Response of Circuit Elements

Prelab Exercises

Phy 123 L

Cal Poly Pomona - Physics Department 



Good circuit

BASIC CIRCUIT


Electrical current will flow across any wire when a high voltage appears at one end, and a low voltage appears at the other.   The voltage difference is provided by a battery in our experiment.       Battery's want nothing more than to send current from their high side, to their low side and will do so whenever you give it the opportunity to do so.   


SHORT-CIRCUIT:     If an electrical path appears in the circuit for the current to flow form high to low voltages, WITHOUT going through a component, the battery will output all of its rated current through the current path.     This can damage the wire and cause a spark.   That's what goes ZAP.
bypass


OPEN-CIRCUIT:   When the electical path between the high and low voltage is physically broken open, no current flow can flow, because the gap of air has too much resistance.    That's when things go dark.
open



OHM'S LAW


The amount of current that flows across a path from one side of the battery to the other,  is determined by the resistance of the components in the path.
  • Some devices such as resistors and wires obey Ohm's Law:  The amount of current (I) that flows across a device is proportional to the voltage (V) that's placed across that device.           
          Ohms law
  • The symbol R refers to the Resistance of the device.      The resistance, as the name implies, describes how difficult it is for current to pass through the device.  The higher the resistance, the less current flows through the device.    R depends on the size, material and temperature of the device.   
    • The fatter the device, the lower the resistance.
    • The longer the device, the higher the resistance.
    • The hotter the device, the higher the resistance.
    • Materials with a higher conductivity (related to the arrangement of electrons in the atoms) will have lower resistance.

  • Simulation of Ohm's Law:    Use this virtual circuit from the physics site of German high school teacher Walter Fendt.  It might be useful in taking the online quiz.

Ohmic Devices


If a device follows Ohm's law at all voltages that are placed across it, the device is called an Ohmic device and the resistance is said to have static resistance.      In this case, the plot of V(I) is just a straight line with the slope =R.


Descriptive Ohmic

Here the resistance is easy to calculate...it's just the slope of the line.   Since one of these points lies on the (0,0) point.   The slope calculation is very easy.


Ohmic slope


Non-Ohmic Devices

  • If a device behaves in a way that is NOT described by Ohm's law, (i.e. The resistance is not constant, but changes in a way that depends on the voltage across it.)  the device is said to be non-Ohmic.   In that case V(I) is not a straight line, but has some curvy shape.     In this case, the resistance is called dynamic resistance because it's constantly changing.  


Non-Ohmic V(I)

You can calculate the dynamic resistance at any point by drawing a line that's tangent to the point in question, and calculating the slope of that line.    The dynamic resistance is equal to the slope of the tangent line.

So for example, in the device above, the dynamic resistance at I = .4 Amps is calculated by a line tangent to the point A, and calculating the slope of that tangent line.     Not that this slope is NOT the same as the slope at any other point in the distribution.     Furthermore, it is NOT equal to just R=V/I, but rather...

slope calc.   I


Simulation

Check out this nice simulation from Oregon, to help you answer the quiz questions.