Coulomb’s Law Last lecture: Electric Charge Charge conservation Charge quantisation Conductors and insulators

EXERCISE: Draw a + on the figure below to show the position or positions where a proton would experience no net force. 4++

Would the force on an electron at this position be A. to the left, B. to the right, or C. zero? Answer Answer: C. the force on an electron would be zero, the same as for the proton. So what is special about this position?

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? Charges and Forces: a closer look 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

The Electric Field

Gravitational and Electric Fields

A small test charge q 0 near a system of charges q 1, q 2,….., experiences a force F that is proportional to q 0. The ratio F/q 0 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 (b) point R? What is the direction of the net electric field at (c)point S (d) point R? A. Right or B. Left A. Right 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

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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) Do on board Problem 51: Electric field on the x axis - compare results with the electric field on y axis.

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.