Upload
whitney-poole
View
216
Download
0
Tags:
Embed Size (px)
Citation preview
front plate slightly chargedinduces opposite charge on back plate.
Brushes pull off chargescharges collected in leyden jar (capacitor)W
imsh
urs
t M
ach
ine
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 fieldE=0 inside a metal (faraday) cageField lines go from + to – charges
Closer field lines are stronger
Electric Field between 2 unlike charges
+ -
A
B
C
Note that the distance betweenelectric field lines at C is shorterthan that at B.
Distance between electric fieldlines at B is shorter than at A.
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
POSITIVE charge moving in an E field.
(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
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 9x104 joule energy
0.001 C of charge 1x104 C of charge
1/.0001 = 1000 volts 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 2 situations Point Charges
Force: F=q*E Force= k q1*q2 / d2
Electric potential energy Electric potential energy
PE= F*d = q*E*d PE = F*d = k q1*q2 / d
Voltage (electric potential) Voltage (electric potential)
V= PE/q1 = q*E*d /q = E*d V= PE/q1 = k q2 / d2
Voltage difference: Voltage difference:
DVe = D PE/q1 = E* Dd 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
• PEfinal
• +++++
• +
• Peiniitial work = gain in PE = F*d
work
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) =qE*d = mg*h
Potential V= PE/q1 = kq2 / d U= PE/m1 = G m2 / d
(Using field) = E*d = g*h