ACTIVITY: Resistance and Ohm’s Law
EQ: What is the relationship between current and voltage?
BACKGROUND INFORMATION: Electrical devices use energy. They get the energy from the current that flows
through them. When designing an electrical device, or a circuit, it is important for the proper amount of
current to flow for the voltage that will be applied. This investigation looks at resistance, which is the property
that relates current and voltage in a circuit or electrical device.
PROCEDURE:
Part 1: Resistance Measurements
Table 1: Resistance Measurements
Object
Nail
Plastic straw
Rubber band
Wire
String
Paper clip
Part 2: Mystery Resistors
Table 2: Resistor Currents
Resistance (Ω)
5
10
20
Conductor or Insulator
Resistor Color
Green
Blue
red
Part 3: Resistance and Potentiometers
Table 3: Pot settings and resistance
Pot Dial Position
Resistance (Ω)
1
10
2
40
3
80
4
100
Battery Voltage (V)
1.54
1.54
1.54
Resistance (Ω)
0.00
Too high to measure
Too high to measure
0.00
Too high to measure
0.00
Current (A)
.233
.131
.072
Part 4: The Bulb Dimmer Circuit
Build a dimmer circuit with two batteries, the pot, a switch, and 1 bulb. Close the switch and observe how the
brightness changes as you change the dial on the pot.
Observation The higher the setting of the potentiometer, the less the current in the circuit and the dimmer
the bulb.
Part 5: The Voltage Drop
Table 4: Pot settings and voltage drops.
Voltage Drop Across Pot
Pot Dial Position
(V)
1
1.05
2
1.82
3
2.63
Voltage Drop Across Bulb
(V)
2.01
1.25
.457
Observed Light output
Bright
Dim
Barely glowing
THINKING ABOUT WHAT YOU OBSERVED:
Part 1: The Meaning of Resistance
1. What relationship did you notice between the resistance and whether a material was a conductor or a
resistor?
2. Describe the ability of an object to conduct electrical current in terms of its resistance.
3. Two circuits are identical except for the size of the resistors. Which circuit has more current, the one with
the 100 Ω resistor or the one with the 1000 Ω resistor?
Part 2: Mystery Resistors
1. Why did the resistance you calculated not come out to exactly 5, 10, or 20 ohms?
Part 3: Resistance and Potentiometers
1. What is a variable resistor?
2. What is one use in a home for a variable resistor?
Part 4: The Dimmer Circuit
1. How does the potentiometer control the brightness of the bulb
Part 5: The Voltage Drop
1. What relationship did you observe between the voltage drop across the pot and the voltage drop across
the bulb?
2. What does the voltage drop tell you about the electrical energy carried by the current?
3. What relationship did you observe between the measured voltage drops and the battery voltage?
Taken from Investigations Manuel, CPO Science, 2008