Download Physics 272: Electricity and Magnetism

Physics 272: Electricity and
Magnetism
Mark Palenik
Wednesday June 13th
Todays topics
• Polarization and electric field
• Polarization of atoms
• Transfer of charge
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
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
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
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?
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
Neutral atom and point charge
• What happens when a neutral atom gets near a point
charge?
E1 polarizes atom
E1 attracts the
dipole
Polarized atom
produces field E2
E2 attracts the
charge
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
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
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 Fnet = 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).
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
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
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