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General Physical Science Chapter 8 Electricity and Magnetism Electrical Charge Electrical charge is one of the 7 fundamental quantities. Two types of electrical charge – Positive (+) – Negative ((-) – 3 types of subsub-atomic particles Protons (+) ((-) Neutrons (no charge) Electrons Electrical Charge The magnitude of the proton charge is equal to the magnitude of the electron charge. – Charges will exactly cancel. Coulomb – Unit of electrical charge – +q = greater + charge = electron deficient – -q = greater - charge = electron rich 1 Electrical Force Exists between two charged particles Law of charges – Unlike charges attract and like charges repel. Magnitude of the force = Coulomb’ Coulomb’s Law – The force of attraction or repulsion between two charged bodies is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them. Coulomb’ Coulomb’s Law Mathematically F = kq1q2 / r2 – F = Force (N) – k = proportionality constant 9.0 x 109 N m2 / C2 Similar to gravitational law, however… however… – Much higher value for the constant. – Based on charges, not masses. – can be an attraction or repulsion Static Charge Excess or deficiency of electrons – Excess; negative charge – Defiency; Defiency; positive charge Walk across carpet in winter Polarization – separation of charge – rubbing rubber balloon in hair Induced polarization – A charge causes charge separation – reason balloon sticks to wall 2 Photocopier Se drum + charge Light reflected off paper removes some of the drum charge Toner has a - charge – sticks to + areas on drum Paper + charge gets toner from drum Toner ‘fused’ fused’ by heat Electrical Charge and Electrical Force Learning Goals – Describe electrical charge in terms of protons and electrons – State similarities and differences between Newton’ Newton’s Law of Universal Gravitation and Coulomb’ Coulomb’s Law Questions: Problems: 11-4 1, 3 Electrical Current Current – Rate of flow of electrical charge – Measured in Amperes (A) 1 A = 1C / 1s I=q/t General: – I=Current – q=charge – t=time q =Ixt 3 Current Conductors – current flows readily Does not ‘flow’ flow’ in the traditional sense – metals are good electrical conductors – ‘loose’ loose’ electrons Also reason for good thermal conductivity Insulators – current d/n flow – electrical wire coating Current Semiconductors – intermediate between conductor and insulator – allows some (but not free) current flow – graphite (resistors) – Silicon/Germanium mixtures computer the chips ‘semiconductor’ semiconductor’ industry Example A current of 0.50 A flows in a wire for 2.0 minutes. (a) How much (net) charge goes past a point in the wire in this time? (b) How many electrons make up this charge? What do we know? – a) 0.50 A (C/s), 2.0 min (120 s); q = I xt – q = 0.50 C/s x 120 s = 60C 4 Example 60 C Charge on a single electron is 1.6 x 10-19C How many electrons = 60 C – 60 C = x(1.6 x 10-19C) – x = 60 C / 1.6 x 10-19C – x = 3.8 x 1020 electrons – 380,000,000,000,000,000,000 electrons! Voltage Greater separation of charges means more work required to separate more charges. Electrical potential energy – Voltage (potential) – Work per unit charge – PE per unit charge Voltage Measure of electrical potential is the volt. (Work per unit charge) –1 V = 1 J / 1 C Measurement of the ‘friction’ friction’ for electrical current flow is called resistance. – Ohm - Ω Ohms Law - relationship between voltage, current, and resistance – V = IR 5 Current Flow Voltage - water pressure; Current = water flow; Resistance = water wheel Current Flow You must have a complete circuit for electricity to flow By convention, current flow is the way that positive charges would flow. – convention was developed before our understanding of protons and neutrons. – No practical difference because of this convention. Electrical Power Power = current x voltage – P = IV But V = IR – P = I2R Heat loss – All metals have some resistance Light bulbs (watts) – Lower T, lower resistance – Superconductivity at very low temperatures. 6 Example Find the current and resistance of a 60W, 120V light bulb in operation. What do we know? – Power = 60W, Voltage = 120 V – P = IV means 60 = I x 120 – I = 60/120 = 0.50 A – P = I2R means 60 = 0.502 x Ω – Ω = 60 / 0.502 = 240Ω 240Ω Current, Voltage and Electrical Power Learning Goals – Define current. current. – Define voltage, voltage, and state how Ohm’ Ohm’s law relates it to current and resistance. – Explain electrical power and identify the parameters used to describe it. Questions: 55-8 Exercises: 55-23 odd Simple Electrical Circuits Two types of current – Direct (battery) – Alternating (wall outlet) US Two ac typically 60 Hz basic types of electrical circuits – Series – Parallel 7 Series Circuit Hooked up as a chain – Same current at all points. – Total resistance sum of individual resistances. – Total voltage drop sum of individual drops. – Rt = ΣRi Parallel Circuit Hooked up like a rope. – Voltage across each resistance the same. – Current will vary in each branch. – 1/Rt = Σ(1/Ri) – For two resistors: Rt= (R1R2) / (R1+R2) Example Three resistors have values of 6.0Ω, 6.0Ω, 6.0Ω, and 3.0Ω. 3.0Ω. What is their total resistance when connected in parallel, and how much current will be drawn from a 12V battery if it is connected to the circuit? What do we know? – 6.0Ω, 6.0Ω, 6.0Ω, and 3.0Ω 3.0Ω – 1/Rt=1/R1 + 1/R2 + 1/R3 = 1/6 + 1/6 + 1/3 = 2/3 – Rt = 3/2Ω 3/2Ω – V = IR means I=V/R = 12/1.5 = 8.0A 8 Parallel Circuits Total resistance always less than the lowest resistance Household wiring – all in parallel – rest will work if one fails – Christmas lights ‘Shunt resistor’ resistor’ Electrical Safety Fuses – Edison base end can similar to lightbulb switch fuse ratings – TypeType-S bases different switch fuse ratings cannot – Circuit breakers – Fuses/CB always on ‘hot’ hot’ side 9 Electrical Safety Case grounding – ‘3rd’ wire Polarized Plug Simple Electrical Circuits and Electrical Safety Learning Goals – Define voltage and state how Ohm’ Ohm’s Law relates it to current and resistance – Explain electrical power and identify the parameters used to describe it. Questions: 99-13 Magnetism Lodestones – 6th Century BC – Magic Artificial Magnets – Chinese – Made from natural magnets 10 Magnetism Two regions of magnetic strength – North pole (N) points north – South pole (S) points south Law of Poles – Like poles repel and unlike poles attract. – Attraction/repulsion inversely related to the square of the distance between the poles. Magnetism All magnets contain both N and S pole – Dipoles Magnetic field – Imaginary lines indicating the direction a compass would point if it were placed near a magnet. – Lines of force Magnetism Use metal filings to ‘see’ see’ magnetic field. – iron filings become magnetized induced magnetism Screwdrivers – serve to show lines of force. 11 Magnetism Source of magnetism is moving/spinning electrons! – Can generate magnetic field with moving electric field Strength directly proportional to current – Electromagnets Electromagnetic Fields Magnetism Ferromagnetic materials – internal magnetic domains – Random arrangement means nonnon-magnetic – Can induce alignment with external magnet Tends to randomize with removal of external magnet. 12 Magnets – ‘Soft’ Soft’ iron temporary magnetism – ‘Hard’ Hard’ magnetic material = permanent magnet ‘Hard’ Hard’ iron Co, some other elements Ceramics Ni, – Electromagnets Magnets Curie Temperature – will convert a permanent magnet to a nonnonmagnetic material. Earth’ Earth’s magnetic field – Proposed by William Gilbert – Aurora Borealis and Aurora Australis – Origin unknown Theorize due to Earth’ Earth’s rotation Earth’ Earth’s Magnetic Field Approximates bar magnet Known to ‘shift’ shift’ with time Magnetic and true poles d/n coincide – must apply magnetic correction for ‘true’ true’ direction 13 Isogonic Lines in Aviation Magnetism Learning Goals – State the law of poles and describe the magnetic field. – Identify the cause of magnetism, and tell why some materials can be magnetized and others cannot. – Analyze some aspects of Earth’ Earth’s magnetic field. Questions: 1414-18 Electromagnetism Interaction of electrical and magnetic effects. – Moving electric fields generate magnetic fields – Magnetic fields deflect a moving electric charge. Telephone receiver. 14 Telephone Receiver Microphone – Diaphragm that can vibrate and change resistance (carbon mic) mic) Varies electric current Speaker – electromagnetic coil – current varies strength – diaphragm vibrates Magnetic Force on a Moving Electric Charge. Motors and Generators No moving current - no force current - force on wire Moving 15 Motors and Generators Brushes – reverse current – cause attraction to become repulsion and repulsion to become and attraction. Generators Same basic principle as motors AC - rotate a wire loop between magnets – Faraday How do we transmit? Electrical Transmission Transformers – Cause change in voltage – windings – iron core Use to decrease current – Decreases I2R loss 16 Transformers For a transformer: – V2 = (N2 / N1) x V1 N1 = number of windings in primary coil = number of windings in secondary coil V1 = voltage at primary coil V2 = voltage at secondary coil N2 Will not work with dc Electronics Emission and control of electrons Television Diodes and Transistors Integrated circuits Electromagnetism Learning Goals – Identify some electromagnetic interactions and applications. – Distinguish between motors and generators. generators. – Explain the principle and use of transformers. 17 Electromagnetism Questions: 1919-24 Exercises: 25, 27 Key Terms; Matching, Multiple Choice, and FillFill-inin-thethe-Blank Questions; Visual Connection and Applying your Knowledge General Physical Science Chapter 8 Electricity and Magnetism 18