PART1: INTRODUCTION -
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U-tube video with experiments with static electricity
http://www.youtube.com/watch?v=QxZ6AWLpnUw&feature=related
1) 2 rods (or rulers) same material, 1 lens , 1 wool cloth
First rod can freely rotate on a glass lens (in equilibrum). both rulers are rubbed by a wool cloth.
The rulers repel each other. They are similarly charged. (both + or - )
2) 2 rods , 1 lens, 1 wool cloth, 1 silk cloth
Rub the second rod with silk instead of wool. The rod attract each other. They have opposite charges.
3) 1 rod, 1 plastic marker, 1 lens, 1 wool cloth
Rub the 1st rod with woll again. It is chargd and in equilibrum on the lens.
The marker second one should be neutral. (touch your face with it, to uncharge it).
They should attract each other. Because of polarity . we will talk about it.
4) empty coke can, platic bottle, charged rod
The neutral empty can or empty bottle are attracted by the charged rod and roll on the table.
5) meter stick, charged rod, lens
1 meter stick in equilibrum on a lens can be repelled by a charged rod and rotates. Even if it is neutral. (polarization again)
can do with the finger too.
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PART2: COULOMB'S LAW - I replaced the base 10 by E (like in the TI) for convenience
1) The size or magnitude of the electrostatic force Fe between 2 points charges q1 and q2 is directly proportional to the product of the charges
and inversely proportional to the square of the distance d between them. This relationship is called coulomb's law.
The constant of proportionality is called the electrostatic constant and noted k.
Can you translate the law in a mathematics formula?
Fe = k ___________. Use k, q1, q2. d in the formula, k is the constant of proportionality.
k= 8.99 109 Nm2/C2
2) FOR THE COMING PROBLEMS, USE YOUR TI. to write 8.99 109 with TI write 8.99 E9 (E means base 10 for TI )
A)What is the magnitude of electrostatic force Fe between 2 small sphere possessing net charge of
+ 2 10-6 and -3 10-6, respectively, if the distance between them is 10 meters? (when plugging in the equation, forget about the sign
of the charges. use q1 = 2 10-6 , q2=3 10-6 , d= 10 m)
(use Fe= Kq1q2/d2 )
B) Is the force attractive or repulsive?
3) A point charge of 3 10-7 C (or 3E-7 C) is placed 2 10-2 meters (or 2 E -2 meters) from a second point charge of 4 10-7 C ( or 4 E -7C ).
A) What is the magnitude of the electrostatic force Fe between the charges?
B) is the force attractive or repulsive?
4) Two electrons in an atom, each with charge - 1.6 E -19C, are separated by 1.5 E -10m.
A) What is the force between them ? B) The force attracts or repels ?
(q1 = 1.6 E -19C, q2 = 1.6 E -19C, d=1.5 E -10m)
5) A charge of 6 E -6C exerts an attractive force of 65N on a second charge 0.005m away.
A) What is the magnitude of the second charge?
6) The hydrogen atom contains a proton and an electron separated by 5.0 E -11m.
Find the force of attraction between the two particles.
7) How far apart are 2 electrons if they exert a force of repulsion of 1.0N on each other ?
8) A force of 4.4 E3N exists between a positive charge of 8 E -4 C and a charge of 3 E -4C. What distance separates the charges in meters?
in centimeters ?
9) The electric force, between 2 charges, on an atomic scale, is much stronger than the gravitational force between these 2 charges.
Let's compute the electric forces between 2 protons in an nucleus. Then let's compute the gravitational force between these 2 particles.
(the distance between the 2 charges is given)
Here what you need:
mass proton =m1 = m2 = 2.7 10-27 kg , charge proton =q1 = q2 = + 1.6 10 -19C, distance = 10-14 m K = 9 109, G = 6.7 10-11
2 protons will repell each other with an electric force of : Fel = K q1 q2 / (distance)2
This is Coulomb's law.
2 protons will also attract each other with a gravitational force of : Fg = G m1 m2 / (distance)2
THis force of gravity was derived by Newton after he invented calculus. He stated that 2 masss attract each other.
The magnitude of the force is proportional to the product of the masses and inversely proportional to the distance squared.
This is why we don't fly off the EArth. The force of gravity is always pulling on us.
Note that the formulas (Coulombs and gravity) look very much alike.
The forces increase with charges/masses and decrease with the distance squared between the particles.
The electric force can repel or attract but the gravitational force can only _______________.
and K is ____________ than G.
Compute Fel = _________________ (use your TI and don't forget to put () )
Compute Fg = ___________________ (use your TI and don't forget to pu () )
Find Fel/Fg = _____________. So the electric force is _______ order of magnitude more than gravity.
So we neglect gravity in an atom. If the electric force is so strong in the nucleus, why the protons stay together? instead of flying off ?
Also, if you bring 2 protons close to each other and then let go, they will repel each other and they will accelerate.
you can find the acceleration using the formula F = m a
(you will learn it in 10th grade, that force (pull/push) is what causes an object to accelerate or speed up)
Find a = ____________
(a=F/m use what you got before. with F = Fel and m2 = m1 = mass proton)
compare to g the acceleration due to gravity on EArth. (9.8ms/s).
An acceleration due to an electric force is ____________ order of magnitude higher. (find the ration a/9.8).
On an atomic scale, it is the electric force that keeps our word together. The electric force keeps the electrons around the nucleus and
glue atoms together to form molecules and molecules to make every thing else.
however, on a large scale , the force of gravity (mostly dark matter) uholds the world together. Us on the platen, Moon around planets, Planets around the Sun,
our system in the galaxy ... This is because the masses are huge (Earth) and almost neutral (no charge or very little).
CHECK:
http://sol.sci.uop.edu/~jfalward/physics17/chapter8/chapter8.html
10) In an hydrogen atom, the distance between the proton and the electron is about 50pm. (1pm is 10-12 m).
THe mass of the proton is mp = 1.7 10-27 kg and the mass of the electron is 9.1 10-31 kg.
A) what are the 2 forces acting between the 2 particles ?
B) Compute the electric force Fe = _________ N
C) Compute the for of gravity Fg = ___________N
D) compare these 2 forces. What can you conclude ?
11) Interestingly the force of gravity and the coulombs law are inverse squared law. It makes sense if you
think of these forces exerted by a source (mass or charge) as light produced by a source that spreads in 3D.
As you move away from the source the " influence from the source" is more diluted over a sphere.
(same force on a given sphere because the distance is the same).
If you multily the radius by 2 , the area is multiplied by 4.
So the influence is divided by 4. (you have less light or less " influence per unit square" )
Same idea for a paint that would be spray in all direction. EXPLAIN
or the brightness of a star. EXPLAIN why we can use the brightness of a star (like a Cepheid)
to find its distance. (they are called standard candles)
see this u-tube video to understand the square law.
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