Foundation year General Physics PHYS 101 Lecture 9: Coulomb’s law and Electric field Instructor: Sujood Alazzam 2015/2016 1

CHAPTER OUTLINE  Electric Charge.  Insulators and Conductors.  Coulomb’s law.  Electric field and Electric Potential.  Currents and Resistance.  Magnetic field.  Generators and Motors.  Transformers.  Galvanometer.  Mass Spectrometry. 2

3.3 Coulomb’s Law Coulomb’s law gives the force between two point charges: The force is along the line connecting the charges, and is attractive if the charges are opposite, and repulsive if the charges are like. 3

Coulomb’s law (force between 2 point charges) : [q] = Coulomb = C 4

Example 3.1: Force Between Two Charges A 1.0  C charge is at x = 1.0 cm, & a  1.5  C charge is at x = 3.0 cm. What force does the positive charge exert on the negative one? How would the force change if the distance between the charges tripled? (  C=10 -6 C) 5

Example 3.2: The electron and proton of a hydrogen atom are separated (on the average) by a distance of approximately. Find the magnitudes of the electric force between the two particles? 6

Example 3.3: Consider three point charges located at the corners of a right triangle as shown in this Figure, where q 1 = q 3 = 5.0 μC, q 2 = -2.0 μC, and a = 0.10 m. Find the resultant force exerted on q 3.? 7

8

3.4.1 The Electric Field Definition of the electric field: Here, q 0 is a “test charge” – it serves to allow the electric force to be measured, but is not large enough to create a significant force on any other charges. 9

The electric field of a point charge points radially away from a positive charge and towards a negative one. Cont. 10

Just as electric forces can be superposed, electric fields can as well. Cont. 11

3.3.2 Electric Field Lines Electric field lines are a convenient way of visualizing the electric field. Electric field lines: 1.Point in the direction of the field vector at every point 2.Start at positive charges or infinity 3.End at negative charges or infinity 4.Are more dense where the field is stronger 12

The charge on the right is twice the magnitude of the charge on the left (and opposite in sign), so there are twice as many field lines, and they point towards the charge rather than away from it. Cont. 13

Combinations of charges. Note that, while the lines are less dense where the field is weaker, the field is not necessarily zero where there are no lines. In fact, there is only one point within the figures below where the field is zero – can you find it? 14 Cont.

A parallel-plate capacitor consists of two conducting plates with equal and opposite charges. Here is the electric field: 15 Cont.

Example 3.4: A charge q 1 = 7.0 μC is located at the origin, and a second charge q 2 = -5.0 μC is located on the x axis, 0.30 m from the origin (see this Figure). Find the electric field at the point P, which has coordinates (0, 0.40) m. 16

17