Download Electric Field Lines: Rules

Electric Field Lines
Q1
Which of the following best
represents the electric field
experienced at point P?
-a tool used to visualize the electric field in a region of space
Metal
plate
P
q2
(a)
(b)
(c)
(d)
(e) None of the above
Electric Field Lines
Electric Field Lines: Rules
• Field lines go away from positive charges and
towards negative charges
• Field is stronger where field lines are closer
together
• Number of lines leaving/entering a charge is
proportional to magnitude of charge
• Field lines don't cross
Electric Field Lines
For the electric field shown below, which way would
a positive charge placed at the point shown move?
A) up
B) down
C) left
D) right
E) it will not move
Electric Dipole
Two Identical Charges
Parallel plates
+
+
+
+
+
-
Electric Field Inside a
Conductor: Shielding
Neutral Conductor in a Field
• Excess charge inside a conductor will
quickly move to the conductor's surface
• The charge redistributes itself so that
there is no electric field inside the
conductor!
• If there were an electric field inside the
conductor, then the charges would move
until equilibrium is established
• In equilibrium, the electric field inside
a conductor is zero
Neutral Conductor in a Field
External electric field does not enter conductor
Conductor shields charge inside from external fields
+
+
+
+
+
-
+
-
+
-
+
-
+
+
+
+
+
-
For the charge distributions shown on the spherical
conductors below, which field lines are most
reasonable?
Electric Potential Energy (19-1)
• Gravity and electrical force are conservative: both
forces have an associated potential energy
• ∆(GPE) = mghB – mghA
= GPEB – GPEA = -WAB
(WAB = work done by gravity)
• ∆(EPE) = EPEB – EPEA = -WAB
(WAB = work done by electric force)
Electric Potential V (19-2)
The electric potential V at a point is the electric
potential energy EPE of a charge (q0) placed there
divided by the charge:
V =
EPE
q0
Units: J/C = V (volts)
V is a scalar
V can be + or -
Potential difference, VB – VA, between two points:
V = V B V A =
W AB
 EPE
=
q0
q0
ex. An electron in a TV picture tube is accelerated from
rest through a potential difference of 5000V.
a) What is the change in potential energy of the electron?
b) What is the work done by the electric force when the
electron goes from the -ve to the +ve terminal?
c) What is the speed of the electron as a result of this
acceleration? (m = 9.1x10-31 kg)
5000V
High
Voltage