Le Champs Électrique Flashcards
What does Coulomb’s Law state?
Give equation
Magnitude of force btw 2 pt charge is directly proportional to pdt of charges & inversely proportional to square of distance btw them
F = q1q2/(4πε0d²)
Charge, like temperature, is …?
Scalar
What is the constant ε0?
Permittivity of free space
(This value and value of permittivity of air is similar so can use this same value in vacuum or air questions)
Define Electric Field
Region in space where stationary charge experience electric force
Describe electric fields
- invisible, so represented by electric lines of forces
- based on interact n w +ve test charge
- never touch or cross one another
- direction at a pt is along tangent to field line at that pt
- strength increases as lines are closer
Define Electric Field Strength (E) and give equations and SI unit
- at a pt is electric force per unit +ve charge acting on small stationary charge placed at that pt
*NOTE: only +ve charge
- E=F/q
=> E = Q/(4πε0d²) - SI Unit: N C^-1 or V m^-1
Define electric potential energy (U). Give equation
(Electric potential is defined to be zero at pt infinite dist away)
=> electric potential energy of charge placed at that pt in e field is,
-work done by external agent in bringing charge from infinity to that pt w/o change in Ek of charge
U = Qq/(4πε0r)
What must NOT be omitted when using electric potential energy formula?
Sign!
Sub value of charge WITH sign of charge to get correct sign for U!
U is +ve for like charges
U is -ve for unlike charges
Define Electric Potential (V)
Give equation
V at a pt in e- field is work done per unit POSITIVE charge by external agent in bringing small test charge from infinity to that pt w/o change Ek of charge
V=U/q=Q/(4πε0r)
*Must sub Q w sign so V hv correct sign!
What is the relationship between Electric potential (V) and Electric Field Strength (E) as well as between Electric Potential Energy (U) and Electric Force (F)?
What does negative sign show in second equation?
F = -dU/dr
Since F=qE and U=qV,
qE = -d(qV)/dr (can cancel off ‘q’)
=> E = -dV/dr
- negative sign shows direct n of e field point fr high to low potential
How is magnitude of electric field strength related to potential gradient at a point?
*Potential gradient: rate change potential w.r.t. dist
Mag of e field strength is numerically equal to potential gradient at that pt
How does electric field differ at surface of and within a charged conductor?
At surface:
- field is perpendicular to surface
- hv net e force
Within:
- no resultant e field
- no e field inside so equi potential region (same potential as surface)
- no net e force
What are equipotentials?
Lines or surfaces linking up all pt w same e potential
*not to confuse w field lines. In fact, equipotential lines always intersect field lines perpendicularly
What does separat n btw equipotential lines show?
Potential gradient
- closer lines, steeper gradient
- for single charge, separat n btw equipotential lines increase w dist, shows decreasing field strength as dist fr charge increase
For multiple charges, where can neutral point be found? (0 net e force)
two like charges:
Neutral pt btw charges
Two unlike charges:
Neutral pt outside dist btw charges (OF DIFF magnitude oni)
