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Generators and Motors

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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

Rather than using batteries, a Faraday flashlight is powered by mechanical means. The flashlight is shaken by the users hand, and this energy is used to create a stored charge. It operates by the laws of induction and stores the current that is created in a capacitor.

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  • Mechanically powered flashlights 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}

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

  • The shake weights in the flashlight 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 (a slinky shaped wire). 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 lamp, light is produced.
  • Watch this simple video which demonstrates Faraday's Law: 

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

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Using the information provided above, answer the following questions.

  1. Why does a stationary magnet in a coil not produce a current?
  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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