lecture outline chapter 20 college physics, 7 th edition wilson / buffa / lou © 2010 pearson...
TRANSCRIPT
Lecture Outline
Chapter 20
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.
20.1 Induced emf: Faraday’s Law and Lenz’s Law
• Emf – what is it???
• It is capable of producing what???
20.1 Induced emf: Faraday’s Law and Lenz’s Law
We observe that, when a magnet is moved near a conducting loop, a current is induced. When the motion stops, the current stops.
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20.1 Induced emf: Faraday’s Law and Lenz’s Law
On the other hand, when a loop moves parallel to a magnetic field, no current is induced.
© 2010 Pearson Education, Inc.
20.1 Induced emf: Faraday’s Law and Lenz’s Law
Changing current in one loop can induce a current in a second loop. (3rd way)
a.) When the switch is closing the buildup of current produces a changing magnetic field in the other loop (inducing current)
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20.1 Induced emf: Faraday’s Law and Lenz’s Law
We conclude that current is induced only when the magnetic field through the loop changes. This is called…
An induced emf is produced in a loop or complete circuit whenever the number of magnetic field lines passing through the plane of the loop or circuit changes.
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20.1 Induced emf: Faraday’s Law and Lenz’s Law
In order to measure the change in the magnetic field through a loop, we define the magnetic flux:
SI unit of magnetic flux: the weber, Wb
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20.1 Induced emf: Faraday’s Law and Lenz’s Law
Faraday’s law for the induced emf:
The minus sign indicates the direction of the induced emf, which is given by Lenz’s law.
In words: this is the amount magnetic flux changes in a certain amount of time.
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20.1 Induced emf: Faraday’s Law and Lenz’s Law
Lenz’s law:An induced emf in a wire loop or coil has a direction such that the current it creates produces its own magnetic field that opposes the change in magnetic flux through that loop or coil.
So if the magnetic field is increasing, the induced current will produce a field in the opposite direction, tending to decrease the field.Aka…if flux is positive, current is negative.
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20.1 Induced emf: Faraday’s Law and Lenz’s Law
The direction of the induced current is given by a right-hand rule.With the thumb of the right hand pointing in the direction of the induced field, the fingers curl in the direction of the induced current.
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20.1 Induced emf: Faraday’s Law and Lenz’s Law
• A) The south end of a bar magnet is pulled far away from a small wire coil.
• B) Looking from behind the coil toward the south end of the magnet what is the direction of the induced current: clockwise, counterclockwise, or no induced current?
• C) Suppose the magnetic field over the area of the coil is initially constant at 40 mT, the coil’s radius is 2.0 mm, and there are 100 loops in the coil. Determine the induced emf in the coil if the magnet is removed in 0.750s.
20.1 Induced emf: Faraday’s Law and Lenz’s Law
• In rural areas where electric power lines carry electricity to big cities, it is possible to generate small electric currents by means of induction in a conducting loop. The overhead power lines carry alternating currents that periodically reverse direction 60 times per second. How would you orient the plane of the loop to maximize the induced current if the power lines run north to south:
• Parallel to Earth’s surface• Perpendicular to Earth’s surface• Perpendicular to Earth’s surface in
the east-to-west direction
20.1 Induced emf: Faraday’s Law and Lenz’s Law
• Suppose an electromagnet exposes a speaker to a maximum magnetic field of 1.00 mT that reverses direction every 1/120s. Assume the speaker’s coil consists of 100 circular loops (each with radius of 3.00 cm) and has a total resistance of 1.00 ohm. According to the manufacturer of the speaker, the average current in the coil should not exceed 25.0 mA.
• A) Calculate the magnitude of the average induced emf in the coil during the time period.
• B) Is the induced current likely to damage the speaker coil?
20.4 Electromagnetic Waves
James Clerk Maxwell showed how the electric and magnetic fields could be viewed as a single electromagnetic field, with the following properties:A time-varying magnetic field produces a time-varying electric field.A time-varying electric field produces a time-varying magnetic field.
For the first time, both fields thought of as ONE!
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20.4 Electromagnetic Waves
An accelerating charge produces an electromagnetic wave. The electric and magnetic fields are perpendicular to each other and to the direction of propagation of the wave.
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20.4 Electromagnetic Waves
All electromagnetic waves travel at the same speed in vacuum:In a vacuum, all electromagnetic waves, regardless of frequency or wavelength, travel at the same speed, c = 3.00 × 108 m/s.
This finite speed of electromagnetic waves leads to delays in transmitting signals over long distances, such as to spacecraft.
All self-propagating; act as transverse
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20.4 Electromagnetic Waves
An electromagnetic wave transmits energy; its electric and magnetic fields are capable of accelerating charged particles. It will exert a force on any surface it intercepts; this phenomenon is called radiation pressure. It is negligible in everyday experience, but could be used to power “solar sails” for interplanetary travel.
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20.4 Electromagnetic WavesElectromagnetic waves can have any frequency. Different frequencies have been given different labels.
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20.4 Electromagnetic Waves
• Frequency and wavelength are inversely related.
• 2 formulas! SUPER SIMPLE!
• Categories of spectrum: – Power Waves– Radio Waves– Microwaves– Infrared Waves– Visible Light Waves– UV Rays– X-Rays– Gamma Rays
20.4 Electromagnetic Waves
• The first successful Mars landings were the Viking probes in 1976. They sent radio and TV signals back to Earth. How much longer would it have taken for a signal to reach us when Mars was farthest from the Earth than when it was closest to us? The average distance of Mars and Earth and the Earth from the Sun are 229 million km and 150 million km respectively. Assume that both planets have circular orbits, and use the average distances as the radii of the circles.