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MAGNETIC FIELDS Pre-Class for Wednesday 5-2-12 True or False: Electricity and Magnetism are related. • Begin next chapter: Ch24: Magnetic Fields – Brief history of magnets Some Definitions • Magnetic Force – The interaction when two magnets are brought close together – Can be attraction or repulsion • Magnetic Field – Represents the effect of a magnet on the area around it • Magnetic Poles – Two for every magnet – Strongest field found here – North (out) and South (in) Observing Magnetic Interference Equipment needed: paper clip, ring stand, clamp, bar magnet, string Material tested Result Paper Fabric Scissor blade Aluminum foil Other: Procedures a. Clamp a bar magnet vertically to the ring stand. b. Tie string around a paper clip. c. Anchor the end of the string under a book. Slowly pull the string until the paper clip floats in the air. d. Without touching the paper clip, slide various objects between the magnet and paper clip. e. Record observations Pre-class for Thursday 5-3-12 The location of the strongest magnetic field on a magnet is at the _____. A. Magnetic force B. Magnetic poles C. Magnetic domains D. Electromagnets Why a magnetic field? • The mantle is on a tectonic plate, which floats on a river of liquid rock/metal • The magma is flowing, just like a river • Friction between the plates and magma cause charges to move, creating a magnetic field MAGNETIC FORCE DUE TO A CURRENT-CARRYING WIRE F=BIL F force of magnetic field (N) B magnetic field strength (T or gauss) I current in wire (Amp) L length of wire (m) #1 Right Hand Rule B field I • Fingers point direction of magnetic field (curved around the wire) • Thumb point direction of current MAGNETIC FORCE DUE TO A CHARGED PARTICLE IN A MAGNETIC FIELD F=qvB F q v B magnetic force (N) charge (C) velocity (m/s) magnetic field (T or gauss) #2 Right Hand Rule • Fingers point direction of magnetic field • Thumb point direction of motion for the particle • Force on charge due to magnetic field comes out of the palm Works for positive charges only!!!(use left hand for negative) B field P+ e- X F F What direction will the force be? B field P+ e- X X X X X X X X X X X X X X X X X X X X F F What direction will the force be? Pre-class for Monday 5+7=12 Objects that keep their magnetic properties for a long time are called _____. a. Permanent magnets b. Electromagnets c. Magnetic domains d. Temporary magnets CH 24 HW CH 25 HW 59. move left or spin 63. 0.04 T 65. ccw 66. into page; out of page 74. 2.4 N 78. 3000 Amps 84. .742 T; 2.66x1016 m/s2 92. 2.1x10-12 N, upward 60. 0.5 T 61. 0.89 Volts 73. 0.13 Volts; 0.13 Amps; cw; opposite 75. 1,800 Volts; 30 Amps; 3,600 Watts 76. 36 turns; 9.4 mAmps 77. 2:1; 5 Amps 80. 3600 Volts, 90 Amps; 1.1x104 Watts Electromagnetic Induction The process of producing a current in a wire using magnetism Electromagnet • A magnet produced by current electricity • No current, no magnet • Wire must be wrapped to combine the magnetic field produced • A metal (Iron, etc.) core can help, but is not necessary • How magnets are made Making Your Own Electromagnet • Materials – – – – – Wire Battery 10 paper clips Nail or bolt Christmas light # of turns # of paper of wire clips 5 10 20 30 • Procedures – Make an electromagnet – Do not complete the circuit until you are ready! Battery will get hot! – Fill out the chart with the proper number of turns and paper clips. • Objective – Construct an electromagnet and pickup paper clips. Electromotive Force • The voltage produced by a battery to move energy around a wire • It is not a force—it was named this because it was originally thought charges were being “forced” to move around a wire • Units: Volts EMF = B l v sinq Pre-class for Tuesday 5-8-12 Find the direction of the magnetic field on the inside and outside of the solenoid shown. 2 TYPES OF CURRENT • Alternating (AC) Current – Current changes direction – Current flows both ways • Direct (DC) Current – Current flows one direction only – Defined by Ohm’s Law But, not these Transformers!!! TRANSFORMERS • A device used to increase or decrease the voltage in an AC circuit • 2 basic types – Step-up: increase voltage – Step-down: decrease voltage • Used to change the voltage sent from the power company to your house CH 24 HW CH 25 HW 59. move left or spin 63. 0.04 T 65. ccw 66. into page; out of page 74. 2.4 N 78. 3000 Amps 84. .742 T; 2.66x1016 m/s2 92. 2.1x10-12 N, upward 60. 0.5 T 61. 0.89 Volts 73. 0.13 Volts; 0.13 Amps; ccw; opposite 75. 1,800 Volts; 30 Amps; 3,600 Watts 76. 36 turns; 9.4 mAmps 77. 2:1; 5 Amps 80. 3600 Volts, 90 Amps; 10800 Watts for both Pre-class for Wednesday 5-9-12 The function of an electric motor is to _____. a) b) c) d) Change electrical energy to chemical energy Change chemical energy to electrical energy Change mechanical energy to electrical energy Change electrical energy to mechanical energy • Materials – – – – Electric Motor or Generator Battery Hand crank generator Light bulbs Switch • Objective – To make an electric motor and generator • Questions – Which part models an electric motor? Explain. – Which part models an electric generator? Explain. – What would happen if a battery with a larger voltage was used? – Where is the magnetic field the strongest? Explain. Procedures A. Connect the hand generator, light bulb, and switch in series. Turn the hand crank and record the light bulb brightness. B. Remove hand generator and connect battery instead. Record light bulb brightness. C. Connect battery to hand generator and switch. Close switch. Record observations.