Measurements of Current, Potential Difference and Resistance

Cautions:   

• The electrical current available from ordinary electrical outlets  and also the amounts used in some laboratory devices are capable of doing serious harm to you.   Treat them with healthy respect. The amount of current which can be produced by flashlight batteries (1.5 v) is quite harmless.
• Electrical meters can be seriously damaged by overloading them.  Always measure an known voltage or current on the highest scale available.  After you have a rough measurement, you can select a more appropriate (more sensitive) scale if necessary.
• 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.

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.
• Move the ammeter so that it measures the current through the resistor instead.   Record both current and potential difference.
• What conclusions can you draw from these measurements?

Part 3:

• Connect two batteries, the 47 ohm resistor the ammeter and the variable resistance box in a closed loop series circuit.  Connect the voltmeter to measure the potential difference across the 47 ohm resistor again.
• Set the variable resistance to 450 ohms.  Measure the potential difference across the resistor and the current through it.
• Gradually reduce the variable resistance to 10 ohms.  Measure the current and potential difference each time you change the resistanace.  Take about a dozen data points.
• Make a graph of potential difference versus current. (You can do it by hand or use Graphical Analysis.)
• Determine as much as you can about the relationship between V and I in this circuit.

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.   This is not true of all materials.  Replace the 47 ohm resistor in your circuit with a flashlight bulb.
• Adjust the variable resistance so that a current of about 20 mA passes through the bulb.   Measure the current and potential difference as you slowly increase the current to about 250 mA.  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?