Ohm's Law: Running into Resistance.
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)
- DC power supply (or battery)
- wires to connect to battery terminals
- 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.
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?
Ohm's law relates the voltage, current, and resistance of a circuit. The voltage at any particular time equals the current times the resistance.
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