Resistance using Ohm's Law

Learning Objectives

Measure Voltage and Current with instruments.

Using Ohm's Law to identify the relationship between:

	Voltage and Current at constant Resistance, and
	Resistance and Current at constant Voltage.

Determine how Current and Voltage are affected in:

	Series circuit, and
	Parallel circuit.

Equipment

Volt Meter, Amp Meter, resistors, power supply, and leads with clips.

Procedure

This lab works best with a group of two students.

	Make sure your Amp meter is in the circuit with the power supply. Start with the input from the power at the 500 mA terminal. If the reading is less than 50 mA, then move to the 50 mA terminal for a more accurate reading.
	The Volt meter is not in the circuit, it monitors the voltage outside of the resistor. Start with the red probe on the 15 V terminal.
	Keep the power supply off until your setup is checked for safety.

Part I Unknown Resistor - How does V affect I?

From your envelope remove the Unknown Resistor card. Keep the resistor on the card. Place the Volt meter red terminal (+) on the red (+) side of the resistor.

This Volt meter is on the 15 V scale and reads 3 V.

This Amp meter is on the 500 mA scale and reads 100 mA.

	Turn on the power supply and adjust the output to 1 Volt across the circuit.
	Refer to Part I in Resistance Data Collection Tool.
	Adjust the voltage from 1 to 6 V
	Record the current from the meter as mA
	Turn off the power supply
	Finish the calculations

Part II Known Resistors - How does R affect I?

From your envelope remove the Known Resistor card. Keep the resistors on the card. Place the Volt meter red terminal (+) on the red (+) side of the resistor.

	Turn on the power supply and adjust the output to 3 Volts across the circuit.
	Refer to Part II in Resistance Data Collection Tool.
	Record the resistor value and the current from the meter as mA for each of the four resistors.
	Turn off the power supply
	Finish the calculations

Part III 3V Circuit with 2 Resistors in Series

From your envelope select the joined Resistors. Keep the Volt meter red terminal (+) on the red (+) side of the resistor.

	Turn on the power supply and adjust the output to 3 Volts across the circuit.
	Refer to Part III in Resistance Data Collection Tool.
	Record the current across both resistors from the meter as mA. Both probes will be on opposite sides of the V.
	Move the Volt meter probes to read across each resistor. Both probes will be on the same side of the V.
	Turn off the power supply
	Finish the calculations

Part IV 3V Circuit with 2 Resistors in Parallel

Connect the joined Resistors at both ends. Keep the Volt meter red terminal (+) on the red (+) side of the resistor.

	Turn on the power supply and adjust the output to 3 Volts across the circuit.
	Refer to Part IV in Resistance Data Collection Tool.
	Record the current across both resistors from the meter as mA
	Move the Volt meter probes to read across each resistor
	Turn off the power supply
	Finish the calculations

Equations

Ohm's Law

	Current = Voltage/Resistance
	I = V/R
	Amps = Volts/Ohms

Resistors in series

R_both = R_eq = R1 + R2 + R3 + …

Resistors in parallel

1/R_both = 1/R_eq = 1/R1 + 1/R2 + 1/R3 + …

3. PHY111 and 112 Lab Homework and the Lab Report are due at the next class.