Ch. 5: Electrostatics and Magnetism Flashcards
defn: electrostatics
the study of stationary charges and the forces that are created by and which act upon these changes
what are the two types of charged subatomic particles?
- proton
- electron
defn: proton vs. electron
proton: positive charge
electron: negative charge
defn: attractive vs. repulsive forces
attractive forces: opposite charges
repulsive forces: like charges
is the electrostatic force attractive or repulsive?
can be either depending on the signs of the charges that are interacting
defn: ground
a means of returning charge to the earth
aka: static electricity
static charge buildup
SI unit: charge
coloumb
the proton and the electron share the same magnitude of charge, do they share the same mass?
no, the proton has a much greater mass than the electron
defn: insulator
+ explain on a molecular level
will not easily distribute a charge over its surface and will not transfer that charge to another neutral object very well
molecularly: the electrons of insulators tend to be closely linked with their respective nuclei
defn: conductor
+ explain on a molecular level
when given a charge, the charges will distribute approximately evenly upon the surface of the conductor
are able to transfer and transport charges
molecularly: conceptualized as nuclei surrounded by a sea of free electrons that are able to move rapidly throughout the material and are only loosely associated with the positive charges
what types of materials are generally conductors? what types are generally insulators?
insulators: nonmetals
conductor: metals, ionic (electrolyte) solutions
defn: Coulomb’s law
quantifies the magnitude of the electrostatic force between two charges
how do you determine the direction of the electrostatic force?
remember that unlike charges attract and like charges repel
the force always points along the line connecting the centers of the two charges
where do electric fields come from and how do they make their presence known?
- every electric charge sets up a surrounding electric field, just like every mass creates a gravitational field
- electric fields make their presence known by exerting forces on other charges that move into the space of the field
how is it determined if the force exerted through the electric field is attractive or repulsive?
it depends on whether the stationary TEST charge q and the stationary SOURCE charge Q are opposite or like charges
defn: test charge q vs. source charge Q
test charge = the charge placed in the electric field
source charge = the charge which actually creates the electric field
is the electric field a vector or scalar quantity?
vector
what are the two methods for calculating the magnitude of the electric field at a particular point in space?
- place a test charge q at some point within the electric field, measure the force exerted on that test charge, and define the electric field at that point in space as the ratio of the force magnitude to test charge magnitude
- we need to know the magnitude of the source charge and the distance between the source charge and point in space at which we want to measure the electric field
what is one disadvantage to the first method of calculating the magnitude of the electric field?
a test charge must actually be present in order for a force to be generated and measured
what is the direction of an electric field vector by convention?
the direction that a positive test charge would move in the presence of the source charge
(if the source charge is positive –> test charge experiences repulsive force, accelerate away from source charge)
(if the source charge is negative –> test charge experiences attractive force, accelerate toward the source charge)
based on the convention of electric field direction, what is the direction of electric field vectors for positive and negative charges?
positive charges have electric field vectors that radiate outward (point away) from the charge
negative charges have electric field vectors that radiate inward (point toward) the charge
defn + char: field lines
imaginary lines that represent how a positive test charge would move in the presence of the source charge
they are drawn in the direction of the actual electric field vectors and indicate the relative strength of the electric field at a given point in the space of the field
where the lines are closer together, the field is stronger, where they are farther apart the field is weaker
what is the net electric field equal to at a point in space for a collection of charges?
equal to the vector sum of all the electric fields
what is the direction of the electrostatic force when the test charge is positive? negative?
test charge within a field is positive: the force will be in the same direction as the electric field vector of the source charge
test charge within a field is negative: the force will be in the direction opposite to the field vector of the source charge
defn: electric potential energy
a form of potential energy that is dependent on the relative position of one charge with respect to another charge or to a collection of charges
what will the sign of electric potential energy be if the charges are like? unlike?
like charges: potential energy positive
unlike charges: potential energy negative
defn: electric potential energy for a charge at a point in space in an electric field (in terms of work)
the amount of work necessary to bring the charge from infinitely far away to that point
will the electric potential energy of a system increase or decrease when two like charges move towards each other or apart? when two opposite charges move towards each other or apart?
LIKE TOWARD: increase EPE
LIKE AWAY: decrease EPE
UNLIKE TOWARD: decrease EPE
UNLIKE AWAY: increase EPE
defn + unit + scalar or vector: electric potential
the ratio of the magnitude of a charge’s electric potential energy to the magnitude of the charge itself
unit: volts (V)
scalar
how is the sign of electric potential determined?
by the source charge Q
if positive source: V positive
if negative source: V negative
what is the electric potential at a point in spacefor a collection of charges?
the scalar sum of the electric potential due to each charge
defn + aka: voltage
aka: potential difference
there is a potential difference between two points that are at different distances from the source charge because electric potential is inversely proportional to the distance from the source charge
is electrostatic force conservative or nonconservative?
conservative!
if allowed, charges will move spontaneously in whatever direction results in a decrease in electric potential energy, what does this mean for a positive and negative test charge in terms of electric potential, voltage, and work?
positive test charge
- move from higher electric potential to lower
- voltage is negative
- work is negative
negative test charge
- move from lower electric potential to higher
- voltage is positive
- work is negative
overall this means: positive charges move in direction that decreases their electric potential, negative charges spontaneously move in the direction that increases their electric potential but in BOTH cases, the electric potential energy is decreasing
defn: equipotential line
a line on which the potential at every point is the same (the potential difference between any two points on an equipotential line is zero)
what do equipotential lines look like on paper? in 3-D?
on paper: concentric circles surrounding a source charge
3-D: spheres surrounding the source charge
is work done when moving a test charge from one point on an equipotential line to another point on the same line? between different lines?
same line, different point: no work
different lines: work done
what does the work done in moving a test charge from one equipotential line to another depend on?
the potential difference of the two lines NOT on the pathway taken between them
what does an electric dipole result from?
from two equal and opposite charges being separated a small distance d from each other
example: transient electric dipole, permanent electric dipole
TRANSIENT: moment-to-moment changes in electron distribution that create London dispersion forces
PERMANENT: the molecular dipole of water or the carbonyl functional group
what is a way of visualizing the electric dipole?
as a barbell
the equal weights on either end of the bar represent the equal and opposite charges separated by a small distance (the length of the bar)
defn + SI unit + direction: dipole moment (p)
the product of charge and separation distance
SI unit: coloumb meter
direction: (physics): along the line connecting the charges (the dipole axis) with the vector pointing from negative to positive
(chemistry): points from positive towards negative usually
defn: perpendicular bisector of the dipole
direction of electric field vectors?
a very important equipotential line –> the plane that lies halfway between +q and -q
electric field vectors point in the direction opposite to p
why is the electric potential at any point along the perpendicular bisector of the dipole 0?
because the angle between this plane and the dipole axis is 90 deg (and cos 90 = 0)
explain how torques can act on dipoles (3)
- when the electric dipole is placed in a uniform external electric field, each of the equal and opposite charges of the dipole will experience a force exerted on it by the field
- the forces acting on the charges will be equal in magnitude and opposite in direction, resulting in a situation of translational equilibrium
- there will be a net torque about the center of the dipole axis
what effect does torque have on the dipole?
causes the dipole to reorient itself so that its dipole moment, p, aligns with the electric field, E
how are magnetic fields created?
by any moving charge
what is the SI unit for magnetic fields?
tesla (T)
what are the 3 ways magnetic fields can be set up?
- movement of individual charges
- mass movement of charge in the form a current through a conductive material
- permanent magnets
what is a gauss? why do we use it?
a unit for magnetic fields
used because Teslas are quite large
defn + char (2) + ex (5): diamagnetic materials
made of atoms with no unpaired electrons and that have no net magnetic field
char: slightly repelled by a magnet, can be called weakly antimagnetic
ex: common materials you wouldnt expect to get stuck to a magnet (wood, plastics, water, glass, skin)
defn + char (3) + ex (3): paramagnetic materials
made of atoms that have unpaired electrons and thus have a net magnetic dipole moment
char: atoms are usually randomly oriented so that the material creates no net magnetic field, become weakly magnetized in the presence of an external magnetic field (aligning the magnetic dipoles of the material with the external field)
ex: aluminum, copper, gold
what happens to paramagnetic materials upon removal of an external magnetic field?
the thermal energy of the individual atoms will cause the individual magnetic dipoles to reorient randomly
defn + char + ex: ferromagnetic materials
made of atoms that have unpaired electrons and permanent atomic magnetic dipoles that are normally oriented randomly so that the material has no net magnetic dipole
char: become strongly magnetized when exposed to a magnetic field or under certain temps
ex: bar magnet
what does the configuration of the magnetic field lines surrounding a current-carrying wire depend on?
the shape of the wire
what is the shape of magnetic fields for straight wires? how do you determine the direction of the field vectors?
concentric rings
right-hand-rule (point your thumb in the direction of the current and wrap your fingers around the current-carrying wire) –> your fingers mimc the circular field lines (curling around the wire)
what is the difference in the equations for the magnitude of a magnetic field curling around a straight wire vs. at the center of a circular loop wire that is determined by the difference in the “r”?
straight wire: gives the magnitude of the magnetic field at any perpendicular distance r from the current-carrying wire
circular loop: gives the magnitude of the magnetic field only at the center of the circular loop of current-carrying wire with radius r
what can we assume in our discussion of the magnetic force on moving charges and on current-carrying wires and why?
WHAT: the presence of a fixed and uniform external magnetic field
WHY: magnetic fields exert forces only on other moving charges, they do not “sense” their own fields
defn: Lorentz force
the sum of the electrostatic and magnetic forces acting on charges at the same time
the magnetic force is a function of the sine of the angle, what does that imply about the charge in order for it to experience a magnetic force?
the charge must have a perpendicular component of velocity in order to experience a magnetic force
if the charge is moving parallel or antiparallel to the magnetic field vector, will the charge experience a magnetic force?
no!
what are the two right-hand rules in magnetism? (3 and 4 steps)
direction of magnetic field from current carrying-wire:
1. point your thumb in the direction of the current
2. wrap your fingers around the current-carrying wire)
3. your fingers mimic the circular field lines (curling around the wire)
direction of magnetic force on a moving charge:
1. position your right thumb in the direction of the velocity vector
2. put your fingers in the direction of the magnetic field lines
3. your palm will point in the direction of the force vector for a positive charge
4. the back of your hand will point in the direction of the force vector for a negative charge
