Electricity and Magnetism
Two sides of the same force
Outline
•	Review of electric potential and current.
•	Ohm’s law and power loss.
•	Magnetic force
•	Separating charge with magnets.
•	Electric and magnetic fields.
Review
•	Electric charge (+ and -) is conserved.
•	The amount of energy required to separate a unit of charge is called the electric potential (V).
•	A circuit allows separated charge to recombine.
•	The rate of flow of charges in a circuit is the current (I).
•	A current I driven by a voltage V provides electrical energy at a rate P = I V, where P is Power, measured in watts (J/s).
What’s in a circuit?
•	Charge flows through a circuit like water flows in a river.

•	The battery or other voltage source supplies the “pressure”, and the circuit supplies the movable charges.
•	A CONDUCTOR (usually a metal) has lots of free charges for making a current. 

Ohm’s Law
•	The amount of current that is produced for a given voltage difference depends on the RESISTANCE (R), measured in Ohms (Ù)
•	CONDUCTORS generally have LOW resistances, while INSULATORS (rubber, glass, etc.) have HIGH resistances.
•	Ohm’s law:         V = I R    or     R = V / I
•	Example: A 12 V battery produces 0.5 A of current in a certain wire.  What is R for the wire?
•	R = 12 V / 0.5 A = 24 Ù
Joule heating and Power loss
•	Resistance reduces the rate at which the charges can flow.
•	Sometimes good, when I don’t want all the charge to flow at once (a short-circuit!). 
•	Sometimes bad, because energy is lost due to Joule heating. 
•	P lost = I2 R  (“Power lost” = “Current squared” times “Resistance”)
•	Example: We found previously that R = 24 Ù, when V =12 V and I = 0.5A.  
•	Then P lost = (0.5 A)2 x 24 Ù = 6 W or 6 J every sec.
Historical note
•	James Joule discovered Joule heating in the 1840’s while working on a new kind of engine: the electro-magnetic engine.
•	Instead of using heat to cause the expansion of gases and push a piston, he used electricity to cause magnets to rotate.
•	But they got very hot, due to Joule heating!

Magnetism
•	Magnetism: A force between “poles” (north and south) of magnets.
•	Opposite poles attract, like poles repel.
•	The earth is a magnet, but the North Pole is really a “south” magnetic pole.  It just attracts “north” poles.
•	Certain materials are always attracted by magnets (e.g. iron, nickel), and can be made into magnets.
Electricity and Magnetism
•	Can make an electric current act like a magnet.
•	Demonstration: A Solenoid picking up paper clips.
•	Can use an electric current to make magnets.
•	Demonstration: A magnetic “recharger”.
•	Can use a magnet to make an electric current.
•	Demonstration: An electric generator.
•	Can use a current to make a magnet move.
•	Demonstration: An electric motor.