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Transcript
III. Electromagnetic Induction. force on a magnet Oersted: Moving charge (I) exerts a _____________________. N’s 3rd law: A magnet exerts a __________________________ force on a moving charge . magnet Dq S A: a wire at rest ________ w/ current N v v q moving B: __________ charge __________ C: wire with no current ____________ but moving ____________ all 3 cases The charges in ____________________ experience magnetic a _________________ force Fmag. For any charge q moving with a velocity v through a B: Magnitude (strength) of Fmag: qvB sinq Fmag = ________________, q is angle between v and B. 0 Notice: If q = 0, then Fmag = _______ . Direction of Fmag. perpendicular Fmag is _______________________ to v and B. Ex: A charged particle moves at different angles to B. Assume v is the direction of the charge's motion. 0 45 0 00 q = _____ 90 q = _____ q = _____ z Fmag v x z B Fmag y max. Fmag = _______ z Fmag=0 B B y x v med. Fmag = _______ v x min. Fmag = _______ y Ex: The force between 2 current-carrying wires is used ampere to define the ________________: I wire 1 > Fmag wire 2 I Fmag > ampere 1 ______________ = the amount of current in two infinite wires of negligible diameter separated by a distance 1 meter of _________________ that will produce a magnetic force of -7 N 2 x 10 _______________________ per meter of length. attractive Note: The force is ___________________ when the current is in same direction in both wires. Case C (again): A wire conductor moved in a B field the magnet could be moved or __________________________________ S N E Fmag causes the ______________________________________ electrons to move to one end of the wire. induces (or sets up) This ___________________________ an electric field in the induced potential difference, wire, which results in an _______________________________ induced voltage or _________________________between the ends of the wire. Electromagnetic Induction: Relative “______________ motion between a conductor and a B field potential difference in the conductor.” induces (creates) a ___________________ magnetic force depends on Because the strength of the _________________ v of the charge and the B field, the angle between the __________________ potential difference also does. the amount of induced ______________________ Ex 1. A wire moved between two magnets. 900 Its velocity v is perpendicular to B: q = ______ v N S v directly through The wire "cuts" _____________________ the most B lines max. 900 , so Fmag is ________. The angle between v and B is _______ Maximum potential difference V is induced in the wire. ____________ In symbols: v B max. V Ex 2. A wire moved at an angle: 00 < q < 900 v N S v some The wire cuts ________________B field lines. not a max., Because the angle between v and B is ____________ less than max. Fmag is ____________________ Less V is induced in the wire than in previous case. _______ Ex 3. A wire moved parallel to B: q = 00 N v v S none of the The wire cuts ________________B field lines. zero Because the angle between v and B is ____________ zero Fmag is _____________ No _______ V is induced in the wire. In symbols: v || B min. V (=0) In sum: Case angle between v and B (0) induced potential difference 1 90 max. 2 between 0-90 in between 3 0 min. potential Notice that in all three cases above, a ________________ difference ________________ was set up that is a result of the relative motion ________________________ between the wire and B field. no current But unless the wire is attached to a circuit, _____________ will flow ________________ . Electromagnetic devices use magnetism and electricity: A 1. motors (based on Case _____ from first slide) current A ___________ in wire experiences a force due to B. turn a coil of wire If that force can be made to ____________________, then you can transform: electrical mechanical energy ______________ energy _______________ speaker Another example of this is a ___________________ coil of wire It is a ______________________ that is attached to a paper cone. When current passes through the wire, the magnetic force makes the paper cone move. electrical ______________ energy _______________ mechanical energy _______________ energy sound Simple DC motor F v B v B F Instead of permanent magnets, you can use the current from the battery to create electromagnets to provide the B field. C 2. generators (based on Case _____ from first slide) When a wire with no current is moved, the Fmag electrons to the ends of the wire, but pushes the ____________ no charge flows connected ___________________unless the wire is ______________ to a circuit: __________________ N S v current With a complete circuit, there is ______________, and mechanical energy ______________ electrical energy _____________ A simple AC generator: Instead of permanent magnets, you can use some of the output current to create electromagnets to provide the B field. B field has a constant direction, so Fmag depends on the direction of v: F B v v B F turn counterclockwise The electromagnetic force pushes charges out the "short" wire Now coil has turned so that it is vertical: v B B v turn counterclockwise No force on electrons or induced voltage because v and B are parallel (or antiparallel) Now coil has made another ¼ turn or so… F B v B v F turn counterclockwise The electromagnetic force pushes charges out the long end –in other words…in the opposite direction!!! Then you would see something like this: And, as the coils were turned, it would look like: Now imagine looking straight down the rings B always left This Direction of induced current: I t I t I t I t is how AC is m a d e ! same Electrical motors and generators are basically the _______ devices—they have _________ wires that turn in _____________: B fields motor generator mech. energy ________________ out electric energyout ______________ electric energy in ________________ mech. energy in _______________ Instead of moving a wire through a B field to induce a voltage, you can also change the B field near the wire by moving the magnet ______________________ magnetic strip. 3. Credit cards have a unique ____________ B field in the ATM. Moving the card changes the __________ voltage in a coil in the ATM. This induces a unique ____________ magnets 4. Guitar pickups are coils wrapped around ___________. steel When a ____________ guitar string vibrates, it changes the B field of the magnet. This induces a _______________ voltage __________ speaker in the coil which is amplified and sent to a _____________ .. wire coils 5. Transformers two ________________ wrapped iron core around the same ____________: iron core primary AC A. ______current in _______________ coil causes… B field secondary B. …the __________ at ________________ coil to change… potential difference (voltage) C. …which induces a ______________________________ secondary in the _________________ coil. > N1, then voltage is ___________________________ stepped up (increased) If N2 ___ . stepped down (decreased) . < N1, then voltage is ___________________________ If N2 ___ created destroyed : Energy is neither _______________ nor _______________ Power in = Pin = VinIin = Step up: Power out Pout VoutIout Iin = VoutI Vin Step down: VinI in = out Iout Vout stepped up Voltages are ____________________ for long-distance I2R transmission because less heat or ________ energy is lost. Once they reach their destination, they are stepped down _____________________ for safe use. 6. In antennas: electrons up and down -________________are accelerated _________________ electromagnetic -As a result, ________________________ radiation waves is emitted from the antenna as ____________. -The radiation is made up of: electric 1/ an ____________ field and magnetic 2/ a _____________ field As they move, the fields are constantly changing . As one field changes, ________________ the other field and vice versa . it induces _______________________________ at the speed These waves move out ________________ visible light of light _____________ . Radio waves, __________________ x-rays are all forms of ___________________ electromagnetic and ___________ radiation light _________________ and travel at the speed of __________ . This is an example of electromagnetic ________________ . induction energy E&M In this way, __________ waves can transmit ___________ by means of their _______________ through a vacuum. fields When these fields reach an antenna, the fields force the _________________ in the antenna to accelerate electrons frequency at the exact same _______________ . Cell phones and microwave ovens also transmit waves energy by means of electromagnetic ____________.