Lecture Text, Ohms Law

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Lecture Text. 
Ohms and Kirchhoff's Laws.


Today  Kirchhoff's laws appear fairly intuitive. If water in a pipe branched into two lines we would expect that the sum of the water coming out of the two pipes is the same as the rate of flow of the water in the single pipe.  However, since current was observed strictly in terms of magnetic field this infers that the magnetic fields are directly proportional to the flow of electrons.  Likewise the linear relationship expressed by ohms law was not obvious to Ohm from fluid mechanics, because pressure drop of fluids in a line is not linear with respect to flow rate. 

On a more practical note consider what it means if Kirchhoff's law don't appear to be being obeyed. What if you had three wires attached to a terminal and measurement indicated that the current going into the terminal from one wire did not equal the current leaving the terminal in the remaining two wires.  This would indicate that the current was going to ground or an other line via the terminal and traveling a path not shown on your circuit schematic.  This could of coarse be caused by a obvious solder splash or less obviously  by a contaminant on surface of PC board.  In the case of multi-layer boards a breakdown of insulation between layers is also possible.  

Ohms law states that current in amperes is equal to voltage divided by resistance.   This form of ohms law is most useful for technicians, because it is easy to measure voltage with a single probe of a grounded voltmeter.  Connect negative lead of your voltmeter to ground, circuit common, or power source return line and you can measure the voltage on as many component junctions (commonly called circuit nodes) as are accessible to your meter probe.  You can determine the current through a resistor by measuring the voltage on both sides of the resistor, then calculating the difference and dividing voltage difference by the resistance the resistor.  This resistance is written on the resistor in letters, or frequently it is indicated by a color code.  It is also available from the circuit schematic.  Current calculated in this manner tells the troubleshooter how much current is in this branch of the circuit.  Usually one current calculation and several voltage measurement tell you all you need to know about transistor, diode, operational amplifiers, or even vacuum tube circuits.  Thus, you most be able to use ohms law to analyze what is going on, not only in   resistor circuits but in the more complex circuits that you will studying in future lessons.

Resistance of circuit can be measured with an ohmmeter when the power to a circuit is off.  Sometimes when you measure resistance across any component you may also measure resistance of components in parallel circuit branches.  You can prevent this by lifting one end of the resistor from the circuit board.   



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