Ohm's Law 5F10.10
Electricity and Magnetism DC Circuits Ohm's Law

Concept

This demonstration verifies the relation $V = IR$, where $V$ is potential difference, $I$ is current, and $R$ the resistance. Some texts quibble with this as simply a definition of resistance, and claim that the correct statement of Ohm’s Law is $J = \sigma E$, where $J$ is the current density, $\sigma$ is conductivity, and E the electric field. Perhaps more importantly, only certain materials, called ohmic, obey these relations. Examples of nonohmic materials abound in modern microelectronics; many of the semiconductor components of a computer are prime examples.

Procedure

  1. Using one of the DMMs as an ohmmeter, verify that the variable resistor is set to 10 Ω and the circuit is wired as shown in the picture above.
  2. Tell the class that the resistor is set to ~ 10 Ω and the voltage is set to ~ 1.5 V.
  3. Ask the class to use ohm’s law to predict what the current should be.
  4. Turn the battery eliminator on and reveal the measured current and voltage.
  5. Repeat steps 2-4 after varying the voltage.
  6. Repeat steps 2-4 after varying the resistance (be careful not to exceed 1 A as this could blow the battery eliminator’s fuse).

Equipment

  1. DMM - Voltmeter
  2. Battery Eliminator (1.5, 3, 4.5, 6, 9, 12 V)
  3. DMM - Ammeter
  4. (3) 3 ft, Red Banana-Banana Cables
  5. Variable Resistor
  6. (2) 3ft, Black Banana-Banana Cables
  7. (2) Alligator Clips