Atreyi SahaPhysics 132Prof. HatchFebruary 22, 2016Unit 2, Lecture 3: Electrostatics- Millikan quantized the nature of charge: e (charge of an electron or proton) = 1.60 x 10-19 Coloumbso Charge is quantized = comes in chunkso to find total charge on an object (Q) = number of electrons (n) * e (charge of one electron) questions will be asked like "5000 charges moved across, what is the total charge?" just plug in 5000 for n and e is a constant o find total charge (q)- Coulomb's Lawo if two point charges q1 and q2 are a distance r apart, the charges exert forces on each other of magnitudeo these are an action/reaction pair, equal in magnitude but opposite in directiono the forces are along the line joining the two charges. The forces are repulsive for like charges an attractive for opposite charges.o Limitation: two charges at a timeo breaking down the formula: the two charges feel the same force (same strength) - only the direction of the force is different k is constant = 8.99 x 109 N*m2/C2 absolute value signs around charges because we are just concerned with the magnitude r = distance in meters between the two charges units of q must be in Coulombs- Lets compare th electric and gravitational forces between and electron in a hydrogen atom. Let r= 5.29 x 10-11 m (radius of a H atom)Taking the ratio of the electric force over the gravity force (Felectric / Fgravity) will help you find out how much greater the electric force is compared to the gravitational force = 2.26 x 1039 which is how MUCH MUCH stronger the electric force is!o Why is gravity much more dominant? We don't deal with charges that are very big. They are very very close to neutral. Mass of the earth is very large Still, electric forces are pervasive. They hold so many things together! 1 Coulombhas a VERY BIG charge - enough charge to kill you instantly.- Conceptual questionso Question 1: Answer is 1) Forces oriented away from each other, magnitude is equalo Question 2: Answer is 3) Forces oriented TOWARDS each other, magnitude is equalo Question 3: One charge is 5X bigger than the other; Answer is 2) because the magnitude of force involves both q's (or charges) and they BOTH experience the SAME forceo Question 4: The answer is 1)Ignore the forces on all other particles except the right most one. From the middle particle, it feels a right-ward force. From the left particle, it feels a left-ward force. Now, which force is greater: left or right? Fleft = k (2q) (q) / (2r)2 = k (q2) / 2r2Fright = k (q) (q) / r2 = k (q2) / r2 (bigger) right force is thus bigger!!o Question 5: The answer is ??Key is to pay attention to symmetry so that you are not repeating Coulomb's Law excessively. Net shown in black (to the left)- Question 6: The answer is 4)- Electric Fieldo How does one charge know that another charge exists and experiences a force? Electric force field - another charge modifies the space which it is occupying A given charge experiences the electric field that is createdo Coulomb's law tells us how to find the force between two point charges. The forceacts over a distance so the charges do not need to be in contact with each other. This leads to the question: does the presence of the charge somehow modify the space surrounding it? Yes, and this leads to the concept of the electric
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