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Topic 5Electric currents
TEST next Thursday 2nd
Feb
Topic 6Fields and
Forces
Definitions and copy of the
syllabus
Electric Force and Electric field
We already know that;
Electric Force and Electric field
1. There are two types of electric charge (positive and negative)
Electric Force and Electric field
2. Static charges can be produced by the action of friction on an insulator
Electric force and electric field
3. Conductors contain many free electrons inside them (electrons not associated with one particular atom)
Electric Force and Electric field
4. Charge is conserved. The total charge of an isolated system cannot change.
I’m indestructible!
So am I!
Electric Force and Electric field
The force between two charges was investigated by Charles Augustin Coulomb in 1785
Electric Force and Electric field
Coulomb found that the force between two point charges is proportional to the product of the two charges
F α q1 x q2
and inversely proportional to the square of the distance (r) between the charges
F α 1/r2
Coulomb’s law
It follows that
F α q1q2
r2
or F = kq1q2
r2
Coulomb’s law
F = kq1q2
r2
The constant k is sometimes written as
k = 1/4πεo
where εo is called the permittivity of free space.
Calculations using Coulomb’s law
The force between two charges is 20.0 N. If one charge is doubled, the other charge tripled, and the distance between them is halved, what is the resultant force between them?
q1q2
r
r/2
2q1 3q2
F = 20N
F = ? N
Calculations using Coulomb’s law
F = kq1q2/r2 = 20.0N
x = k2q13q2/(r/2)2 = 6kq1q2/(r2/4) = 24kq1q2/r2
x = 24F = 24 x 20.0 = 480 Nq1
q2
r
r/2
2q1 3q2
F = 20.0N
x = 480 N
Electric field
An area or region where a charge feels a force is called an electric field.
The electric field strength at any point in space is defined as the force per unit charge (on a small positive test charge) at that point.
E = F/q (in N.C-1)
Force on a charge
• This means the force on a charge q is given by
• F = Eq
• If the charge is a proton or electron
• F = Ee where e = 1.6 x 10-19 C
Electric field around a point charge
If we have two charges q1 and q2 distance r apart
F = kq1q2/r2
Looking at the force on q1 due to q2, F = Eq1
F = kq1q2/r2 = Eq1
E (field due to q2) = kq2/r2
q1 q2
NOT in data book
Electric field
Electric field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one charge) must use vector addition.
q1 q2
Field here due to both charges?
Electric field
Electric field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one charge) must use vector addition.
q1 q2
Field here due to both charges?
Field due to q1
Electric field
Electric field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one charge) must use vector addition.
q1 q2
Field here due to both charges?
Field due to q1
Field due to q2
Electric field
Electric field is a vector, and any calculations regarding fields (especially involving adding the fields from more than one charge) must use vector addition.
q1 q2
Resultant field
Field due to q1
Field due to q2
Electric field patterns
An electric field can be represented by lines and arrows on a diagram, in a similar ways to magnetic field lines.
Electric field patterns
An electric field can be represented by lines and arrows on a diagram , in a similar ways to magnetic field lines.
The arrows show the direction of force that would be felt by a positive charge in the field
Electric field patterns
An electric field can be represented by lines and arrows on a diagram , in a similar ways to magnetic field lines.
The arrows show the direction of force that would be felt by a positive charge in the field
Electric field patterns
An electric field can be represented by lines and arrows on a diagram , in a similar ways to magnetic field lines.
The closer the lines are together, the stronger the force felt.This is an
example of a radial field
Field around a charged metal sphere
E = 0 inside the sphere
Field around two point charges
Field around two point charges
Field between charged parallel plates
Uniform field E = V/dV
d
“Edge effects”
NOT in data book
Remember!
The force F on a charge q in a field E is
F = Eq
Parralel plates
• E = V/d and E = F/q
• So V/d = F/q
• Useful!!!!
Electric field hockey!• http://phet.colorado.edu/sims/electric-hockey/electric-hockey_en.jnlp
Let’s do some reading and try some questions
Read pages 285 to 287 and 289 to 295.
Questions:
Page 287 qs 3, 4, 5, 11.
Page 296 qs 4,5,6,7,11
