Physics 212 Lecture 19: Lenzs Law and Motional EMF

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Physics 212 Lecture 19 Lenzs Law and Motional
EMF
Lenzs Law

• Current loops do not like changes in the
  magnetic flux through them.
• Therefore, the induced current due to Faradays
  Law acts to oppose the change in magnetic flux
  through the loop.

Example A simple current loop

• B increases up through loop
• Apply 2nd right hand rule Iinduced makes
  downward B field through loop.
• This opposes the increase in B

Example A metal ring on top of a solenoid

• Turn on solenoid with current I
• B increases up through core
• Apply 2nd right hand rule Iinduced makes
  downward B field through ring, so Iinduced is
  opposite to I
• Opposing currents repel
• Ring experiences upward force and jumps.

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Motional EMF

• 1. Wire moving in a constant magnetic field

Electrons are pushed down qv x B with
q(-) Ions are pushed up qv x B with
q() Equilibrium magnetic force separating
charges equals force attracting charges
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Motional EMF

• 1. Sliding wire in a circuit with changing
  magnetic flux

Changing flux creates EMF Use magnetic flux
definition B constant, area changing Use
velocity definition
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Questions for Groups

• 31.10 (modified) What happens when you drop a
  magnet down a copper tube?
• 30.7 What happens when the rotational speed of a
  generator coil is increased?
• 30.10 Explain why an applied force is needed to
  keep a wire moving at constant speed when
  connected in a circuit with increasing magnetic
  flux through it.
• 30.3 (modified) If a bar moves to the right in a
  magnetic field into the page, explain the
  direction of the electric field created.

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Example Ch 30 21Figure shows bar sliding to
the right without friction. The length is 1.2m,
the resistor is 6 Ohms, and B2.5T into the
page. Find (a) F to move bar at constant v2m/s.
(b) power delivered to resistor.