front plate slightly charged
induces opposite charge on back plate.
Brushes pull off charges
charges collected in leyden jar (capacitor)
Electric Field
Definition
• Electric field is the
strength & direction
of the forces in
space surrounding a
positive test charge
Electric field as coulomb’s law
• F = q1 * (k q2 /d2)
rewrite the force equation
F = q1 * E
q1 is the positive test charge
E is the field created by charge(s) q2
Top tips on electric field
E=0 inside a metal (faraday) cage
Field lines go from + to – charges
Closer field lines are stronger
Electric Field between 2 unlike charges
Note that the distance between
electric field lines at C is shorter
than that at B.
A
Distance between electric field
lines at B is shorter than at A.
B
+
C
-
Electric Field between 2 charged plates
positive
negative
Electric Field between 2 charged plates
+
+
+
+
+
+
+
+
- - - - - - - -
coulombs, joules, volts
• Coulomb= unit of charge
• Joule = unit of energy
• Volt= unit of energy per charge
coulomb
• How many charges are in 1C? coulomb?
• 1C is about 6 billion billion charges)
(1/1.6 x 10-19 = 6.25 x 1018)
• The man,
the law
Charles Coulomb~1750
Joules of energy
• Charges have a type of energy called
electron potential energy (PEe or U)
High energy: + is close to +:
or + is far from –:
++
+
-
It takes work to move charges
against (opposite) an electric field
High energy
work needed to move + charge
(W = F*d)
E
F
+
low energy
(a) When a positive charge moves in the direction of
an electric field, the field does positive work and the
potential energy decreases. Work = qo E d
POSITIVE charge moving in an E field.
Volts is an energy density
• Voltage is also called potential
• 1 volt = 1 joule / 1 coulomb
• Example: 12 V battery: every coulomb of
charge has 12 joules of energy
Examples
• Static balloon
9 V battery (1hr,1A)
1 joule of energy
0.001 C of charge
1/.0001 = 1000 volts
9x104 joule energy
1x104 C of charge
9/1 = 9 Volts
Another c,j,v example
• Van de graaf static generator has..
1,000,00 volts (high)
1 joule of energy (low)
.00001 coulombs of charge
1,000,000 v = 1 j/ .00001c
Uniform electric field
Force:
2 situations
F=q*E
Electric potential energy
PE= F*d = q*E*d
Voltage (electric potential)
V= PE/q1 = q*E*d /q = E*d
Voltage difference:
DVe = D PE/q1 = E* Dd
Point Charges
Force= k q1*q2 / d2
Electric potential energy
PE = F*d = k q1*q2 / d
Voltage (electric potential)
V= PE/q1 = k q2 / d2
Voltage difference:
DVe = D PE/q1 = k q2 / d2 - kq2/ d1
It takes work to move + charge against static
forces like it takes work to lift against gravity
•
•
•
• Peiniitial
PEfinal
+++++
+
work = gain in PE = F*d
Electrostatics
gravitational
Force:
F= kq1 q2 / d2
F= Gm1m2/d2
Field
E= F/q1 = kq2/d2
g = F/m1 = Gm1/d2
potential energy
PE= F*d = kq1 q2 / d
PE = F*d= Gm1m2/d
(using field)
Potential
(Using field)
=qE*d
V= PE/q1 = kq2 / d
= E*d
=
mg*h
U= PE/m1 = G m2 / d
= g*h