1. Physics 121: Electricity & Magnetism – Lecture 2 Electric Charge
Dale E. Gary
Wenda Cao
NJIT Physics Department

2. Electricity in Nature
Most dramatic natural electrical phenomenon is lightning.
Static electricity (balloons, comb & paper, shock from a door knob)
Uses—photocopying, ink-jet printing
September 5, 2007

3. Static Charge
1. How can I demonstrate static charge using an inflated balloon?
Pop it. The sound it makes is due to static charge.
Rub it on cloth, rug, or hair, then it will stick to a wall.
Rub it on a metal surface, then use it to pick up bits of paper.
Drop it and time its fall. If it falls slower than a rock, it is affected by static charge.
Let the air out slowly. It will be larger than its original size due to static charge.
September 5, 2007

4. Demonstrations of Electrostatics
Balloon
Glass rod/silk
Plastic rod/fur
Electroscope
Van de Graaf Generator
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5. Glass Rod/Plastic Rod
A glass rod rubbed with silk gets a positive charge.
A plastic rod rubbed with fur gets a negative charge.
Suspend a charged glass rod from a thread, and another charged glass rod repels it.
A charged plastic rod, however, attracts it.
This mysterious force is called the electric force.
Many similar experiments of all kinds led Benjamin Franklin (around 1750) to the conclusion that there are two types of charge, which he called positive and negative.
He also discovered that charge was not created by rubbing, but rather the charge is transferred from the rubbing material to the rubbed object, or vice versa.
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6. Forces Between Charges
We observe that
Like charges repel each other
Opposite charges attract each other
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7. Electrostatic Induction
Electroscope
Positively charged rod
This is a device that can visually show whether it is charged with static electricity.
Here is an example charged positive.
Notice that the charges collect near the ends, and since like charges repel, they exert a force sideways.
You can make the deflection arm move by adding either positive or negative charge.
BUT, we seem to be able to make it move without touching it.
What is happening?
Electrostatic Induction
September 5, 2007

8. More accurate picture of the atom—the Helium atom
We now know that all atoms are made of positive charges in the nucleus, surrounded by a cloud of tiny electrons.
More accurate picture of the atom—the Helium atom
Electron
Proton
Neutron
Proton charge +e, electron charge -e
where e = 1.60210-19 C
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9. The Atom 2. Which type of charge is easiest to remove from an atom?
We now know that all atoms are made of positive charges in the nucleus, surrounded by a cloud of tiny electrons.
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.
Electron
Proton
Neutron
2. Which type of charge is easiest to remove from an atom?
Proton
Electron
Proton charge +e, electron charge -e
where e = 1.60210-19 C
September 5, 2007

10. The Atom
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.
3. If we remove an electron, what is the net charge on the atom?
Positive
Negative
If we cannot remove a proton, how do we ever make something charged negatively? By adding an “extra” electron.
Proton charge +e, electron charge -e
where e = 1.60210-19 C
September 5, 2007

11. Glass Rod/Plastic Rod Again
We can now interpret what is happening with the glass/plastic rod experiments.
Glass happens to lose electrons easily, and silk grabs them away from the glass atoms, so after rubbing the glass becomes positively charged and the silk becomes negatively charged.
Plastic has the opposite tendency. It easily grabs electrons from the fur, so that it becomes positively charged while the fur becomes negatively charged.
The ability to gain or lose electrons through rubbing is called Triboelectricity.
Tribo means rubbing
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12. Triboelectric Series Most Positive (items on this end lose electrons)
asbestos
rabbit fur
glass
hair
nylon
wool
silk
paper
cotton
hard rubber
synthetic rubber
polyester
styrofoam
orlon
saran
polyurethane
polyethylene
polypropylene
polyvinyl chloride (PVC pipe)
teflon
silicone rubber
Most Negative
(items on this end steal electrons)
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13. 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
Conductor
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14. Insulators and Conductors
4. Which of the following is a good conductor of electricity?
A plastic rod.
A glass rod.
A rock.
A wooden stick.
A metal rod.
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15. Metals and Conduction
Notice that metals are not only good electrical conductors, but they are also good heat conductors, tend to be shiny (if polished), and are maleable (can be bent or shaped).
These are all properties that come from the ability of electrons to move easily.
This iron atom (26 protons, 26 electrons) has two electrons in its outer shell, which can move from one iron atom to the next in a metal.
Path of electron
in a metal
September 5, 2007

16. Van de Graaf Generator Rubber band steals electrons from glass
Glass becomes positively charged
Rubber band carries electrons downward
Positively charged glass continues to rotate
Wire “brush” steals electrons from rubber band
Positively charged glass steals electrons from upper brush
Sphere (or soda can) becomes positively charged—to 20,000 volts!
September 5, 2007

17. 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 = 109 N m2/C2 is the Coulomb constant. q1 q2 r q1 q2
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18. Electric Force and Coulomb’s Law
Although we can write down a vector form for the force, it is easier to simply use the equation for the magnitude, and just use the “like charges repel, opposites attract” rule to figure out the direction of the force.
Note that the form for Coulomb’s Law is exactly the same as for gravitational force between two masses
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.
Note BIG difference,
There is only one “sign”
of mass, only attraction.
Full form of Coulomb’s Law
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19. Spherical Conductors Note, forces are equal and opposite
Because it is conducting, charge on a metal sphere will go everywhere over the surface.
You can easily see why, because each of the charges pushes on the others so that they all move apart as far as they can go. Because of the symmetry of the situation, they spread themselves out uniformly.
There is a theorem that applies to this case, called the shell theorem, that states that the sphere will act as if all of the charge were concentrated at the center.
Note, forces are equal and opposite
These two situations are the same
September 5, 2007

20. Insulators and Conductors
5. Two small spheres are charged with equal and opposite charges, and are placed 30 cm apart. Then the charge on sphere 1 is doubled. Which diagram could be considered to show the correct forces? 2q -q A. B. C. D. E.
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21. Case of Multiple Charges
You can determine the force on a particular charge by adding up all of the forces from each charge.
Forces on one charge due to a number of other charges
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22. Charges in a Line
6. Where do I have to place the + charge in order for the force to balance, in the figure at right?
Cannot tell, because + charge value is not given.
Exactly in the middle between the two negative charges.
On the line between the two negative charges, but closer to the -2q charge.
On the line between the two negative charges, but closer to the -q charge.
There is no location that will give force balance. -q
-2q
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23. 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. -q
-2q L x
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24. 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.
September 5, 2007