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Lecture 26

Lecture 26. Moving On …. Last Time. We demonstrated that rubbing objects can separate charge, There are two types of charge – positive and negative (electrons). The electrons are easy to push around. Charge can be stored on the plates of a capacitor. Also Last time.

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Lecture 26

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  1. Lecture 26 Moving On …

  2. Last Time • We demonstrated that rubbing objects can separate charge, • There are two types of charge – positive and negative (electrons). • The electrons are easy to push around. • Charge can be stored on the plates of a capacitor.

  3. Also Last time • We found that a battery is also a source of charge • The capacitor uses up its limited charge quickly. • The battery makes use of internal chemical reactions to provide a continuous availability of charge • Eventually it, too, runs dry.

  4. Also, also last time • We discovered we could connect light bulbs to a battery and it would light and stay lit. • A second battery in series reduces the amount of current that the battery can squeeze out. The two bulbs (in series) are dimmer than the single bulb. • We can also hook up bulbs in parallel, in which case they are equally bright.

  5. As well as different materials • Conductors • Electrons move easily • Insulators • Electrons are held tightly in place by their chemical bonds. • Semiconductors • Intermediate properties • Basis of transistors.

  6. Also, also, also last time • We defined some new concepts • Potential Difference or Voltage • The work per unit charge required to move a charge from one point to the other • Current • The amount of charge that passes a single point in a circuit per unit time (1 sec). • Capacitance • The ratio of the charge that the capacitor can hold to the voltage that produces that charge.

  7. Plus … • Resistance • The ratio of the potential difference across a resistor (R=Ohms) to the current flowing THROUGH the resistor.

  8. A circuit with resistors and batteries

  9. A circuit with a capacitor V=6 volts

  10. Bulb Demo

  11. The “series” circuit

  12. The “parallel” circuit Brighter Dimmer Equal Brightness

  13. The Need for the Microphone

  14. An Old Carbon Microphone

  15. The Microphone • The microphone is a device that received the sound vibrations • converts it to an electrical “signal” • Which is then sent to the next stage in the process (later). • The signal tends to be small and gets weaker as it travels down a long wire.

  16. The Microphone Process MECHANCAL ---> --------------- ELECTRICAL Microphone Signal on a wire

  17. Consider a powder ofmetal Particles of Metal are pressed closer together. Resistance is reduced

  18. How does it work?

  19. The “Crystal” Microphone

  20. Dynamic Microphone A Magnet???

  21. A Magnet +Q S N OP Survey

  22. Magnets come in all shapes and sizes. Almost every refrigerator door has two to provide that last, snug pull when it closes.

  23. What happens to the poles of the bar magnet if you cut it in half?

  24. Iron filings or other iron objects are attracted to a magnet, since iron (and nickel and cobalt) atoms are naturally magnetic themselves. Putting such items in a magnetic field induces the atomic magnets to align and the object itself becomes magnetic---it will have a net magnetic field---which means it will be attracted to the magnet that induced it.

  25. Magnets • Magnets Do NOT attract chages. • Magnetism is a very different phenomenon. • Magnets have N and S poles • Like poles repel • Unlike poles attract • Where have we seem this before??

  26. Other Observations • A magnet moving into a coil produces an electric current (and voltage!). • A wire moving near a magnet will have a current generated in it. • There is a “magnetic field” around a wire. • A loop of wire acts like a small magnet.

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