TOPIC 6.2: Electric Fields
and Forces
These notes were typed in association with Physics for use
with the IB Diploma Programme by Michael Dickinson
6.2 Electric Force and Field
6.2.1 State that there are two types of electric charge.
Have you ever removed a wool sweater and heard a strange
crackling noise?
Have you ever opened a band aid and seen a flash?
When ever two objects rub against each other there is friction.
This friction can cause “electrification” effect as tiny charged
particles are forced to move form one region to another.
6.2 Electric Force and Field
6.2.1 State that there are two types of electric
charge.
These charged particles are electrons.
Negative charge
Outside of atom
Can easily be stripped away by friction.
“Orbit” the nucleus.
The nucleus contains
Protons, positive charge
Neutrons, no charge.
d
6.2 Electric Force and Field
6.2.1 State that there are two types of electric charge.
UNITS
Electric charge = q or Q. Measured in Coulombs/C
1 Coulomb of charge = the charge carried on 6.25 x 1018 electrons
OR
The charge on 1 electron(e) = 1.6 x 10-19Coulombs
IB GIVEN DATA
Charge on electron – e = 1.60 x 10-19C
Electron rest mass – me = 9.11x10-31kg = 0.000549u = 0.511MeV/c2
Proton rest mass – mp = 1.673x10-27kg = 1.007276u = 938MeV/c2
Neutron rest mass – mm = 1.675x10-27kg = 1.008665u = 940MeV/c2
6.2 Electric Force and Field
6.2.1 State that there are two types of electric charge.
Two types of charges:
Positive and negative.
Uncharged/neutral objects have equal amounts of positive and
negative charges.
Proton and electron have equal amounts of charge
But proton is about 1800 times more massive.
6.2 Electric Force and Field
6.2.1 State that there are two types of electric charge.
Electrification occurs because electrons move.
This creates a negative area and leaves behind an equally positive
region.
Net charge is produced when there is an imbalance in the
number of protons and electrons.
6.2 Electric Force and Field
6.2.1 State that there are two types of electric charge.
Some objects naturally become positively charged. Others
naturally become negatively charged.
Example: Rub an acrylic plastic ruler on a piece of cloth. The friction
between the plastic and the cloth causes some of the electrons to be
stripped from the surface of the plastic and deposite don the wool.
This gives an overall positive charge to the plastic and a negative
charge to the cloth.
6.2 Electric Force and Field
6.2.2 State and apply the law of conservation of charge.
IB Definition
Charge cannot be created or destroyed but it can move from
one place to another or from one object to another.
Charges can move from one place to another.
Charges can NOT be created from nothing.
6.2 Electric Force and Field
6.2.2 State and apply the law of conservation of charge.
Practice 6
A metal sphere carrying a charge of +4.2µC comes into contact
with an identical second sphere which is neutral. Calculate the
charge on each sphere.
Answer = 2.1µC
6.2 Electric Force and Field
6.2.2 State and apply the law of conservation of charge.
Practice 7
Two identical metal spheres carry charges of –3.2µC and –4.8µC
respectively. The two spheres are brought together and the charge
evenly distributes itself so that each sphere carries equal charge.
Determine the charge on each sphere.
Answer = –4µC
6.2 Electric Force and Field
6.2.2 State and apply the law of conservation of charge.
Practice 8
Two metal spheres of different size carry charges of –3.0µC and
+4.8µC respectively. The two spheres are brought together and
the charge distributes itself so that one sphere carries twice as
much charge as the second sphere. Determine the charge on each
sphere.
Answer = +1.2µC and +0.6µC
6.2 Electric Force and Field
6.2.3 Describe and explain the difference in the electrical properties
of conductors and insulators.
Conductors – materials that allow a free movement of electrons
both within the substance and on its outer surface.
Insulators – do not allow electrons to move freely. Do allow the
build-up of electrons over its outer surface.
If a charged insulator is held, it will keep its charge.
If a conductor is held it will lose its charge. Electrons will flow
from the conductor to the person and then to the earth.
Rubber soled shoes don’t allow this.
6.2 Electric Force and Field
6.2.3 Describe and explain the difference in the electrical properties
of conductors and insulators.
Examples of Conductors
All metals – gold, silver, copper, ect.
Carbon in the form of graphite
Examples of Isulators
Plastics – perspex, poythene, PVC
Rubber
Wood
Glass
6.2 Electric Force and Field
6.2.4 State Coulomb’s Law.
Two charged objects are brought together and an electrical force
is created between them.
6.2 Electric Force and Field
6.2.4 State Coulomb’s Law.
Opposites attract/likes repel
***See diagram on board***
Two charged objects are
brought together and an
electrical force is created
between them.
6.2 Electric Force and Field
6.2.4 State Coulomb’s Law.
Size of force depends upon size of
each charge and the distance
between them.
This electrical force is directly
proportional to the product of the
two charges.
This electrical force is inversely
proportional to “the square of the
distance between them”.
6.2 Electric Force and Field
6.2.4 State Coulomb’s Law.
IB Definitions/Data
Coulomb’s Law – the force between two point charges is directly
proportional to the product of their charges and inversely
proportional to the square of the distance between them. The
direction of the force is along the line connecting the two
charges.
Coulomb constant = k = 8.99 x 109 Nm2/C2
Permittivity of free space = ε0 = 8.85 x 10-12 C2/Nm2
6.2 Electric Force and Field
6.2.4 State Coulomb’s Law.
F ∝ q1q2/r2
F = kq1q2/r2
IB Definitions/Data
Coulomb’s Law
F = kq1q2/r2 = 1 / 4πε0 * q1q2/r2 (write this out)
k = a constant of proportionality called Coulomb’s constant
k = 1/4πε0
ε0 = permittivity of free space
ε0 = 1/4πk
6.2 Electric Force and Field
6.2.4 State Coulomb’s Law.
Practice 9
A +4µC charge is placed 20cm away from a –6µC charge.
Calculate the force between these charges. Is this an attractive or
repulsive force?
Answer: - 5.39 Newtons
***Note the negative sign means the force is an attractive force
6.2 Electric Force and Field
6.2.5 Define electric field strength.
We’ve already looked at the idea of a filed when we looked at
gravitational fields.
A field is a region of space where a force acts without contact.
Around any electric charge, there exists an “electric field”
A region of space where a force would exist if another charge
entered the field.
IB Definition
Electric Field – the area around an electric charge where a force
exists if another charge enters the field.
6.2 Electric Force and Field
6.2.5 Define electric field strength.
Point Charge
We use the idea of a “test charge” or “point charge” to investigate
electric field strength.
A test charge is considered to be a positive charge whose charge is
so small that I does not alter the electric field in any way. (The
charge can be ignored.)
Check out this simulation to better under stand a test charge.
http://phet.colorado.edu/en/simulation/electric-hockey
6.2 Electric Force and Field
6.2.5 Define electric field strength.
Field strength of the electric field is proportional to the
magnitude of the force at that point and inversely proportional to
the charge. Also notice the units are N/C or NC-1
IB Definition: E = F/q
Electric Field Strength(E) – the force per unit charge on a tiny
positive test charge.
E = F/q
→
F = Eq
And since F = kq1q2/r2
Eq = kq1q2/r2
E= k(q/r2)
6.2 Electric Force and Field
6.2.6 Determine the electric field strength due to one or more point
charges.
Problem 10
Calculate the strength of the electric field 0.4m away form a point
charge of magnitude +20µC.
Answer: E=1.12 x 106 N/C
6.2 Electric Force and Field
6.2.6 Determine the electric field strength due to one or more point
charges.
Remember that electric field has both magnitude and direction.
This means that we can use vector addition to solve of the net
electric field.
Problem 11
The arrangement shows three point charges q1, q2, and q3. The
sizes of these charges are +6µC, +4µC and -3µC respectively. They
are separated by the distances shown.
Calculate the magnitude and direction of the resulting force on
q1
Answer 21.1N @ 71.6º
6.2 Electric Force and Field
6.2.6 Determine the electric field strength due to one or more point
charges.
Problem 12
Calculate the magnitude and direction of the electric field for a
test charge placed at the position shown due to the two point
charges q1 and q2 of magnitude +5µC and -8µC. The distance
from the test charge to the two individual charges is shown.
Answer: 142 x 103 N/C @ 62º
6.2 Electric Force and Field
6.2.7 Draw the electric field patterns for different charge
configurations.
Radial field - The electric field lines are directly towards or away
from the point charge.
The closer the field line the larger the relative magnitude of the
field.
As the distance away fro the charge increases, the strength of the
field diminishes and the gap between the field lines increases.
6.2 Electric Force and Field
6.2.7 Draw the electric field patterns for different charge
configurations.
Notice electric field lines are directed away from the positive
charge and toward the negative.
6.2 Electric Force and Field
6.2.7 Draw the electric field patterns for different charge
configurations.
Which charge is the strongest? Weakest?
6.2 Electric Force and Field
6.2.7 Draw the electric field patterns for different charge
configurations.
Guidelines for drawing field lines:
Electric field lines never cross each other.
Electric field lines are most dense around objects with the greatest
amount of charge.
At locations where electric field lines meet the surface of an
object, the lines are perpendicular to the surface.
Equipotential Lines
An equipotential is a line or
surface over which the
potential is constant.
Electric field lines are
perpendicular to
equipotentials.
The surface of a conductor is
an equipotential.
Equipotential Lines
6.2 Electric Force and Field
6.2.8 Solve problems involving electric charges, forces and fields..
***See previous problems***
Problem 16
Erin Agin drew the following electric field lines for a
configuration of two charges. What did Erin do wrong? Explain.
6.2 Electric Force and Field
6.2.8 Solve problems involving electric charges, forces and fields..
Problem 17
Consider the electric field lines shown in the diagram below.
From the diagram, it is apparent that object A is ____ and object
B is ____.
6.2 Electric Force and Field
6.2.8 Solve problems involving electric charges, forces and fields..
Problem 18
Consider the electric field lines drawn at the right for a
configuration of two charges. Several locations are labeled on the
diagram. Rank these locations in order of the electric field
strength - from smallest to largest.
Answer: DAECB (with the order of C and B being in question)