Electric Charges, Forces and Fields Chapter 16

Electric Charge Two types of charge, positive and negative Electrostatic Force: Like charges repel, unlike charges attract Charge is measured in Coulombs [C] qe = -1.6 X 10-19 C qp = +1.6 X 10-19 C

Electric Charge Electricity involves the study of electrically charged objects. Static electricity – charges that aren’t moving (static) Electric current – charges that are moving. Conservation of Charge: The net charge of an isolated system remains constant.

Example You shuffle across a carpeted floor on a dry day and the carpet acquires a net positive charge. Will you have a deficiency of electrons or an excess of electrons? If the carpet acquired charge of magnitude 2.15 nC, how many electrons were transferred?

Electrostatic Charging Conductors – have valence electrons which are loosely bound. Electrons can be removed relatively easily, allowing the flow of electric charge. Excess charge added to a conductor will distribute throughout the conductor. Insulators – have valence electrons which are tightly bound. Electrons cannot be easily removed and will not allow the flow of charge. Excess charge added to an insulator will sit in one place and not redistribute. Semiconductors - materials with conductivity between that of insulators and conductors.

Detecting Static Charge An electroscope is a device used to detect static charge. The leaves of the electroscope are made of conducting material. Leaves spread in the presence of electric charge.

Electroscopes

Electrostatic Charging Charging By Friction Charging by Conduction (Contact) Charging by Induction (no contact) Charge separates – polarization of charge

Grounding The Earth is a huge reservoir of electric charge – it will accept a huge amount of excess charge. Items which are grounded are connected to the ground so that excess charge will be removed.

Millikan Millikan devised an experiment to measure the charge on one electron! He used an atomizer to spray droplets of oil. The spray process causes the droplets to be charged due to friction. Millikan balanced the weight of the drop with the force of attraction

Coulomb’s Law – Electric Force Coulomb’s Law is used to calculate quantitatively the electric force (attraction or repulsion) between charged objects. k = 8.99 X 109 Nm2/C2

Coulomb’s Law Force is directly proportional to charge Force is inversely proportional to square of distance Force is negative for attractive, positive for repulsive

Example Two point charges of -1.0 nC and +2.0 nC are separated by a distance of 0.30 m. What is the electric force on each particle?

Example – Coulomb’s Law with Vectors A system of three charges is shown on page 514 , Figure (b). Find the net force acting on charge 3 due to charges 1 and 2.

Homework Read Sections 15.1 and 15.2 Read Examples 15.4, 15.5 on pages 515, 516. Do # 1, 3, 5, 6, 10, 11, 12, 14, 15, 17, 24, 25, 29, 32, 33, 35, 36, 38, 39 page 530

Force Fields A force field shows the force that would exist on a mass or a charge in a region of space, due to the presence of another mass or charge. Gravitational force field is measured in N/kg since Fg is a force between masses. Electric force field is measured in N/C since Fe is a force between charges

Gravitational Force Field Gravitational field points towards the mass since Fg is always attractive. Gravitational field gets weaker as distance increases g = Gm/r2

Electric Field Electric field, E, shows the force that would exist on a positive test charge placed in that region of space. E = Fe/q [N/C] E = kq/r2 E is a vector quantity

Example Two point charges are placed on the x-axis. q1 = 1.5 μC located at x=0 and q2= 6.0 μC at x=0.60m. Find all locations where the electric field is zero.

Example Three charges are located as follows: q1 = 1.00 μC located at (3.0m, 0); q2 = 2.00 μC located at (-5.0m, 0) and q3 = -1.50 μC located at (0, 4.0m). Find the electric field at the origin.

Field Lines Electric field is depicted using ‘lines of force’: Lines closer together shows stronger electric field. Electric field is tangent to the field lines. Electric field lines point from positive charge towards negative charge. The number of field lines entering (exiting) a charge is proportional to the magnitude of the charge. Field lines never cross.

Parallel Plates The electric field between two parallel plates is E = 4πkQ/A where Q is the charge on one plate and A is the area of one plate. Parallel plates are used as capacitors in electronics.

Homework Read 15.4, particularly example 15.9 on page 524. Do # 41, 43, 45, 47, 50, 51, 54, 58, 59, 60

Conductors and Electric Fields Inside a conductor, charges have the ability to move. They will spread out as far as possible in order to minimize forces of repulsion. Any excess charge on a conductor resides entirely on the outer surface only! The electric field inside a conductor is zero.

Conductors and Electric Fields Electric field lines are always perpendicular to the surface of the conductor. On a conducting sphere, excess charge spreads evenly. The electric field lines outside the conducting sphere are evenly spaced. On an irregular shape, the charges accumulate at sharp points or regions of high curvature. Electric field lines in these regions are more dense.

Electric Field near Conductor