Electromagnetic Induction ….the grand finale…. Need to include pictures of Itaipu and Three Gorges Dam!!!!!

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Electromagnetic Induction ….the grand finale…. Need to include pictures of Itaipu and Three Gorges Dam!!!!!

Check it out…. The main idea behind hydroelectric dams….

Three Gorges -located in China -Largest in the world (22,500 MW) capacity -1.24 million people ‘relocated’ Some big ones….

Itaipu -location: Border of Brazil and Paraguay -2 nd biggest in the world (14,000 MW capacity) -90% of Paraguay’s energy needs -19% of Brazil’s needs

Electricity & Magnetism So far…. 1.An electric current produces a magnetic field 1.A magnetic field exerts a force on an electric current or moving charge

Michael Faraday (1791-1867 Smart dude alert!!! Chemist, Physicist, Pilosopher Main Discoveries: -Magnetic Field -Electromagnetic Induction Often referred to as the ‘Best experimentalist in the history of science Ref: http://en.wikipedia.org/wiki/Michael_Faraday

Electromagnetic Induction Definition: Generation of an EMF from changing magnetic fields

Electromagnetic Induction Key Points: 1.EMF is induced by a changing magnetic field. 2.A changing magnetic field can be obtained by i) moving a magnet or coil, or ii) changing electric current 3.A steady current or magnet (constant magnetic field) induces no emf

Magnetic Flux ( Φ) Definition: Number of field lines passing through a coil For a loop of wire in a magnetic field that depends on: 1.Magnetic Field Strength 2.Area of loop 3.Orientation of field

Magnetic Flux Φ = BAcosΘ Φ: Magnetic Flux (Weber - W) (also Tm 2 ) B: Magnetic Field (T) A: Area of the surface (m 2 ) Θ: Angle between B and the normal to the surface

Example A 1.80 m diameter circular coil that contains 50 turns of wire is perpendicular to a 0.250 T magnetic field. If the magnetic field is reduced to zero in a time of 0.100 s what is the average induced EMF in the coil?

Lenz’s Law Copper pipe and magnet Quick Review: How long does it take an object to fall 1.5 m if starting from rest?

Lenz’s Law (page 624) Definition: The induced current will produce a magnetic field (and magnetic flux) that opposes the change in magnetic field (and flux).

Example Label the disk's induced magnetic poles (North and South) as it reacts to the motion imposed by an outside force:

Example What is the direction of the current through each resistor?

Example A circular loop of wire radius 2.5 cm is placed in a magnetic field B = 0.020 T (into the page). The field is then reduced to 0.010 T (into the page) in 0.10 s. a)What is the average induced EMF? b)Which direction does the current flow?

Direction of Current -Induced current will produce a flux to oppose the change -We are decreasing flux into page, so induced current will want to keep this (higher) flux directed into page -RHR: direction of induced current will produce magnetic field -Therefore, conventional current is clockwise

Question If a wire coil with 320 turns is exposed to a magnetic flux decreasing at a rate of 0.03 W/s, find: a)The induced voltage b)Polarity of the solenoid? c)Direction of the current in the solenoid

Moving Conductor ε = Blvε: EMF (V) B: Magnetic Field strength (B) l: Length of conductor (m) v: speed of conductor (m/s)

Derivation (page 627)

Example A wire is in a magnetic field as shown. In which direction could the wire be moved to induce an emf across the length of the wire? A.to the left B.up the page C.into the page D.down the page

Example Four conductors of equal length are each moved through a uniform magnetic field in different directions and with different speeds, as shown. While the four conductors are being moved through the field, in which conductor will the largest potential difference be induced? A.Conductor A B.Conductor B C.Conductor C D.Conductor D

Example A conducting rod 25.0 cm long moves perpendicular to a magnetic field (B = 0.20 T) at a speed of 1.0 m/s. Calculate the induced EMF in the rod.

Example 2 A conducting rod 15 cm long moves at a speed of 2.0 m/s to the right in a 0.30 T magnetic field directed out of the page. If the resistance of the circuit is 4.0 Ω: a)What is the magnitude of the current through the circuit? b)What direction is the current

Try these…. Page 654 (8, 9, 10, 12, 14, 15)

Motor Spinning wires in a magnetic field Need more explanation of motors and back emf.

Back EMF Motors: Electric NRG into Mechanical NRG Generators: Mechanical NRG into Electrical NRG

Generator

Transformers From the dam to our homes….. -When we generate power we ramp up voltage for transmission (up to 100 000 V) -When it arrives at homes, we ramp it back down

Transformers Read pgs. 633-636 In notes: -What is a transformer? -Draw a transformer in your notes (Fig. 21-21) -State the two transformer equations -What is the difference between a step up and a step down transformer?

Transformers A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF), or "voltage", in the secondary winding. This effect is called inductive coupling.

Transformers Composed of two different coils of wire around opposite sides of an iron core. Passing an alternating current through a coil of wire that surrounds a metal core induces a varying magnetic field in the core. This field will cause a responding current flow in a secondary coil wound around the opposite side.

Transformers

Examples #1: A step-up transformer is used to convert 120 V to 15000 V. If the primary coil has 24 turns, how many turns does the secondary coil have? (3000) #2: A step-down transformer reduces the voltage from 120 V to 12.0 V. If the primary coil has 500 turns and draws 0.030 A: a)What is the power delivered to the secondary coil? (3.60 W) b)What is the current in the secondary coil? (0.300 A)

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