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Transcript
Electricity & Magnetism Electrostatics Review Conductors allow charges to flow freely Insulators hinder flow of charges Are all conductors metal? Is earth a conductor or an insulator? Electrostatics Review Law of Electric Charges: Opposite charges attract Similar charges repel Charged objects attract some neutral objects Three ways to charge an object: Friction Contact Induction The Bohr-Rutherford model of the atom is a planetary model in which the electrons orbit around the atomic nucleus Electrons orbit the nucleus in orbits that have a set size and energy. The atoms of a solid are held tightly in place and the nuclei contain all of the protons, the positive charges. The negative charges are free to move within a solid from atom to atom. Vocabulary Electric charge-a basic property of matter described as negative or positive Static electricity-a build-up of stationery electric charge on a substance Elementary charge (e)-electric charge of magnitude equal to the charge on a proton and an electron Charging by Friction Electrostatic Series: A list that ranks the objects ability to take negative charges. · Rubber (Items at top take negatives) · Ebonite · Polyethylene · cotton · silk · wool · glass · acetate · fur/hair (Items at bottom lose negatives) Charging by Conduction Charging by Induction Electric Potential Difference Amount of work done per unit charge to move a charge V W E Q Q or E QV W = amount of work done to move a positive charge Q (J) Q = amount of charge (C) V = electric potential difference (voltage) Where does C come from? e=1.60 x 10-19C Electric Potential Difference Electric potential difference, or voltage, indicates the difference in electric potential energy of the charges (electrons) between two points in a circuit. The amount of charge (Q), given by amount of electrons, is measured in Coulombs 1 electron has a charge of e=1.60 x 10-19C, so Q=Ne, where N is the # of electrons Voltmeters measure electric potential difference and are connected in parallel in a circuit. They have the symbol Sources of electrical energy cause an increase in electric potential (voltage gain), whereas loads cause a decrease in the electric potential (voltage drop) Electric Potential Difference Example 1: Calculate the electric potential difference between the negative and positive terminals of a battery if 1500 J of electric potential energy is transformed to move 125 C of charge between the terminals. Practice Questions: p. 513 #1-5 Electric Current Electric charges moving from one place to another Will only occur in a conductor (e.g. copper wire) It is measured in a unit called amperes (A) Q I t I = electric current (A) Q = total charge (C) Δt = total time (s) Current + the Human Body Electric Current Current flows from a region of _________ charge to a region of __________ charge Here’s an example of a typical circuit diagram. Please label with an arrow of the direction of electric current flow. Just like a voltmeter measures voltage (electric potential difference), an ammeter measures amps (current) Voltmeter: parallel Ammeter: series Example - Calculate the amount of current through a wire that has 0.85 C of electrons passing a point in 2.5 minutes Current Practice Practice Questions: p. 518 # 1 - 9 Pictoral vs. Schematic Electric Circuit Symbols for elements of an electric circuit. Series and Parallel Circuits Equivalent Resistance Series Circuit: Req = R1 + R2 + R3 + … + Rn Parallel Circuit: 1 1 1 1 1 ... Req R1 R2 R3 Rn Series and Parallel Circuits Practice Problems: Pg. 642 # 33, 34 Pg. 646 #36, 37 Kirchhoff’s Laws Kirchhoff’s Voltage Law (KVL) http://www.wisconline.com/objects/ViewObject.aspx?ID=DCE3002 Vseries = V1 + V2 + V3 +.... Vparallel = V1 = V2 = V3 = ... Kirchhoff’s Laws Kirchhoff’s Current Law (KCL) http://www.wisconline.com/objects/ViewObject.aspx?ID=DCE3102 http://www.youtube.com/watch?v=MnJS9RWbZwI Iseries = I1 = I2 = I3 = ... Iparallel = I1 + I2 + I3 + ... Applying the Laws Vsource = 40V Vlamp1=10V Vlamp3=20V Vlamp2=? Vlamp4=? Isource=0.40A I3=0.10A I1=? I2=? Homework P. 522 Practice # 1 – 2 P. 522 Questions # 1 – 2 Ohm’s Law Potential difference between any two points varies directly as the current between the two points. V IR V = potential difference (V) I = current (A) R = resistance (Ω, ohm) Sample problems: Pg. 632 #24, 26 Power Power is the rate at which energy is being used or supplied. Same as previously defined in energy unit. E P t P = power (W) ΔE = energy used (J) Δt = time (s) Other useful power equations derived using: P VI V2 P R Sample Problems: pg. 655 #41, 42 E VIt and V IR P I 2R Cost of Electricity Electricity is charged by the amount of energy used. The rate that the power companies use is cost per kilowatt hour (kW·h) kW·h = energy used in 1 hour by a load with a power of 1 kW For example, it costs $10.87 to operate a 40” LCD television set for 30 days when used only 4.0h per day. Sample Problems: pg. 655 #41, 42 Electricity & Magnetism Magnetic Resonance Uses Cancer detection and staging Stroke and MS detection Spine evaluation Surgical planning and follow-up Sports injuries Heart disease detection Major Advantages No radiation … non-invasive Soft tissue visualization Image organ structure and function Spectroscopy, MRI Image at any angle (3D) Magnetic Resonance Imaging Magnetic Force & Fields Magnetic Force & Fields Law of Magnetic Forces: N S S N FORCE N S FORCE N S Magnetic Force & Fields Law of Magnetic Forces: 1. Opposite poles attract Magnetic Force & Fields Law of Magnetic Forces: 2. Similar poles repel Magnetic Field Lines Properties of magnetic field lines: Outside the magnet, begin on ________________ and end on ___________________. Inside the magnet, travel from _______________ to __________________. Never _______________. Spacing indicates the ___________________ of the force (i.e. the ______________ the lines, the greater the force.) Magnetic Field Lines Magnetic field around a bar magnet Magnetic field between a pair of opposite poles Magnetic Field Around the Earth Oersted’s Discovery Danish physicist Discovered electromagnetism in 1819 Was demonstrating the heating effects of an electric current in a wire Observed that a currentcarrying conductor caused the needle of a compass to move Electromagnetism Principle of Electromagnetism: Whenever an electric current moves through a conductor, a magnetic field is created in the region around the conductor. Right-Hand Rule #1 If a straight conductor with a current is held in the right hand with the right thumb pointing in the direction of the electric current, the curled fingers will point in the direction of the magnetic field lines. Thumb points in direction of current Magnetic lines of force from current Fingers of right-hand curl around in direction of field Straight Conductor – Top View The shaded inner circle represents the cross-section of a straight conductor carrying a current. Magnetic Field Around A Solenoid Solenoid – large series of coils or loops (of wire). Magnetic field created by a current flowing through a solenoid is similar to the field of a bar magnet. Direction of magnetic field depends on current direction. Right-Hand Rule #2 Fingers curl in direction of current flow Thumb points North The strength of the magnetic field of a coil depends on: Current in the coil Number of loops Type of core material (e.g. air, iron…) Motor Principle A current-carrying conductor crossing an external magnetic field, experiences a force perpendicular to the magnetic field and the direction of the current. Parts of a Motor -Commutator – split ring that rotates with the coil -Brushes – connects commutator and cell -Cell – provides current -Field magnet – provides magnetic field Motor Motor Right-Hand Rule #3 - Thumb in direction of current - Fingers in direction of magnetic field - Palm facing direction of force Faraday’s Law of Induction Law of Electromagnetic Induction An electric current is induced in a conductor whenever the magnetic field in the region of the conductor changes. Faraday’s Law of Induction Lenz’s Law When a conductor interacts with a magnetic field, there must be an induced current that opposes the interaction, because of the law of conservation of energy.