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Coulomb’s Law

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.

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Coulomb’s Law

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

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

  3. 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?

  4. 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? 20.3 The Electric Field 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.

  5. The Electric Field

  6. Gravitational and Electric Fields

  7. 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 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).

  8. SI unit for electric field is the Newton per coulomb (N/C)

  9. CHECKPOINT: • What is the direction of the electric field due to the electron at • point S (b) point R? • What is the direction of the net electric field at • point S (d) point R? • A. Right or B. Left A. Right B. Left A. Right B. Left

  10. If four charges are placed at the corners of a square as shown, the field E is zero at • all points along the sides of the square midway between two charges • the midpoint of the square • midway between the top two charges and midway between the bottom two charges • none of the above Answer: B

  11. 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

  12. Registering for Mastering PhysicsCourse ID: PHYS1022EANDMRegistration code: USMPEY-TTBBO-SPEND-MINNA-WERSH-WIRES On the Access Information page, you will be asked if you have a Pearson Education account. If you have not yet registered – select Noand 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 Yesand 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/

  13. Deadlines Assignment 1 Coulomb’s Law must be completed by midnight on Friday 15 October. 

  14. The Electric dipole • a pair of equal and opposite charges • - molecules can be modelled approximately as dipoles

  15. 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

  16. 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 vectorpof magnitudep = qd in the direction from negative to positive.

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