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Ohm's Law

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Look at the water spraying out of this garden hose. You have to be careful using water around power tools and electric outlets because water can conduct an electric current. But in some ways, water flowing through a hose is like electric current flowing through a wire.

Introducing Ohm’s Law

For electric current to flow through a wire, there must be a source of voltage. Voltage is a difference in electric potential energy. As you might have guessed, greater voltage results in more current. As electric current flows through matter, particles of matter resist the moving charges. This is called resistance, and greater resistance results in less current. These relationships between electric current, voltage, and resistance were first demonstrated in the early 1800s by a German scientist named Georg Ohm, so they are referred to as Ohm’s law. Ohm’s law can be represented by the following equation.

Voltage (Volts) = Current (Amps) x Resistance (Ohms)
                       V = IR

Understanding Ohm’s Law

Ohm’s law may be easier to understand with an analogy. Current flowing through a wire is like water flowing through a hose. Increasing voltage with a higher-volt battery increases the current. This is like opening the tap wider so more water flows through the hose. Increasing resistance reduces the current. This is like stepping on the hose so less water can flow through it. If you still aren’t sure about the relationships among current, voltage, and resistance, watch the video at this URL: http://www.youtube.com/watch?v=KvVTh3ak5dQ

Using Ohm’s Law to Calculate Current

You can use the equation for current (above) to calculate the amount of current flowing through a circuit when the voltage and resistance are known. Consider an electric wire that is connected to a 12-volt battery. If the wire has a resistance of 2 ohms, how much current is flowing through the wire?

Current = \mathrm{\frac{12\;volts}{2\;ohms}} = 6 amps

Q: If a 120-volt voltage source is connected to a wire with 10 ohms of resistance, how much current is flowing through the wire?

A: Substitute these values into the equation for current:

Current = \mathrm{\frac{120\;volts}{20\;ohms}} = 12 amps

Summary

  • According to Ohm’s law, greater voltage results in more current and greater resistance results in less current.

Vocabulary

  • Ohm’s law : Law stating that current increases as voltage increases or resistance decreases.

Practice

Review Ohm’s law and how to calculate current at the following URL. Then try to solve the two problems at the bottom of the Web page. Be sure to check your answers against the correct solutions. http://www.grc.nasa.gov/WWW/k-12/Sample_Projects/Ohms_Law/ohmslaw.html

Review

  1. State Ohm’s law.
  2. An electric appliance is connected by wires to a 240-volt source of voltage. If the combined resistance of the appliance and wires is 12 ohms, how much current is flowing through the circuit?

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