How Things Work - Spring 2018

The sheet of notes is still permitted. The topics for the final exam are: electrical charge proton, neutron, electron, quark - particles; which are fundamental and which are composite  atomic number and elements Coulomb's law (inverse square) charging things - what happens voltage current resistance units of V, I and R series circuit parallel circuit basics of circuits bulb brightness predictions - it's related to current (and series vs. parallel) V = I R basic electrical schematics (and symbols - battery, resistor, wire) magnetism electromagnetism electromagnetic induction compasses finding north magnetic north vs. geographic north motors vs. generators

Electromagnetism Recall that the motion of charges leads to magnetic fields.   If you have charge traveling through a wire, electrons can be thought of as moving together – this causes a magnetic field, also known as electromagnetism.  The magnetic field caused by a current passing through a wire is often small, but if you coil the wire upon itself, the magnetic fields “add up”.  Several hundred turns of wire (with current running through it) can produced quite a strong electromagnet.  A coil with current running through it can naturally react to a permanent magnet – if this is engineered well, we have a motor.  See illustrations and demos in class. Below:  basic motors Electromagnetic Induction So, current causes magnetism – something shown in the early 19 th  century by Hans Oersted.  As it happens, the reverse is also true – magnetism can cause current, but there must be some relative CHANGE in the magnetic field or location of conductor.   There mus

Magnetism! Similar to the case of charge, magnetic poles are divided into North and South poles. A North magnetic pole is one that points toward the Earth's magnetic north pole.  This means that the Earth's magnetic north is ACTUALLY A SOUTH POLE (magnetically speaking). Also: - Like poles repel - Opposite poles attract - Each magnet must have at least one North and one South pole (though they may have more than one of each).  There is NO such thing as a magnetic monopole. - Magnetic fields are real, but the lines are imaginary - Field lines indicate the direction that a compass needle would take in the vicinity of the magnetic field. - There are naturally occurring magnetic minerals - a very common one is called magnetite (Fe3 O4) Magnetic north on the Earth is near Ellesmere Island in Northern Canada, several hundred miles from true (geographic) North (the North Pole).  It is moving toward Russia at several miles per year. For gory detail: http://en.wikipedia.org/wiki/North

1.  Describe the difference between voltage, current, and resistance.  Give the proper units, too. 2.  What is the resistance of a light bulb that allows 2 A of current through it when connected to a 12-V battery?  (6 ohms) 3.  A 5-ohm resistor is connected to a 10-volt battery. What current goes through the resistor?  (2 amps) 4.  In general, what is the difference between resistors in series and in parallel?  Recall the light bulb examples and how the brightnesses compare. 5.  Which has more resistance, 2 identical bulbs in series or the same 2 identical bulbs in parallel? 6.  For question 5, which set-up (series or parallel) would "kill" the battery quicker? 7.  You have 2 bulbs in series - remove one (unscrew it) and what happens? 8.  You have 2 bulbs in parallel - remove one (unscrew) and what happens? 9.  Draw the symbols for battery, resistance and wire.  Draw a schematic for 2 resistors in series.  Draw a schematic for 2 resistors in paral

OK, so about regular circuits: The images represent SERIES CIRCUITS and PARALLEL CIRCUITS. In a  series  circuit, the current is  constant  and is set by the total resistance of the circuit (the sum of the resistors). If you remove one resistor (or light bulb, as in the first image), the current  stops . If the resistors were identical bulbs, having more bulbs would result in dimmer bulbs, since the battery voltage is distributed among them.  Note that the sum of the voltages "over" the bulbs is equal to the total voltage provided by the battery (give or take some minor losses).  Identical bulbs (or resistors) have identical voltages "over" them - 3 identical bulbs connected to a 9-V battery would have roughly 3-V each over them. In  parallel  circuits, current has multiple paths to take, so the total resistance of the circuit is actually LESS than if the resistors were alone or in series with other resistors - see details below. Since the bulbs are c

Circuits 1 Thus far, we have only discussed "static" (stationary) charges.  Static charges alone are useful, but not nearly as much as charges in motion.  As you recall, electrons are the most easily moved particles.  Recall the Galvani experiments discussed in class: https://www.youtube.com/watch?v=o8zNSzbjRLI https://www.youtube.com/watch?v=sJifWqUa2pY So, electrons are moved by the chemistry of the electrochemical/voltaic cell (originally called the "electric pile" and now simply, battery).  Here are pictures/diagrams of Volta's original batteries: There is a chemical reaction between the electrolyte paste (often an acid) and the two metals (often zinc and carbon).  Electrons are given up by one metal (zinc) and accepted by another (carbon or copper).  The motion of electrons is called "current", but we usually imagine that it is positive charge moving (to keep the numbers positive). We can't see electrons, but we can cer