Ch.7 Electric Charges & Electric Fields. 7.1 Properties of Electric Charge

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1 Ch.7 Electric Charges & Electric Fields 7.1 Properties of Electric Charge

2 Introduction to Electric and Magnetic Fields The radiation produced by the magnetic and electric fields of electron accelerators is used to treat tumours to increase the survival rate of cancer patients. But this must be properly used to ensure that patients receive safe doses of radiation as do medical personnel. Radiation Therapy

3 Applications of Electric and Magnetic Fields Electrostatic precipitators or scrubbers remove particulate matter in the gas emissions of industrial processes to reduce air pollution. An electrocardiogram (EKG or ECG) interprets the electrical activity of the heart Photocopiers and fabric softener sheets both use principles of electric charges Safety grounding methods: o o static wicks on airplanes help to discharge the plane of charge build-up due to air friction while flying Ground plugs and ground fault circuit interrupters (GFCIs) in plugs/outlets help to send excess current to the ground to avoid electrical shock

4 The Laws of Electric Charges Opposite electric charges attract each other. Like electric charges repel each other. Charged objects attract some neutral objects.

5 What is Electric Charge? Neutral atom has a charge of zero b/c # of e- = # of p+ Ions can be +ve charged: # of p+ > # of e- called a cation (meow) Ions can be -ve charged: # of e- > # of p+ called an anion

6 What is Electric Charge? The basic unit of electric charge is the Coulomb (C) where e denotes elementary charge (either electron or proton) e = x C The total charge on an object is denoted by the symbol q 1 C is apprx. the amount of electric charge that passes through a 60 W light bulb in 2s

7 Types of Electrical Material Conductors Electrons are able to move easily from atom to atom Ex: copper, gold, aluminum, saltwater, humid air Insulators Electrons are not free to move easily from atom to atom Ex: plastic, glass, rubber, pure water, dry air Semiconductors Have very few conduction electrons in their intrinsic state, but can be highly conductive after the process of doping Ex: silicon, germanium

8 Methods of Charging Friction Depends on the force of attraction between a nucleus and its outer electrons. i.e. electron affinity Induction When a charged (+/-) object is brought close to a neutral object, without contact. Contact When a charged (+/-) object makes contact with a neutral object.

9 The Law of Conservation of Charge The Law of Conservation of Charge: Charge can be created or destroyed but the total charge (the difference between the amounts of positive and negative charge) within an isolated system is always conserved. In all methods of charging, 1 object gains e- while another loses the same amount

10 Ch.7 Electric Charges & Electric Fields 7.2 Coulomb s Law

11 Brief History Charles-Augustin de Coulomb ( ) experimented with the forces that exist between any two electric point charges. Point charges are extremely small particles (ie. no measurable dimensions) that carry a charge. Electric charge (q) is measured in Coulombs (C)

12 Coulomb s Law The force between two point charges is inversely proportional to the square of the distance between the charges and directly proportional to the product of the charges. F E = magnitude of electric force (N) F E = kq 1 q 2 r 2 This applies when the 2 charges are very small compared to the distance between them. k = Coulomb s constant = 9.0 x 10 9 N m 2 /C 2 q 1 & q 2 = magnitude of the charges (C) r = distance between the centers of the two charges (m)

13 Coulomb s Law By Newton s 3 rd Law, the electric force exerted on charge A by charge B is equal in magnitude and opposite in direction to the force exerted on B by A. A q F E F E 1 q 2 r B

14 Comparing Laws Coulomb s Law Electric Force (F E ) Newton s Law Gravitational Force (F g ) repulsive q 1 or q 2 m 1 attractive m 2 attractive F E q 1 q 2 F g m 1 m 2 F E 1 r 2 F g 1 r 2 F E = kq 1 q 2 r 2 F g = Gm 1 m 2 r 2

15 2.7 N (Note: B/c charges are +, they would repel each other.) Example 1 Two point charges, q 1 = 4.0x10-6 C and q 2 = 3.0x10-6 C, are 0.20 m apart. What is the magnitude of the electrostatic force between them? q 1 q m

16 Vector Nature of Electric Force Coulomb s law only describes the force that exists between two point charges. For more than two charges, we must first consider two charges at a time. Once we have calculated the forces between charge pairs, we can determine the overall force on any one charge by calculating the vector sum of all the forces. (called superposition principle)

17 Example 2 Three point charges, q 1 = 4.0x10-6 C and q 2 = -2.5x10-7 C, and q 3 = 6.4x10-6 C, are arranged in a one-dimensional line, as shown. Find the total force on q 3. q 1 q 2 q cm 10.0 cm 1.1 N [right]

18 Example 3 The diagram shows three excess electrons inside a conductor with a circular crosssection. Find the net force, including direction, on each electron. q 1 = q 2 = q 3 = 1.602x10-19 C 0.75cm q 3 q cm 7.1x10-24 N [outward, 150 away from sides] 0.75cm q 2