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Resisting CurrentResistance, any kind of resistance, is when one force slows down or gets in the way of another force. Electrical resistance is the force that gets in the way of the flow of current. Scientists measure resistance in ohms (rhymes with homes). There is a magical little formula used to figure out the resistance in an electrical system. That formula is called Ohm's Law.Measuring Resistance
The value "V" is something called the potential difference. The potential difference is the amount of work created by the movement of a charge. The potential difference is measured in volts. "I" is the current and "R" is the resistance for the system. Current is measured in amperes and resistance is measured in ohms. On an atomic level, resistance is thought of as the impedance (blockage) to the flow of charges through the object. Impedance is the resistance of all the parts of a system added together.
How can you think of resistance? Have you ever gone to a baseball game? Between innings, we like going to get some food. There are always people between the counter and us. Resistance to current is similar to you trying to make your way through the crowds to get your hot dog. You have to weave your way through the people to reach your goal. The more people in your way, the more resistance. If everyone is in their seats it is super-easy to get your food. There would be very little resistance. Let's go back to that equation and look at it in terms of resistance. When you move the values around you get R=V/I. In English that means the resistance of a system is based on voltage and current. Not all conductors follow Ohm's law. 
Resistance is also based on the resistivity of a material. The resistivity of a material changes because of chemical makeup or the temperature. Copper is a better conductor than wood so copper would have lower resistivity. That resistivity combines with (1) the distance and (2) the space that charges have to move in (thin vs. thick wires) to affect the "R" value.
Knocking Electrons AroundIn metals, electrons carry the charges of the current as it flows. What stops the electrons? What offers the resistance to that current? Nothing allows a perfect flow of current, not even super-conductors. In metal, there are tiny flaws. You can't see them because they are on a molecular level. Those imperfections cause the electrons to collide with the metal atoms. When they hit the metal, the electrons lose energy. Where does that energy go? It is usually turned into heat. You can watch a hot plate heat up, or maybe a stovetop. They heat up because of the friction between electrons and the metal. Imperfections mean collisions; collisions mean heat.
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Go for site help or list of physics topics at the site map!©copyright 1997-2007 Andrew Rader Studios, All rights reserved. Current Page: Physics4Kids.com | Electricity & Magnetism | Resistance |
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