MadSci Network: Engineering

Re: Why does heavy resistance draw little current from a circuit?

Date: Tue May 9 13:05:58 2006
Posted By: Chris Seaman, Staff, Electrical Engineering, Materials Engineering, Alcoa Technical Center
Area of science: Engineering
ID: 1144852152.Eg

There are two fundamental measurements to make in an electronic circuit: voltage and current. Voltage is a measure of the "force" pushing the electrons, current is a measure of the "flow" of the electons.

When you measure voltage, you are making a measurement "across" the circuit. Some of the current is "leaking" out of the circuit through the voltmeter. You don't want a measuring device that affects what you are trying to measure. If the resistance of the voltmeter is low, it will change how the circuit behaves. As the resistance of the voltmeter increases, the less "leakage" of current. Usually, a voltmeter is rated in meg-ohms, or millions of ohms; a very high resistance.

When you measure current, you must make the measurement "within" the circuit. The ammeter must be made part of the circuit. Since the ammeter must "resist" the current to make the measurement, an ammeter will have a very low resistance, probably milli-ohms or millionths of an ohm.

It is much easier to measure voltage than current, because you don't have to break into the circuit.

The purpose of resistance is to control current and voltage. If your goal is to get electricity from one point to another, such as delivering power, you want the resistance to be as low as possible so as not to waste energy.

However, you may need to control the electricity. An example of this is a volume control on a radio. If you want loud volume, you send more electricity to the speakers by making the resistance high in a portion of the circuit. When the neighbors complain and you need to turn the volume down, you make the resistance low, sending less electricity, and power, to the speakers.

The following link takes you two a webpage explaining a "voltage divider", one of the most common types of control circuits:

On this page, the load could be your speaker, where you want low resistance, or your voltmeter, where you want high resistance. Try different values and see the effect on the circuit.

Chris Seaman
Alcoa Technical Center

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