Transcript best electric field

Force Fields
• Gravitational fields
-act on masses
-Strength ~ Mass / d2
-attractive
-cannot be shielded
-can store energy
-overall very weak force
• Electrical fields
-acts on charges
-Strength ~ Charge & 1 / d2
-attractive or repulsive
-can be shielded
-can store energy
-generally stronger than gravity
Electric Field
Definition
• Electric field is the
strength & direction
of the forces in
space surrounding a
positive test charge
Top tips on electric field
Field lines go from + to – charges
Closer field lines are stronger
field lines can’t cross another!
Question 1
• Suppose an electric field points south
• Which way would a proton move:
a. south
• B. north
• C. west
• D. east.
Question 1 answer
• Suppose an electric field points south
• Which way would a proton move:
A. south
Since electric field shows direction of a
positive charge from high to low
energy
Question 2
• Suppose an electric field points south
• Which way would an electron move:
a. south
• B. north
• C. west
• D. east.
Question 2 answer
• Suppose an electric field points south
• Which way would an electron move:
B. north
Since electron is a negative charge it
moves opposite direction of field
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
Question 3
• The electric field in a certain region of
space is 40 N/C.
A. What is the force on a -10 C
charge placed in the region?
B. Is it attractive or repulsive?
Question 3 Answer
• F=q*E
• F= (40 N/*C)(-10 C)
• F = - 400 N
(attractive force since
negative answer, +*- = - )
Question 4
• Imagine a metal ball placed in an electric field
• IF an electric field moves charges and conductors are full of
electrons which can move, where will all the electrons go?
a. stay on left side
• B. spread out on outer surface
• C. stay in middle
• D. don’t move
Question 4 answer
• Imagine a metal ball placed in an electric field
• IF an electric field moves charges and conductors are full of
electrons which can move, where will all the electrons go?
B. spread out on outer surface
since the charges move away from another to lower their energy
• the charges on opposite sides cancel out forces so E=0 inside
Electric Shielding- Faraday Cage
• Inside a Conductor, the Electric Field MUST be ZERO!
• Any net charge on the inside of a conductor will be
pushed to the surface & distributed to equilibrium!
• Electric field E is perpendicular to the outer surface,
and zero inside any conductor (metal)
Faraday cage
• Don’t try this at home!
Electric field demos
Pith balls inside the metal
screen don’t move since
there is no electric field
inside a faraday cage
Metal cage blocks
out the electric field
from the radio
station or cell phone
tower-so no music!
Common Faraday Cages
Purpose:
• Keep in or out electro-magnetic fields used in
electronics/telecommunications
Examples:
• Metal foil around wires inside phones
(so less pick-up of unwanted noise/calls)
• Rebar in concrete or bridges- bad reception
• Metal boxes around electronic trigger switches
coulomb
Charles Coulomb~1750
• How many charges are in 1C? coulomb?
• 1c is about 1 million charges!
• The man,
the law
Joules of energy
• Charges have a type of energy called
electron potential energy
High energy: + is close to +:
or + is far from –:
++
+
-
Question 5
• Imagine moving a positive charge in an electric field
E
+
• If you move the + charge to the left against the electric field, what
happens to the energy of the + test charge?
a. no change
• B. increases
• C. decreases
• D. not enough info
Question 5
• Imagine moving a positive charge in an electric field
E
+
• If you move the + charge to the left against the electric
field, what happens to the energy of the + test charge?
B. increases
since field points from high to low energy
Question 6
• Imagine moving a positive charge against an electric
field
E
+
• If it takes 2000 Joules of work to move the charge..?
a. how much energy does the charge gain?
• B. how much work does the field do to move the charge
back to the right?
Question 6
• Imagine moving a positive charge against an electric
field
E
+
• If it takes 2000 Joules of work to move the charge..?
a. 2000 J----positive work needed to add energy?
• B. -2000 J--- negative work done by the field,
(field loses energy while charge gains)
Electric Potential (Voltage)
•V= PE/Q
•voltage= energy/Charge
•Volts= joules/coulombs
•Analogies:
voltage is like water pressure
energy= height
water= charge
Question 7:coulombs, joules, volt
match these!
• Coulomb
unit of energy/charge
• Joule
unit of charge
• Volt
unit of energy
Question 7:coulombs, joules, volt
match these!
• Coulomb
unit of charge
• Joule
unit of energy
• Volt
unit of energy per charge
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
Example of high voltage, low charge
• What’s the voltage on a balloon if it has…
1 joule of energy (medium)
.001 coulombs of charge (low)
voltage= energy/charge
v = 1 J / .001C
= 1000 Volts !
Question 8
• What’s the voltage on a
Van de Graaf static generator
1 joule of energy (medium)
.00001 coulombs of charge (v.low)
v=?
Question 8
• What’s the voltage on a
Van de Graaf static generator
•
1 joule of energy (medium)
.00001 coulombs of charge (v.low)
v = 1 j/ .00001c
= 100,000 Volts ouch!