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Ohm's law
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If you are given the voltage and current in a circuit, what formula can you use to calculate the resistance?
Hi, Kim! What’s in the bag? Batteries, cords, a lamp, a switch, a voltmeter, an ammeter. Oh! You’re going to do an experiment.
Nice! Do you have a hypothesis? What do you want to show? There is a relationship between voltage, current and resistance. Interesting.
Okay, you connect a battery to a lamp and a switch. And you’ll measure the voltage across the battery terminals and the current in the circuit. All set? Good! Turn on the current.
Mm, the voltage is one-point-five volts, and the current is zero-point-two amps. You write it down, huh, Kim? What are you doing now? Two batteries? Connected in series?
And you’re going to measure the same way. Now the voltage is three volts, and the lamp shines brighter. That’s because the current is larger. Zero-point-four amps. Take a note of that too.
Are you adding yet another battery? Connecting them all in series? Okay, now the voltage is four-point-five volts. The lamp is shining really brightly now. Because the current is zero-point-six amps.
Write that in the table, too. Okay. So you’ve done the lab work. What next? Yes, draw a graph.
The current on one axis, the voltage on the other. Oh! Look. There is a relationship. The hypothesis is correct!
The graph starts at the origin and is a straight line! It’s a linear relationship. The larger the current, the larger the voltage. The voltage is therefore proportional to the current. In other words, the voltage equals a constant times the current.
That’s how linear relationships work. That constant, where do you find it, Kim? Yes, in the table. One more column. The voltage divided by the current.
And it is seven-point-five, seven-point-five and seven-point-five. Seven-point-five, regardless of the current and voltage in the circuit. There it is: the constant. But what is it really, Kim? It’s the resistance.
The resistance in this circuit is seven-point-five ohms. The voltage equals the resistance times the current. Voltage equals resistance times current. This is called Ohm’s law. Voltage equals resistance times current, and you showed it with your experiment, Kim.
The hypothesis is correct. As a memory-aid, when calculating using Ohm’s law, try this triangle. If you need to know the voltage, then cover that with a finger. What do you see? Resistance times current.
Voltage equals resistance times current. Want to determine the resistance? Cover that with a finger. Resistance equals voltage divided by current. See?
And to calculate the current, cover that with a finger. Current equals voltage divided by resistance. See how the triangle works? Pause the film if you need to let it sink in. Now we let Ohm’s law and the memory triangle sit there, while we look at an example.
In this circuit we have measured the current as zero-point-five amps. The voltage is nine volts. What’s the resistance in the circuit? Ohm’s law tells us that voltage equals resistance times current. Then resistance equals voltage over current.
So the resistance is nine volts divided by zero-point-five amps. Eighteen ohms. Ohm’s law describes the relationship between voltage, resistance, and current. The voltage equals the resistance times the current. Well done, Kim!