Electrostatics & Magnetism Flashcards
Calculating electric field (E)
E = F/q
F is the electrostatic force experienced by a charged particle
q is the charge of the particle
SI Unit: N/C
Electrons are free to move in a (conductor/insulator)
conductor; they distribute evenly over the surface
Example: metals
Insulator
Electron’s don’t move, charges don’t distribute evenly over surface
Example: nonmetals
SI unit of charge
coulomb (C)
Charge of an electron & proton
e = -1.6 x 10^-19 C
p = +1.6 x 10^-19 C
True or false: A neutral object is made of an equal amount of positive and negative charge.
True
Can a charged object attract a neutral object?
Yes.
Coulomb’s Law (calculating electrostatic force on a particle)
F = (Kq1q2)/r^2
K is the electrostatic constant:
9.0 x10^9 N*m^2/C^2
Remember, even if the charges of the two objects aren’t equal. The force that object 1 exerts on the object 2 = force object 2 exerts on object 1
Relationship between distance and electrostatic force:
Inverse relationship; F = 1/r^2
As distance increases x2, force goes down by 4
Additional things to know about electric field:
Electric field vectors always point away from regions of higher voltage to lower voltage.
-Electric field vectors always point away from regions of higher voltage to lower voltage.
-The electric field induces positive particles to move in the same direction as the E field & negative particles to move in opposite directions as E field lines
Question from PHYS Lecture:
Diagram of electric field on a positive charge
Diagram of electric field on a negative charge
Calculating electric field of a point charge: (any point in the field)
E = (K*|q|)/r^2
q is absolute value of charge on the particle
r is the distance between the charged particle and the particular point in the electric field
If the charged particle is negative, the vectors in the electric field point (away/towards) the particle.
negative = towards
positive = away
What is a parallel plate capacitor?
Two plates of opposite charges parallel to each other which create a uniform electric field (E).
E travels from positive plate to negative plate.
Calculating electric field (E) in a parallel plate capacitor
E = Q/ε0A
ε0 is a constant:
8.85 x 10^-12 C^2/N*m^2
Q is the charge of the capacitor
A is the area of the capacitor
Electric force and electric field are (scalars/vectors) while electric potential energy and electric potential are (scalars/vectors).
vectors, scalars
As a proton enters a parallel plate capacitor from the positive plate, how does its electric potential energy and kinetic energy change?
∆K increases, ∆U decreases as it moves to the negative plate
As an electron enters a parallel plate capacitor from the positive plate, how does its electric potential energy and kinetic energy change?
∆K decreases, ∆U increases because the electron doesn’t want to move towards the negative end, so its speed slows down
Calculating electrostatic force of a parallel plate capacitor
Felectro = q*Electric field
Calculating electric potential energy of 1 electron/proton
∆U = q * ∆V
∆V is the electric potential difference/change: Vfinal - Vinitial
Protons spontaneously move from (high/low) electric potential to (high/low) electric potential.
Move from higher to lower electric potential.
*Spontaneous means W=0
Electrons spontaneously move from (high/low) electric potential to (high/low) electric potential.
Move from lower to higher electric potential
*Spontaneous means W=0
Energy of conservation for electric potential
∆K + ∆Uelectric = W
Electric potential energy conservation problem
Calculating electric potential (V) by a point charge
∆V = Kq/r
Electric potential is inversely related to distance of point charge
The further the charge gets, the more the electric potential decreases
Calculating electric potential (V) inside a capacitor
∆V = E x d
d is the distance between two plates
E is the electric field inside the capacitor
*Can also use this equation to calculate electric field
Calculating electric potential energy between 2 point charges
∆U = (Kq1q2)/r
K is the electrostatic constant:
9.0 x10^9 N*m^2/C^2
Trend:
A proton moving from high –> low electric potential, ALWAYS decreases its electric potential energy.
An electron moving from high –> low electric potential, ALWAYS increases its electric potential energy.
Equipotential lines
-Patterns of lines which represent the electric potential.
-The further the lines are from the charged particle, the lower the electric potential.
-Equipotential lines have equal potential differences between (every increases by 50V)
True or false:
Electric field lines are always perpendicular to equipotential lines.
Yes they are perpendicular, and point in the direction of high EP to low EP
*Note here, the distance between equipotential lines decreases, which means electric field vector increases
*Electric field always points in the direction of lower voltage
Photoelectric effect
The ejection of electrons from a substance due to the absorption electromagnetic radiation.
True or false
For the ejection of an electron to occur, the absorbed electromagnetic radiation (E) must exceed the work function (W)
Equation relating E, W, and kinetic energy
(1/2)mv^2 = E - W
or
(1/2)mv^2 = hf - W
Kinetic energy of an electron increases when E is (greater than, less than, equal to) W.
E is greater than W.
True or false:
Frequency of the wave absorbed by the electron is directly proportional to its kinetic energy.
True
Magnetic field
Field created by a moving charge, originates at North pole and travels to South pole
Unit: Tesla
SI Unit: 1 Ns/mC
Diamagnetic
Atoms only have paired electrons, no net magnetic field, slightly repelled by magnet
Example: wood, plastic, water, glass
Paramagnetic
Atoms which have unpaired electrons
Example: aluminum, gold, copper
Lorentz force equation
Sum of the electrostatic and magnetic forces acting on a body
Equation for magnetic force on a particle in magnetic field
Magnetic force = qvBsinθ
θ is the angle between velocity vector and magnetic field vector
right-hand-rule #3
(determines magnetic force)
Additional things to know:
-Magnetic force is perpendicular to the plane of the velocity and magnetic field vectors
-Magnetic force does NO work on a charged particle
-Magnetic force only affects the direction of the particle, it doesn’t alter the KE or the speed
When is the magnetic force 0?
- When the charged particle is at rest (it’s not moving).
- When the charged particle is moving parallel to the magnetic field (θ=0˚)
- When the charged particle moves completely opposite of mag. field (θ=180˚)
In which of the situations is the magnitude of the magnetic force maximum?
B. 90˚ angle between velocity vector & magnetic field vector
Equation for a charged particle moving in a circular motion
r = mv/qB
r is the radius of the particle’s path
m is mass of particle
q is charge of particle
B is magnetic field of particle
For a charged particle traveling a circular path, magnetic force always points inward towards the center of the circle.
Based on the direction of velocity and magnetic field vectors given, how do you determine the charge of the particle traveling in a circular path?
Do RHR#3.
If magnetic force points towards center of circle, charge is positive.
If magnetic force points outwards from the circle, charge is negative.
How do magnetic fields apply to mass spectrometers?
Ions with different masses which enter the spectrometer follow paths of different radii. Only some ions reach the exit slit to continue on to the actual detector.
RHR2
Point thumb towards direction of traveling current and curl fingers around wire.
Fingers can curl either CW or CCW.
x is into the page, simple dot is out of the page
Equation for magnetic force on straight current-carrying wire
Magnetic force = ILBsinθ
I is current in the wire
L is length of wire (m)
B is magnetic field (T)
θ is angle between wire and B
For direction of magnetic force, use RHR#3
Equation for magnetic field of straight current wire
B = µ0*I/2πr
µ0 is 4π x 10^-7
r measured in (m)
For direction of magnetic field, use RHR#2
For a straight current wire, what if you aren’t given direction of magnetic field and you need to figure it out?
Use RHR#2.
Draw the arrows around the wire pointing in either the determined CW or CCW from the rule.
*On different sides of the wire, the direction of mag. field is different.
*“The page” is always on the right side of the wire for reference
Tips to memorize for determining mag. field w/o doing RHR2 (its so confusing)
Current in horizontal direction:
Current points right - Above the wire is out of page
Current points left - Above the wire is into page
Current in vertical direction:
Current points up - On right of wire is into the page
Current points down - On right of wire is out of page
True or false
If Fnet = 0, magnetic force and electrostatic force should cancel.
True
Equation for magnetic field of a current loop wire
B = µ0*I/2r
µ0 is 1.26 x 10^-6 T*m/A
r measured in (m)
Magnetic force between parallel wires/loops
If current points in same direction, they attract each other.
If current points in diff. directions, they repel each other.
