Physics 272: Electricity and Magnetism. Mark Palenik Wednesday June 13 th

Transcription

1 Physics 272: Electricity and Magnetism Mark Palenik Wednesday June 13 th

2 Todays topics Polarization and electric field Polarization of atoms Transfer of charge

3 Charged Particles Net charge: sum of all individual charges Charge cannot be created or destroyed (net charge is always conserved) An object with a charge of zero is called neutral Usually, we ll be talking about atoms/molecules A charged atom (ion) with a negative net charge has more electrons than protons A charged atom (ion) with a positive net charge has fewer electrons than protons

4 Neutral objects Can a neutral object ever produce an electric field? If yes, draw an example of such an object along with its electric field If no, explain why not Is there an object that we ve seen already with no net charge but an electric field? As we will see, atoms can have dipole moments

5 Electric fields on charges Electric fields point in the direction positive charges are pushed, and opposite for negative charges Materials are made of bound atoms, negative and positive charges attract each other through their fields to hold atoms together We could also apply an external electric field to an atom

6 Effect of electric fields on atoms What happens when we apply an electric field to a bound atom? What happens if we make it stronger/weaker? External E What happens?

7 Polarization Applying an electric field polarizes the atom Atom remains netural, charges are displaced A neutral atom without a dipole moment has no field (sphere inside of sphere) No E E The polarized atom now has a dipole moment The dipole moment is given by P = αe, where a is material dependent constant Since atoms are small, we can use the point dipole formulas (recall last lecture) Polarization goes away if the field is removed

8 Neutral atom and point charge What happens when a neutral atom gets near a point charge? E1 polarizes atom Polarized atom produces field E2 E1 attracts the dipole E2 attracts the charge

9 A little quantum (not a pun, quantum has nothing to do with small) According to QM, both electrons and protons do not have well localized positions or momenta Protons are more localized because of mass Electrons occupy large cloud E field doesn t move protons much because they re heavy and attached to each other. Electron cloud is distorted by E field. No external E E field applied Probability of finding electrons near + charge is higher

10 iclicker question Many neutral molecules have a natural, permanent dipole moment. What happens when a neutral dipole gets close to an neutral atom without a dipole moment? a) Nothing happens, since both particles are neutral b) The dipole polarizes the neutral atom, but there is no force between them, since both particles are neutral c) The dipole polarizes the neutral atom, and there is a repulsive force between them d) The dipole polarizes the neutral atom and there is an attractive force between them

11 The rules of attraction: Is that a movie or something? Repulsion can only happen between like sign charges Attraction can happen between: unlike charged objects neutral matter and charged objects Remember F net = qe -- the NET force on an unpolarized object is only non-zero if the electric field acting on it is not uniform (i.e. constant everywhere).

12 Transfer of charge Charges can be transferred by rubbing one object on another the only charged objects that can be transferred by rubbing are positive or negative ions, or electrons Amount of energy required to remove bare nuclei or protons from inside the surface atoms is enormous

13 iclicker question: Transfer of charge Two neutral pieces of tape are put together When they are pulled apart, the top piece is negatively charged. Which charges have moved? a) Electrons have moved from the bottom tape to the top b) Protons have moved from the top to the bottom c) Electrons and protons have moved d) No charges have moved

14 iclicker question: Charge of tape In the same scenario described on the previous slide, what is the net charge of the TWO pieces of tape after they have been pulled apart? a) Negative, because electrons have been added b) Positive, because electrons have been lost c) Neutral, because charge is conserved