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 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.

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.

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.

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.

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

A circuit with resistors and batteries

A circuit with a capacitor V=6 volts

Bulb Demo

The “series” circuit

The “parallel” circuit Brighter Dimmer Equal Brightness

The Need for the Microphone

An Old Carbon Microphone

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.

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

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

How does it work?

The “Crystal” Microphone

Dynamic Microphone A Magnet???

A Magnet +Q S N OP Survey

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

Like poles repel and unlike poles attract Like poles repel and unlike poles attract. The magnetic field lines point away from the north pole and point toward the south pole. But they don’t start or stop on those poles, they form continuous loops (inside the magnets).

What happens to the poles of the bar magnet if you cut it in half? Magnetic poles always come in pairs. If you continue cutting the magnet in half you will eventually get down to an iron atom with a north and a south pole—these too cannot be separated. There are no magnetic monopoles as far as we know.

The north pole of a compass will actually point to the Earth’s south magnetic pole (which is about 1300 miles away from the geographic north pole—more on that on a later slide in this chapter’s slides.)

If magnet A is held stationary while the smaller magnet B is free to rotate, B will twist and align with the field of A.

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.

In 1820 Hans Christian Oersted was showing his students in a lecture that the electric current in the wire did not deflect the compass needle (Figure a). When he held the compass above the wire as in Figure (b) he saw to his amazement that the compass needle deflected. Figure (c) shows the magnetic field created by the current. It is vertically downward where the compass in figure (a) was located and so couldn’t rotate the compass needle.

Exercise 35 Chapter 20. Electrical energy has to come from somewhere Exercise 35 Chapter 20. Electrical energy has to come from somewhere! Here the falling magnet generates current in the coil. The current has a magnetic field that interacts with the magnetic field of the falling magnet and resists its downward motion. It is conservation of energy again, the energy generating the electric current comes from the gravitational potential energy being lost by the falling magnet.

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??

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.