1. STATIC ELECTRICITY
1. Understand the nature of electric charge in terms of electron loss or gain
2. Define an electric field in terms of electric forces
3. Draw electric field lines
4. Describe the electric field between two parallel charged plates in terms of
uniform electric field lines
5. Use E = F/q to calculate the strength of an electric field
6. Use E = V/d to calculate the strength of a uniform electric field
7. Use change in kinetic energy, Ek = 1/2mv2 to calculate the velocity of a
charged particle inside an electric field
8. Define electric potential in terms of work and energy
9. Use V = W/q and V = ∆Ep/q to calculate change in electric potential
energy
10.Describe some practical applications of an electric field such as the Van
der Graaf generator, Millikan’s Experiment and the electron gun
Read Chapter 15 (p171 to 180)
Wednesday, 25 August 2010
2. Demo THE VAN DER GRAAF
Label the picture of the Van der Graaf (left) using the labels in
the box below
______________
Lower roller
______________
Belt - A piece of surgical tubing
______________
Output terminal - an aluminium or steel sphere
Upper roller - A piece of nylon
______________
Motor
______________
Upper brush - A piece of fine metal wire
______________ Lower Brush
______________
• When the generator is turned on, the electric motor begins turning the belt.
• Since the belt is made of rubber and the lower roller is covered in silicon tape.
Silicon has a greater affinity for electrons than rubber and so it captures electrons
from the belt. The belt in turn must capture electrons from the dome, leaving the
dome positively charged.
Reference: http://science.howstuffworks.com/vdg3.htm
Wednesday, 25 August 2010
3. THE PROCESS OF CHARGING
Based on atomic structure
Electron
Neutron
Proton
Empty space
Two steps
1. Charge transfer - Charges are transferred to other objects. This occurs by a
process called adhesion. Adhesion is a chemical bonding process. One material will
have a greater affinity for electrons than the other and so will capture electrons.
The outer electrons are relatively easily lost to atoms in other materials
2. Charge separation - When the material and the object are moved away from each
other the process of charge transfer is unable to be reversed
Once charge separation has taken place, the objects carrying the charge can
become attracted to each other (since opposite charges attract)
Wednesday, 25 August 2010
4. FIELDS & FORCES
• An electric field is a space in which an electric charge will experience a force
• A field is represented using field lines (these lines are just used to model the field -
a representation only)
• Arrows on these lines represent the direction of the field
• Density of the lines represent the strength of the field
Parallel Plates
Van der Graaf generator Field lines are from + to - in
The dome has a radial keeping with conventional
electric field around it. current. Lines are parallel
Wednesday, 25 August 2010
5. DRAWING FIELD LINES
Rules
1. For single charges electric field lines are drawn away from the positive charge or
towards the negative charge.
2. For interacting positive and negative charges the field lines are drawn from
positive to negative.
3. The lines are at right angles to the charge at their origin.
Examples: Draw the electric field lines in each of the following situations
1 2 3
+ + + + -
4 5 6
+ - + -
+ -
+ -
+ -
+ + -
+ +
+ - + -
+
+ - + -
http://surendranath.tripod.com/Applets/Electricity/FieldLines/FieldLinesApplet.html
Wednesday, 25 August 2010
6. FIELD STRENGTH
• Electric field direction is in the direction of motion of positive charge in the field.
(electrons however will travel in the opposite direction of the electric field)
• Electric field strength is the force per unit charge:
where F = the force on a charge (N)
E= F
q = the size of the charge (C)
q
E = Electric field strength (NC-1)
(ie. the greater the force on a given charge => the greater the electric field strength
will be)
• The Electric field strength is related to voltage between the plates
where V = the voltage across 2 plates (V)
E = V
d = the distance between the plates (m)
d
E = Electric field strength (NC-1)
The greater the voltage then the greater the electric field strength will be. Also the
smaller the distance between the plates then the stronger the electric field will be.
1 Coulomb, C is the term used to describe a large number of charges.
1 C = 1.6 x 1019 electrons so charge on an electron is 1.6 x 10-19 C
Wednesday, 25 August 2010
7. Example
The diagram shows an electrostatic precipitator use to collect ash. It is made from 2
parallel plates that carry a high voltage across them:
(a) Show the shape of the electric fields between the plates (including
the ends)
(b) Calculate the strength of the electric field at point X and Y
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A tiny piece of ash rises vertically up the chimney at constant speed. At
point X it becomes charged by losing 100 electrons. The charge on an
electron is 1.6 x 10-19 C
(c) Calculate the total excess charge of the piece of ash.
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(d) Calculate the magnitude and direction of the electric force on the piece of ash.
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(e) Draw and describe the path of the ash.
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Wednesday, 25 August 2010
8. WORK & ENERGY
Consider a positive charge in a uniform electric field between plates A and B :
d
+ -
+ v
+ -
+ -
plate A + - plate B
+ -
V
Definition: “Voltage is the work done per unit charge”: V=W
q
Work is done on the charge to move it from plate A to plate B => W = Vq
This is equivalent to the potential energy that the charge has at plate A => EpA = Vq
The potential energy that the charge has at A is changed to kinetic energy when the
charge reaches plate B.
Since Energy is conserved: Ep (A) = Ek (B)
Vq = 1 mv2
2
Wednesday, 25 August 2010
9. Example VELOCITY OF CHARGES
The diagram shows two parallel plates, 2 cm apart , connected by
a 200V power supply:
(a) What is the size of the electric field, E between the plates?
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A small sphere of mass 5.0 x 10-26 kg, carrying a charge of
-8 x 10-19 C is placed between the plates.
(b) Which direction will the small sphere move? ______________
(c) What is the size of the electric force on the sphere? _________
(d) How much kinetic energy does it gain when it moves from one plate to the other?
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(e) If it started at one plate, what speed would it have when it reached the other
plate?
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10. POTENTIAL ENERGY IN A UNIFORM FIELD
The electric field is uniform in
this (shaded) region
(indicated by a uniform density of field lines ..... in
other words, field lines are parallel and straight)
So the force acting on a given charge is the same
anywhere in the uniform electric field.
V
The relationship between Potential energy, Ep and plate separation, d:
The following equations apply for the uniform field: E = F = V
=> V = Ed 1
q d
When we consider the voltage across the plates: V = Ep => Ep = Vq 2
q
Substituting equation 1 into equation 2 gives: Ep = Edq
This equation is analogous to the equation
for gravitational potential energy: Ep = mgh = g h m
Wednesday, 25 August 2010
11. THE STRENGTH OF A UNIFORM FIELD
Remember that E = F and V
q d
Example
Two metal plates are set at 2 mm apart and connected to a 12 V
battery. This creates an electric field between the two plates. - +
(a) Draw in the electric field lines and their direction.
- +
(b) Use E = V/d to calculate the electric field strength inside the
plates. - +
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(c) What is the other expression for electric field strength? ________
(d) A 20 mC charge is placed inside the field. Calculate the electric
12 V
force on the charge.
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(e) What is the direction of this force? ______________________
15A: Q.1 to 10
Wednesday, 25 August 2010
12. Practical applications THE TV TUBE
Demo: Picture tube from a small TV set
1. Study the diagram of the electron gun
(left)
2. Label the diagram from the list of labels
provided (below)
A. Positively charged anode attracts electrons (which
accelerate towards the hole)
B. Phosphorescent paint glows when the electron beam
strikes it (Ek -> Light E)
C. Electrons emerge as a solid beam
D. Thermionic emission - hot filament glows -> electrons
released
E. Parallel plates provide an electric field that can change
the direction of the beam.
Wednesday, 25 August 2010
13. EXAMPLE
The diagram shows an electron gun, where electrons are emitted by a hot
filament.
(a) What is the name of the process?
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(b) The electrons accelerate towards the screen. Explain the acceleration of the
electrons.
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(c) Explain how the beam changes direction.
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(d) Calculate the work done on each electron in the electric field (to accelerate the
electron)
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