Ohms Law

Chapter 8

8.1 Ohm’s Law

You know about three important electrical quantities: voltage, current, and resistance. Of the three, current is the one that carries energy through a circuit. How does the current depend on the voltage and resistance? In this section you will learn the fundamental relationship for circuits known as Ohm’s law.

What is Ohm’s law?

The relationship If you have been working with circuits, you probably have an idea of how voltage, between amps, current, and resistance are related. You know that if you increase the voltage, the volts, and ohms current goes up. You know that if you increase the resistance by adding a second light bulb, the current goes down. Ohm’s law German physicist Georg S. Ohm (1787-1854) experimented with circuits to find the exact mathematical relationship present in most circuits. The relationship that he discovered is called Ohm’s law.

Equation I = V/R V = IR R = V/I

Gives you... current (I) voltage (V) resistance (R)

If you know... voltage and resistance current and resistance voltage and current

Figure 8.1: Ohm’s law in a circuit.

8.1 Ohm’s Law

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Using Ohm’s law to analyze circuits

Ohm’s law can be used to predict any of the three variables given the other two. Sometimes you want to know the current in the circuit. Sometimes you want to know voltage or resistance. Use the problemsolving steps to help set up and work through problems. Example A light bulb with a resistance of 2 ohms is connected to a 1.5 volt battery as shown. Calculate the current that will flow. Example A light bulb requires 3 amps to produce light. The resistance of the bulb is 1.5 ohms. How many batteries do you need if each battery is 1.5 volts? We are asked for the number of batteries, which means we need to know the voltage since each battery is 1.5 volts. We know current and resistance. Use the formula V = IR. Plug in numbers. V = 3 A × 1.5 Ω = 4.5 V Each battery can produce 1.5 volts so we need three...