Measurements of Current, Potential Difference and Resistance

Cautions:   

• The electrical energy available from ordinary electrical outlets and also some laboratory devices are capable of doing serious (even fatal) harm to you.   Treat them with healthy respect. The amount of energy which can be produced by flashlight batteries (1.5 v) is quite harmless, except they can cause a wire to heat up enough to burn you.
• Electrical meters can be seriously damaged by overloading them.  Always measure an unknown voltage or current by starting on the highest scale available.  After you have a rough measurement, you can select a more appropriate (more sensitive) scale if necessary.  The accuracy of measurements using an analog meter is greatest when the needle has significant deflection.  So you want a large deflection for good results, but choosing a too sensitive range can destroy the meter.
• Meters, like batteries have a positive and negative terminals. 
  • For Ammeters and Voltmeters the positive side of the meter should go closer to the positive side of the battery than the negative side.
  • If a meter tries to deflect to the left, that indicates the meter is hooked up in reverse.  Remove the power, reverse the connections to the meter and try again.
  • If a meter needle is ever pinned all the way to the maximum end of the scale, turn off the power in the circuit immediately.
  • Never connect an ammeter in parallel with a battery!

Part I:

• Measure the potential difference between the battery terminals by connecting one lead from the meter to each end of the battery.  Do this for both batteries.
• Put both batteries together connecting the positive of one to the negative of the other.   This is called a series connection.  Measure the voltage of the two batteries together.  How is this related to their individual measurements?

Part 2:

• Connect the battery, ammeter, and a 47 ohm resistor in a closed loop series circuit. Connect the voltmeter to measure the potential difference between the ends of the resistor. The ammeter will measure the current leaving the battery.  Record both the current and the potential difference indicated.  Why do we say the circuit in the figure below measures the current leaving the battery?  Is that necessarily the same as the current through the resistor in the figure? Explain before continuing.

   

  The figure below shows pictorially and schematically how to set up the current leaving the battery on the left and right sides respectively.  Note: Schematics do not normally indicate the polarity of meters.  The positive side of the battery is the longer of the two parallel bars.

• Draw the pictorial and schematic diagrams for a circuit in which the ammeter measures only the current through the resistor.
• Construct the circuit where the ammeter measures only the current through the resistor.    Record both current and potential difference.
• What conclusions can you draw from these measurements?

Part 3:

• Set the variable resistance box to 450 ohms.
• Draw a circuit that consists of the two batteries, the 47 ohm resistor, the ammeter, and the variable resistance box in a closed loop series circuit. 
• Wire your circuit and then connect the voltmeter to measure the potential difference across the 47 ohm resistor.
•  Measure the potential difference across the resistor and the current through it.
• Gradually reduce the variable resistance to around 10 ohms.  Hint: make each new value of the resistance box about 75% of the previous value each time.  Measure the current and potential difference each time you change the resistance.  Change range on the ammeter and voltmeter is necessary - see the cautions above. You should have about 13 or so data points.
• Make a graph of potential difference versus current. (You can do it by hand or use Graphical Analysis.) Be sure to turn the 'connect points' feature off, and include the linear fit line.
• Use the graph to determine as much as you can about the relationship between V and I in this circuit. Specifically, what is the resistance of the nominal 47 ohm resistor?

Part 4:

• For a carbon resistor (what you have been using), the resistance is determined by the physical properties of the resistor and does not depend on the current flowing through it (unless its temperature appreciably changes).   This is not true in all cases.  Replace the 47 ohm resistor in your circuit with a flashlight bulb.
• Adjust the variable resistance (try about 150 ohms) so that a current of about 20 mA passes through the bulb.   Measure the current and potential difference of the light bulb as you slowly increase the current to about 250 mA by reducing the value of the resistance box.  You should take at least a dozen data points.  Note the brightness of the bulb at each stage, i. e. not glowing, very dim, bright, etc.
• Plot a graph of potential difference versus current.
• What conclusions can you draw from your graph and your observations?

Last modified Mar, 2016