Download Lecture 2 ppt version - University of Southampton

Last lecture:
Electric Charge
• Charge conservation
• Charge quantisation
Conductors and insulators
Coulomb’s Law
+
EXERCISE: Draw a
on the figure below to show the
position or positions where a proton would experience no
net force.
4+
+
Answer
Would the force on an electron at this position be
A. to the left, B. to the right, or C. zero?
Answer: C. the force on an electron would be zero, the same as
for the proton.
So what is special about this position?
20.3 The Electric Field
Charges and Forces: a closer look
Questions:
How does q ‘know’ of the presence of any
of the Qs?
If we move one of the Qs, the force on
q changes. Does it change immediately?
An electric field is set up in the space surrounding a charge. It has both
magnitude and direction, ie it is a vector field.
Information about the move of a charge travels outward (in all directions)
as an electromagnetic wave at the speed of light c.
The Electric Field
Gravitational and Electric Fields
A small test charge q0 near a
system of charges q1, q2,…..,
experiences a force F that is
proportional to q0.
The ratio F/q0 is the electric field
at that point (the field point).
The net electric field at a point P
due to a distribution of point
charges is found by summing the
fields due to each charge
separately
SI unit for electric field is the
Newton per coulomb (N/C)
CHECKPOINT:
What is the direction of the electric field due to the electron at
(a) point S
A. Right
(b) point R?
B. Left
What is the direction of the net electric field at
(c) point S
A. Right
(d) point R?
A. Right or B. Left
B. Left
If four charges are placed at the corners of a
square as shown, the field E is zero at
A. all points along the sides of the square
midway between two charges
B. the midpoint of the square
C. midway between the top two charges and
midway between the bottom two charges
D. none of the above
Answer: B
Example 20.4 p 335
Finding the field of two protons
Two protons are 3.6 nm apart. Find the electric
field at a point between them, 1.2 nm from one of
the protons. Then find the force on an electron at
this point.
Go over this example
Registering for Mastering Physics
Course ID: PHYS1022EANDM
Registration code: USMPEY-TTBBO-SPEND-MINNA-WERSHWIRES
On the Access Information page, you will be asked if you have a
Pearson Education account.
If you have not yet registered – select No and enter your log in
name (ie your university email address), and follow instructions
for setting your password.
If you have already registered for the e-Book - select Yes and
enter your log in name, and the same password.
The link to the presentation with all instructions for registering is
on the front page of my course.
http://www.phys.soton.ac.uk/teach/year1/notes/phys1022/
Deadlines
Assignment 1
Coulomb’s Law
must be completed by midnight on Friday 15
October.
The Electric dipole
- a pair of equal and opposite charges
-- molecules can be modelled approximately as dipoles
Example 20.5 p 335
A molecule may be modelled approximately as a positive
charge q at x = a and a negative charge –q at x= -a.
Find an expression for the electric field on the y axis.
Find an approximate expression valid at large distances
(y>>a)
Problem 51:
Electric field on the x axis - compare results with the
electric field on y axis.
Do on board
The Electric dipole moment
The field of a dipole depends on
both the distance and the
orientation of the dipole, since it is
not spherically symmetric.
Hence the dipole moment is defined
as a vector p of magnitude p = qd in
the direction from negative to
positive.