### The Idea

*Ohm's law* forms the basis for understanding how electricity flows through circuits. This is a very simple relationship that involves three things: 1) the *voltage* or the push that move electrons through the circuit, 2) the *current* (or amps), which is a measure of how much electricity is flowing through that circuit as a result of that push, and 3) the *resistance* (in ohms), which does all it can to make it difficult for the electricity to flow.

Because of its simplicity, this experiment is a good one for you to discover the law for yourself, based on your measurements.

### What You Need

- one 100 ohm resistor rated for 0.5 watt (other resistor values can work, but the resistor must be rated to handle the wattage that will be applied to it; the wattage is supplied by the resistor manufacturer and is often marked on the resistor)
- ammeter
- voltmeter
- DC power supply (or battery)
- wires to connect to battery terminals

### Method

- Connect a circuit, as shown in Figure 99-1. This is a circuit consisting of a resistor in series with a DC power supply and an ammeter with a voltmeter connected by jumper wires to each of the ends of the resistor. A drawing called an
*electrical schematic*is shown in Figure 99-2. This is equivalent to Figure 99-1, but it shows the electrical connections without regard to the actual physical layout of the components. - Turn the DC power supply to zero.
- Set the ammeter to read milliamps. Set the voltmeter to read 0–10 volts.
- Increase the DC power supply to give a voltage reading of 0.2 volt.
- Read the current.
- Do the same with a voltage of 0.4, 0.6, and 0.8 volts.
- Graph current versus voltage. Draw a line that best fits the data. What is the significance of the slope of the line?
- Repeat this with a 200 and a 300 ohm resistor.

### Expected Results

For a given resistor, the greater the voltage, the more current flows.

As resistance increases, less current flows for a given voltage.

Voltage increases linearly with current. The slope of the line is the resistance the current is flowing through.

### Why It Works

Ohm's law is given by volts = resistance (ohms) × current (amps).

From this, you can see that the slope of the volts versus the current graph is resistance.

### Other Things to Try

What happens if you have two or three resistors of the same resistance in a row, one connected to the next? This is called a *series circuit* and is shown in Figure 99-3. For a given voltage, is the current greater or less than for a single resistor? What happens if you take those same three resistors and connect them in a parallel circuit, as shown in Figure 99-4?

### The Point

Ohm's law relates the voltage, current, and resistance of a circuit. The voltage at any particular time equals the current times the resistance.