Electrostatics Flashcards Preview

MCAT Physical Sciences > Electrostatics > Flashcards

Flashcards in Electrostatics Deck (25):

Electrostatic Force

The force existing between stationary charges


How do electric charges react to one another?

Like charges repel, while unlike charges attract.


What happens when you rub a glass rod with a piece of silk?

The electrons flow from the glass rod to the silk cloth due to friction, leading to static cling.



SI unit of charge


Fundamental Unit of Charge

e = 1.60 X 10^-19 C (the charge of an electron when negative and proton when positive)


Coulomb's Law

F = k q1 q2/ r^2


Coulomb's Constant

k = Coulomb's Constant = Electrostatic Constant = 1/ (4πε0) = 8.99 X 10^9 N m^2/ C^2


Permittivity of Free Space

ε0 = 8.85 X 10^-12 C^2/ (N m^2)


What happens when the distance between two charges doubles?

The force reduces to 1/4 of the original force because F = 1/r^2


How do the gravitational and electrostatic forces compare for the electron and proton in a Hydrogen atom?

Gravitational Force = 3.63 X 10^-47 N
Electrostatic Force = 8.22 X 10^-8 N
Electrostatic Force stronger than Gravitational Force by approximately 10^40


Electric Field

A vector quantity that every charge exerts on other electric charges defined by the force a stationary positive test charge feels. The units for electric field are N/C = V/m.
E = F/q0 = k q/ r^2


What force does any charge feel due to an electric field?

F = qE, a conservative force (independent of pathway)


Which direction would the force a positive charge feels due to an electric field point?

A positive charge in an electric field feels a force in the direction of the field, while a negative charge feels a force opposite of the direction of the electric field.


Electric Field Lines

Imaginary lines that show the direction a positive test charge would be accelerated in the electric field.


How are the electric field and electric field lines related?

The direction of the electric field is tangent to the electric field line at the given point, pointing the same direction. Furthermore, like topographical maps, the distance between field lines indicates relative strength, where closer field lines indicate a stronger electric field.


What do you do for an electric field due to a collection of charges?

Do a vector sum of the electric fields of each of the charges.


Electric Potential

V = k Q/ r = W/ q = U/ q
Units: J/ C = V
A scalar quantity


Potential Difference

ΔV = Vb - Va = W(from a to b)/ q0


Equipotential Lines

Equipotential lines cross the electric field lines perpendicularly and represent areas of the same potential, meaning that any two points on the same equipotential line will have a potential difference of 0. No work is done when moving a charge along an equipotential line, because work depends only on the potential difference, not on the path.


Electric Potential Energy

U = W = qV = q ΔV {change in electric potential energy} = k qQ/ r = (1/ (4πε0)) qQ/ r
Units: Joules


Electric Dipole

Two charges of equal magnitude and opposite sign separated by a distance


What is the potential at any point P due to a dipole moment?

V = k q/ r1 - k q/r2 = k q ((r2 - r1)/ r1 r2)
When P >>> d, V = k (q d/ r^2) cos θ = k (p/ r^2) cos θ


What is the magnitude of the electric field along the perpendicular bisector of a dipole?

E = (1/ (4πε0))(p/ r^3), because θ is 90 degrees and this plane lies halfway between the two charge of the dipole


Dipole Moment

A vector quantity that is equal to qd with units C m, with a direction that points towards the negative charge away from the positive charge.


What happens when a dipole is placed in a uniform external electric field?

The net force on the dipole will be zero, because each of the charges feels an equal and opposite force, meaning that no translational movement occurs, but the dipole will spin about its center so that its dipole moment will align with the electric field.
τ = F d/2 sin θ + F d/2 sin θ = Fd sin θ = qEd sin θ = pE sin θ