PES 1120 Spring 2014, Spendier
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Lecture today: Chapter 21
1) Electric charge
2) Conductors and Insulators
3) Induced charge
Course Introduction:
Electricity and magnetism will be studied in this course. They are studied together
because they are closely related to one another. In higher level physics courses, they are
actually related as one phenomenon known as "electromagnetism"
Electricity and magnetism is all around us. We have electric lights, electric clocks. We
have microphones, calculators, televisions, VCRs, radio, computers. Light itself is an
electromagnetic phenomenon as radio waves are. The colors of the rainbow in the blue
sky are there because of electricity. Cars, planes, trains can only run because of
electricity. Horses need electricity because muscle contractions require electricity. Your
nerve system is driven by electricity. Atoms, molecule, all chemical reactions exist
because of electricity. You could not see without electricity. Your heart would not beat
without electricity. And you could not even think without electricity.
I will show you that physics is beautiful. And you may even start to like it.
Electric charge:
We have learned in Physics 1 that mass is both a fundamental property of matter, and a
consequence of how fundamental particles are put together to form a macroscopic object
(when particles bind, the binding energy is reflected in the mass of the composite object).
There is another fundamental property of matter which we will now investigate, called
electric charge.
There is a lot that can be understood about nature by just understanding the basic
behavior of the interaction between electric charges and charges themselves. [Note: I will
use the term “charge” to denote “electric charge”, although there are other types of
charge in nature.]
DEMO: (discovered 600 B.C): Rub a rubber rod on a fur cloth and you can pick up
small pieces of paper.
The rod has developed a "static electric charge" or a static electricity (static electricity is
different from electricity that travels through wires.)
PES 1120 Spring 2014, Spendier
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More Demos:
To understand what is going on to create static electricity, we need to learn more about
the properties of charges:
Charge: Property of matter that allows it to exert a force on other charged objects
We know there are two different types of charge due to the different direction of the force
Charge comes in two distinct forms:
- positive charge (+, i.e proton)
- negative charge (-, i.e. electron)
These are, of course, just names, but the connotation that they carry will prove useful.
Opposite charges attract:
Like Charges Repel:
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Atoms
Charge comes from the subatomic particles inside of the atom.
The electrons in an atom are attracted to the positively charged nucleus. The protons in
the nucleus repel each other, but are held in place by an even stronger force (the so-called
“strong force”). Nucleus also contains Neutrons = neutral particles
The unit of charge is called the Coulomb (C). The "elementary charge" , "e" is the
magnitude of charge on the electron (and proton).
Every proton has charge: +e = +1.602 x 10-19 C
"e" stands for electron
Every electron has charge: -e = -1.602 x 10-19 C
A neutral atom (that is how atoms typically are) has an equal number of protons
and electrons. Example of a neutral atom
In mass, electrons and protons are completely different!
Every proton has mass: mp = 1.67 x 10-27 kg
Every electron has mass: me = 9.11 x 10-31 kg
(protons are much bigger than e)
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Going back to Physics I, Newton's 2nd law is:
Ftotal = ma
Forces cause acceleration. The bigger the mass the bigger the force needed to cause a
given acceleration. So electrons will be easier to move. So that is why in chemistry, we
always worry about electrons, since electrons get transferred from on object to another.
So this brings us back to our demo: How did I make the rod actually charged when
I rubbed it with fur? I charged the rubber rod by using frictional force
A rubber rod is rubbed with animal fur. During the rubbing process, the atoms of the
rubber are forced into close proximity with the atoms of the animal fur. The electron
clouds of the two types of atoms are pressed together and are brought closer to the nuclei
of the other atoms. The protons in the atoms of one material begin to interact with the
electrons present on the other material. The atoms of one material - in this case, the atoms
of rubber - are more serious about their claim for electrons. As such, the atoms of rubber
begin to take electrons from the atoms of animal fur. When the rubbing has ceased, the
two objects have become charged. The rubber has an excess of electrons and the animal
fur has a shortage of electrons. Having an excess of electrons, the rubber is charged
negatively. This process is called charging by frictional force.
Conductors and Insulators:
When many atoms are placed close to each other in a solid, a completely new electron
structure emerges! Depending on how easily electrons can move (charge can move) we
classify materials as:
Conductors - materials that have “free electrons" = electrons which are free to move
from nucleus to nucleus (metals such as copper)
Insulators - materials without free electrons. Insulators are materials that prevent
electrons from moving freely (rubber, plastic, glass)
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In between conductors and insulators are "semiconductors". We will not talk about these
in this course but they are very important for computer chips.
Induction:
The slight separation of charge in conductors and “weak" insulators when a charged
object is brought near. This is why neutral objects can be attracted to charged objects due
to Induction.
Negative charges closer to rod ==> attraction
Example 2:
DEMO: ballon (http://phet.colorado.edu/en/simulation/balloons)
Rub balloon on sweater - balloon becomes negatively charged
1) What do you think happened to make the balloon negatively charged?
2) After rubbing them together, how does the charge on the balloon compare to the
charge on the sweater?
3) Why does the balloon want to “stick” to the sweater in some cases and stick to the wall
in others? Explain the differences.
==>
Bees navigating to charged flowers:
http://phenomena.nationalgeographic.com/2013/02/21/bees-can-sense-the-electric-fieldsof-flowers/
As bees fly through the air, they bump into charged particles from dust to small
molecules. The friction of these microscopic collisions strips electrons from the bee’s
surface, and they typically end up with a positive charge.
Flowers, on the other hand, tend to have a negative charge, at least on clear days. The
flowers themselves are electrically earthed, but the air around them carries a voltage of
around 100 volts for every metre above the ground. The positive charge that accumulates
around the flower induces a negative charge in its petals.
When the positively charged bee arrives at the negatively charged flower, sparks don’t fly
but pollen does. “We found some videos showing that pollen literally jumps from the
flower to the bee, as the bee approaches… even before it has landed,” says Robert. The
bee may fly over to the flower but at close quarters, the flower also flies over to the bee.