chapter 22 - electric fields Flashcards
electric field
region of space where a charged object experiences a force due to another charged object
observing electric fields
- metal foil between two charged plates
- charged balloons suspended from a point by string
- gold leaf electroscope
electric field strength
the force exerted per unit charge on a small positive test charge placed at that point
is a property of a field and not the particular charge in it
E = F/q
unit = N?C
vector - points the direction a positive charge would feel a force
due to coulombs law E (at q) = Qk/r²
electric field lines
show electric field strength and direction of the field
- +>-
- perp to surface of a conductor
- closer = stronger
- equally spaced and parallel = uniform field
show direction a + charge would go
electric field patterns
+ point charge > radiates out all directions (- opposite)
+ charge and - > goes from + to -
+ and + > radiates out from both - null point in the middle which has no force due to electric field
force on a charge due to electric field
F = Eq
coulombs law
F ∝ Qq
F ∝ 1/r²
the force between two point charges (q and Q) r apart in a vacuum = kQq/r²
where k = 9x10⁹ Nm²/C² = 1/4𝞹ε
where ε = permittivity of free space
coulombs law spheres of charge
if a sphere of charge assume all charge is at the centre and use that assumption to find the distance from one centre to another
testing coulombs law
set up a sphere attached to a rod and a sphere suspended from thread of length 1m
measure distance r from the balls at max repulsion and d (r/2)
calculate F as W = Tcosθ
F = Tsinθ
so F = Wtanθ - tanθ = d/1 = d
plot F against 1/r^2
grad = kQq
validity of assume charge is at the centre of a sphere
charges will redistribute due to repulsion
like charges will be repulsed to opposite sides so in reality the distance (r) will be larger
energy / work done
E = V/d
electron gun
velocity of electrons accelerated by an electron gun
1/2mv² = qV
v = √2qV/m
oscilloscopes
- electron gun produces a beam of electrons
- there are y plates (horizontal) and x plates (vertical)
- pd across the y plates deflects the trace vertically
- pd across x plates deflects the trace horizontally
- light produced on screen by electron beam
electric potential energy
work done to bring q from infinity to a defined point (Q)
(work done to overcome the repulsion or attraction between charged points)
energy is transferred to the moving charge (its electric potential increases)
EPE (of q in the field of Q) = Fd = kQq/r
EPE when charges are the same
EPE increases (positive work is done bringing a test charge from infinity to a point of positive charge)
EPE when charges are opposite
EPE decreases (negative work is done bringing a test charge from infinity to a point of negative charge)
electric potential
work done per unit charge in brining a positive test charge from infinity to a point
electric potential energy per unit charge
V = EPE/Q
= kQ/r
EPE/ V/ E / F
scalar or vector
EPE / V - scalar - can add up individual potentials to find total potentials of multiple charged
E/F - vector - must consider direction when adding individuals to find resultant
equipotential lines
surfaces of constant potential - a test charge moving along an equipotential has constant EPE
represented by dotted lines
no work is done on the charge as it moves along an equipotential as the force is at right angles to the equipotential
right angles to field lines
all equations
F (2 charges) = kQq/r²
F (1 charge) = EQ
E (field strength) = V/d
W = QV
E (radial field field strength) = kQ/r²
V = kQ/r = Er
