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UMass Amherst PHYSICS 132 - Unit 2, Lecture 2

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Atreyi SahaPhysics 132Prof. HatchFebruary 19, 2016Unit 2, Lecture 2: Electrostatics- Both insulators and conductors have electrons. o An insulator is a material that will not conduct anything because its electrons are held tightly together. Electrons feel a force like conductors but are NOT able to moveo Conductors (metals) have valence electrons that are free to move. Such electrons move to distribute themselves equally around.  Like charges repel and unlike charges attract They move in response to other charges i.e.1 like with the plastic rod w/positivecharge, electrons moved towards it because ofattractive forcesi.e. 2 like with the plastic rod w/negativecharge, electrons moved towards away becauseof repulsive forces- Charging by Inductiono Two metal spheres of equal size that are grounded (meaning that the charges are evenly dispersed/distributed)o Charge up the rod (positive), bringit close to one of the metal spheres,a bunch of electrons will gotowards the positive side rodo Charge up the rod (positive), bringit close to one of the metal sphereswhile the two spheres are beingseparated What have we done?You have charged those twoballs. They are equal and exactly opposite to each other.- Why do neutral things get attracted to charged objects?o Positively charged rod and a piece of aluminum foil (metal, neutral)o Neutral object still has plusses and minuses - electrons are free to move since it is a metal Charges were separated on the tin foil such that negative charges of the foil were attracted to the positive charges of the rodo Forces acting on the foil: Pair 1: Gravity (points downward); object accelerated up - thus electrical force Pair 2: You feel upward force (attractiveforce due to electrons), you feeldownward force (repulsive force due topositive charges on foil repellingpositively charged rod)Fnegative > Fpositive : even though the foil isthin, in the perspective of the atoms -the electrons are very far away from thepositive charges The attractive force is also GREATERthan gravity because the foil had a netacceleration in the upwards directiono What would happen if the rod has the opposite charge? Bunch of negatives The foil would have still accelerated upwards, just the plusses and minuseswould switch sides on the foil (the plusses would be on top)- Polarization: a conductor may remain neutral but charge may be separatedo Easy to polarize charges because of the availability of electrons that can move freelyo Polarize or separate charges big "atomic" distanceso Why didn't tin foil touch and then get repelled? tin foil not completely wrapped around where the two surfaces touch - that is where the charge will be neutralized where the surfaces don't touch, those electrons are still separated/polarizedthus feeling attractive forces and they overcame the bit of neutralization thus not bounce off like the ball- How do we explain the attraction of a neutral insulator?o Example - a balloon to a wallo Balloon is not a metal - when you rub a balloon and then put it against a wall, it stickso For Insulators: In this case, you are not moving the charges but rather polarizing the individual atoms themselveso Proton in middle and electrons on outside. When you bring up a negatively charged rod to an atom, the protons get attracted to the rod and the electrons get repelled and go to the other side. You have polarized an atom.o The strength will be less because it is a single electron versus a whole bunch of them like with metals. Weak attractive force.- Summary of Neutral Objectso Neutral metal will feel much stronger attractive force because of how you can get a stream of electrons to move and be polarizedo An insulator will feel a weakattractive force because you arejust adjusting the electronorientation of single atomso Metals will always have a strongerattractive force because ofpolarizationWater is polarizedbecause oxygen ishighlyelectronegativemeaning that ithogs the electronsbecause it has 8Xmoreprotons/electronsthan hydrogen.The polarization ofatoms like oxygenleads to hydrogenbonding which occursbetween hydrogenions and negativeions. H ends up becomingH+ and attractsnegatively chargedatoms- WimshurstMachineo There are two wax/plastic plates that aresandwiched by metalo There is no such thing as a perfectly neutral object- there is always going be an imbalance matched - imbalance will cause polarizationo sharp points draw charge - if voltage is strongenough, electrons will jump across thereby creatinga static sparko capacitors store charge o How it works: Rotation will cause polarization ---> Pointy things draw charges off ---> brushes neutralize ---> as charges get drawn off, they get stored in capacitors the more the capacitors are charged up, the more it induces polarization once enough charge builds up, you will see a


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UMass Amherst PHYSICS 132 - Unit 2, Lecture 2

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