Electricity & Magnetism

Transcription

1 Electricity & Magnetism Chapter 8 Student Learning Objectives Recall properties of charge Characterize static electricity Differentiate between series and parallel circuits Describe properties of magnets Analyze electromagnetic systems What is charge? All matter is composed of atoms which contain charged particles. electron proton neutron Charge Charge 1.6 x C +1.6 x C 0 Mass 9.1 x kg 1.67 x kg 1.67 x kg Practice: Why do the electrons stay in orbit around the nucleus? Law of conservation of charge: The total amount of electric charge in the universe remains constant. Charge is never created or destroyed. Neutral atoms have the same number of electrons and protons. 2 Atomic Number He

2 Objects become charged when they lose or gain electrons. (ionized) Opposite charges attract Like charges repel F = k(q1)(q2) d 2 The electric force and the gravitational force are both inverse square laws, and both apply mutual forces between two quantities. Electric Force Attract & Repel Strong Gravitational Force Attract Weak l k is a constant = 9.0 x 10 9 Nm 2 /C 2 l q = charge l d = distance between charges Repulsive and Attractive Electrical Forces Two negative charges repel Two positive charges repel One negative and one positive attract Repulse Repulse Attract Section 8.1 Practice 1) Does an object that has lost electrons have a positive or negative charge? Does it have more or less mass? 2) If an object has lost 100 electrons, what is the object s charge in Coulombs? 3) What is the electric force between the electron and the proton in a hydrogen atom? The average separation between the electron and proton is 5.29 x m. 4) Rubbing a balloon on your hair results in the transfer of 2000 electrons from your hair to the balloon. What is the force between your hair and the balloon if the balloon is 2 cm from your hair? 2

3 How do objects become charged? Electrons are lost or gained, or are redistributed within the object. Friction: electrons are knocked loose and transferred Contact: electrons are transferred Induction: charge is rearranged A net electric charge on an object results in static electricity. Examples: Cars & Clothes Practice 1) Is an object charged by induction ionized? 2) Static electricity on clothes and us is less common on humid days. Why? 3) Why does a plastic slide retain static charge while a metal slide does not? 4) How do you think fabric softener works? What is an electric field? An electric field surrounds charge, modifying the space near the charge. An electric field explains the action-at-a-distance between charged objects. Charge affects the space around itself. Objects are affected by the field established in that space. Lines of Force Electric fields interact. 3

4 What are the properties of electric circuits? Electric current is the net movement of electrons. Electrons are present in all electrical systems. When they begin to move, there is current. Current = I = q/t Two different electric potentials set-up an electric field, which causes existing electrons to flow from the high potential to the low potential. E Field Current EPE KE as charge is accelerated through the system. No difference in electric potential = no charge will flow. Examples: Wall Outlets (ac), Batteries (dc), Lightening, "Shock War", Bird on a Wire Voltage is created by a difference in electric potential energy. Electric resistance is a measure how difficult it is for electrons to move within the system. 1. Conductivity 2. Length 3. Diameter 4. Temperature Simple Electrical Circuit The light bulb offers resistance. The kinetic energy of the electric energy is converted to heat and radiant energy. Section 8.2 4

5 Electrical Circuit & Waterwheel Analogy Section 8.2 Ohms Law V = IR Power is the rate at which the electric energy is used by a system. P = IV Practice 1) A flashlight uses a 1.5 volt battery and has a resistance of 2 ohms. What is the current within the flashlight circuit? How many Watts will this flashlight supply? 2) A 2000 Watt hair dryer, a 100 Watt radio, and five 60 Watt light bulbs are being used simultaneously. Will this cause a 12 A safety switch to open? (V =110 volts) 3) An Xbox 360 console is attached to a 55 inch plasma TV. Each of these devices uses about 135 Watts. If you play the Xbox 360 on this TV for 4 hours, what is the cost? The current average utility rate is 22 /kwh. 4) A person with dry skin has a maximum resistance of 500,000 ; moist skin has a resistance of If the metal prongs of a plug are touched as the plug is placed into a 110 volt wall outlet, how much current could pass through the person s body? Is this dangerous? Don't try it! A Noticeable A Painful A Muscle spasms A Loss of muscle control Disrupts heartbeat 5

6 How are circuits wired? Series Circuits: single continuous loop Voltage is divided. (energy shared) Resistance is additive; it increases with each added piece. R = R 1 + R 2 + Parallel Circuits: multiple parallel loops Voltage is supplied to each parallel loop. (energy not shared) Resistance will drop; it decreases with each added piece. 1 = R R 1 R 2 Series Circuit R s = R 1 +R 2 +R 3 + If one bulb were to burn out the circuit would be broken, and all the lights would go out Same current all the way Section 8.3 Parallel Circuit I = I 1 +I 2 +I 3 + for R in parallel: 1/R p = 1/R 1 +1/R 2 + for 2 R in parallel: R p = (R 1 R 2 )/(R 1 + R 2 ) Current Divides, Voltage is the same Section 8.3 6

7 Practice: What do you think an electrical switch does to the circuit? Why is too much current in a wire dangerous? What are the properties of the magnetic field? All magnets have two poles; this is where the magnetic field is strongest. A north-seeking pole A south-seeking pole The poles are not charged! Like magnetic poles repel Opposite magnetic poles attract A magnetic field surrounds a magnet, modifying the space near the magnet. Practice: Compare and contrast electric charges and magnetic poles. What produces magnetism? The motion of electric charge is what makes a material magnetic. Magnets: Intrinsic magnetic moments of the electrons align. (spin) 7

8 Wires: Electric current forces magnetic moments to align. Convection: ionized material in motion produces a magnetic field. I B-Field qyhktwj4 The Earth is surrounded by a magnetic field. Protects us from solar wind Produces aurora And the poles switch! How are electric and magnetic effects related? Electric charge moving induces magnetism. Magnetic field changing induces current. E Fields and B Fields interact. Faraday s Law: Induced voltage in a coil is proportional to the number of loops, multiplied by the rate at which the magnetic field changes. More Loops + Turning Faster = More Voltage 8

9 Motors/Generators Motors Current induces magnetism Magnetic fields interact Electric energy Mechanical energy (Rotation) Generators Magnetic field induces current Coils turned in B Field Mechanical energy Electrical Energy Generators do not create energy, they convert it! Maxwell s Law: A magnetic field is induced in any region of space in which an electric field is changing with time. Current = Magnetism Example: Electric Transformer Electric transformers transform the voltage. Step down: fewer turns in secondary Step up: more turns in secondary V 2 =N 2 V 1 N 1 Practice: If a laptop needs only 12 volts and the wall outlet supplies 110 volts, what is the relative number of turns needed in each coil? What type of transformer is this? 9