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Lenz's Law

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Faraday Flashlight

Faraday Flashlight

Credit: Chetvorno
Source: http://en.wikipedia.org/wiki/File:Linear_induction_flashlight.jpg
License: CC BY-NC 3.0

Powered by mechanical means, this flashlight has no batteries. It operates by the laws of induction and stores the current that is created in a capacitor.

News You Can Use

  • Mechanically powered flash lights rely on an external source such as human muscle to generate the electricity needed to power the light bulb. These types of flashlights do not need any batteries or external electrical connections. Faraday flashlights or “shake flashlights” generate electricity based on Faraday's law. This law states that an induced emf is equal to the rate change of the magnetic flux.
  • Mathematically this is represented as

\epsilon=\oint - \frac{d\phi_m}{dt}

where the right hand side of the equal sign is the time rate of change of the magnetic flux.

  • These shakes weights consist of an electrical generator, a component to store the electricity (usually a capacitor and an LED lamp). The electrical generator consists of a very strong magnet that is able to slide back and forth through the center of a solenoid. As the magnet moves through the solenoid, a current is created and that current charges the capacitor. When an operator pushes the button to connect  the circuit between the capacitor and the LED, light is produced.
  • Watch this simple video which demonstrates Faraday's Law: 

http://www.youtube.com/watch?v=8za6EQQDNzk

Explore More

Using the information provided above, answer the following questions.

  1. Why can't you simply place a stationary magnet in a coil to have current produced?
  2. What is the magnetic flux?
  3. Does the current change direction depending on which way the magnet is moving with respect to the coil?

Image Attributions

  1. [1]^ Credit: Chetvorno; Source: http://en.wikipedia.org/wiki/File:Linear_induction_flashlight.jpg; License: CC BY-NC 3.0

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