01_Electric_Forces
March 6, 2023
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Lecture 01. Electric Forces
1.1
Activity 1. Force
What is the force we need to exert to lift an object?
1.2
Activity 2. A simple experiment on electric forces
A simple experiment with a pen and a small piece of paper * Rub a pen against a wool cloth *
then try to lift up the piece of paper
1.2.1
Get a feel of
Can we evaluate the electric force acting on the small piece of paper?
1.3
Two types of electric charges
• Glass
• Hard rubber, plexiglass
Benjamin Franklin (1706-1790) > Positive electric charges glass
Negative electric charges amber
William Gilbert carried out a series of experiments on different materials, including diamond. The
results were published in London in 1600 in a book entitled De Magnete.
1.4
Electric charge is always conserved
The charged state is due to a transfer of charge from one object to another
1.5
Electric charge is quantised
Robert Millikan got the Nobel prize in 1923 “for his work on the elementary charge of
electricity and on the photoelectric effect.”
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The smallest electric charge is the charge of the electron
1.6
Microscopic structure of matter
Particle
Mass
Charge
Electron (e)
Proton (p)
Neutron (n)
9.109 x 10-31 kg
1.672 x 10-27 kg
1.674 x 10-27 kg
-1.602 x 10-19 C
+1.602 x 10-19 C
0
Atomic radius ≃ 0.53 x 10-10 m
Nuclear radius ≃ 10-15 - 10-14 m
1.6.1
Get a feel of
• Calculate the ratio between mp (mass of the proton) and me (mass of the electron)
• How to propose a scale representation for the mass of the electron and the proton? Scaled
mass:
– The mass of a coin corresponding to 20 Euro cents is 5.74 grams If we use the coin to
represent the mass of an electron, how big would be the mass of a proton?
Observation
• The mass of the atom is concentrated mainly in its nucleus
• Scaled radius:
– A man stands with open arms. The distance from one hand to the other is about 1.5
meters If we use this distance to represent the nuclear radius, how big will be the atomic
radius?
Observation
• The volume of the atom consists mostly of empty space
1.7
1.7.1
A classification of the materials
Electrical conductors
In eletrical conductors some of the electrons are free to move inside the material
Examples: metals, including copper, aluminium and silver
1.7.2
Electrical insulators
In electrical insulators all electrons are bound to atoms and cannot move freely
through the material
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Examples: glass, rubber, dry wood
1.7.3
Semiconductors
Examples: silicon, germanium
1.8
Charging by contact
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