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Ohm's Law Lab

Instructor: Amanda Robb
In this lab, we'll describe an experiment you can try at home to demonstrate the principles of Ohm's law. When you're done, you'll be able to explain the relationship between voltage, current and resistance in a circuit.

Introduction

Goal: To understand the relationship between voltage, current and resistance using Ohm's Law
Age: High school and up
Approx Time: 30 min
Safety Concerns: Electrical parts can heat up over time. Make sure to always touch circuits by the plastic insulation and disconnect if the circuit gets too hot.

When you're making a smoothie for breakfast, you probably turn it on low, then increase to high speed. Sure, you turn a switch, but how does the blender actually change speed with the same amount of electricity coming from the wall? The answer to this question lies in Ohm's Law, which says that the voltage of a circuit is equal to the current multiplied by the resistance.

Voltage is equal to current multiplied by resistance
Ohms Law

For a review Ohm's Law, check out the lesson Ohm's Law: Definition & Relationship Between Voltage, Current & Resistance

But what does this have to do with our blender? Well, voltage in our house always stays constant, about 120V in the United States. To make the blender move faster, it needs more current. To run it at a lower speed, it needs less current. If voltage stays the same, then resistance must change in different settings on the blender to allow for different currents. Since current and resistance have an inverse relationship based on Ohm's Law, if we want less current, we have to increase resistance.

Today, we're going to test this by building our own circuit and measuring current using different resistors.

Materials

  • Three different resistors of known resistance (such as 1 Ohm, 5 Ohm and 10 Ohm)
  • Multimeter
  • Three wires
  • One 1.5V battery and holder
  • One switch
  • A table to record your data, like this one. Since we're using the same battery, and you should know the resistance of each resistor, you can enter those in the table before we do the lab.

Voltage in Volts (known) Resistance in Ohm's (known) Current in Amps (calculated) Current in Amps (measured)
1.5 1
1.5 5
1.5 10

Steps

1. To start, connect your battery to one of the resistors using a wire. Connect the other end of the resistor to the switch, and finally connect the switch back to the battery. When the switch is closed, you should have a complete circuit.

Circuit setup
circuit setup

2. Next get your multimeter to measure current. Each multimeter is different depending on the company, so you'll need to read the instructions for the specific settings. When you've set your meter up correctly, open your switch and touch one lead to one end of the switch and the other lead to the other end. Your meter should show the current in amps. Record this in your table in the column labeled 'Current in Amps (measured)'.

Multimeters look different depending on the brand but this is a typical one
multimeter

3. Now, let's analyze our data. First, use the known voltage, resistance and Ohm's law to calculate what the current is expected to be, and then compare it to our measured current.

For example, for the first resistor, we know that the voltage is 1.5 volts and the resistance is 1 Ohm. Dividing 1.5 volts by 1 Ohm gives us 1.5 Amps, the expected current. Record this in your table in the column labeled 'Current in Amps (expected)'.

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