Gravitational and Electric Forces
in the Hydrogen Atom
M m = 9.1 10-31 kg
r12 -e M = 1.7 10-27 kg
+e
m r12 = 5.3 10-11 m
Gravitational force Electric Force
Mm
Fg G 2 r
r12
Fg = 3.6 10-47 N
16.
Gravitational and Electric Forces
in the Hydrogen Atom
M m = 9.1 10-31 kg
r12 -e
+e M = 1.7 10-27 kg
m r12 = 5.3 10-11 m
Gravitational force Electric Force
Mm 1 Qq
Fg G 2 r Fe 2 r
r12 40 r12
Fg = 3.6 10-47 N Fe = 3.6 10-8N
17.
Superposition of forces from two charges
Blue charges fixed , negative, equal charge (-q)
What is force on positive red charge +q ?
y
x
18.
Superposition of forces from two charges
Blue charges fixed , negative, equal charge (-q)
What is force on positive red charge +q ?
Consider effect of each charge separately:
y
x
19.
Superposition of forces from two charges
Blue charges fixed , negative, equal charge (-q)
What is force on positive red charge +q ?
Take each charge in turn:
y
x
20.
Superposition of forces from two charges
Blue charges fixed , negative, equal charge (-q)
What is force on positive red charge +q ?
Create vector sum:
y
x
21.
Superposition of forces from two charges
Blue charges fixed , negative, equal charge (-q)
What is force on positive red charge +q ?
Find resultant:
y
NET
FORCE
x
22.
Superposition Principle
F31 F F31
F31y
q1
F21
F31x
F21
q3 F21y
Forces add vectorially
F21x
F = (F21x + F31x) x + (F21y + F31y) y
23.
Example: electricity balancing gravity
Two identical balls, with mass m
and charge q, hang from similar
strings of length l. l
After equilibrium is reached, q q
find the charge q as a function of
and l m m
24.
Example: electricity balancing gravity
What forces are acting on
the charged balls ? l
q q
m m
25.
Example: electricity balancing gravity
• Draw vector force
diagram while
identifying the forces. T T
• Apply Newton’s 3rd
Law, for a system in FE FE
equilibrium, to the
components of the
forces.
FG=mg FG=mg
• Solve!
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