Saturday, October 8, 2011
Electric Heater Resistance - A Moving Target
Most people know that inductive loads such as electric motors draw more current when they start. Did you know that this is also true of most electric strip heaters? The resistance of a conductor increases as its temperature increases. This is true of the nichrome wire used in electric heaters. The heater resistance is less when the heating coil is cold than after it heats up. The increase in resistance causes a decrease in current. In short: an electric heater draws a little more current when it starts than after it heats up. The difference for an electric strip heater is nothing like the huge surge that a motor has, but it is measurable. For incandescent light bulbs, the difference is huge because the temperature difference between hot and cold is much greater. An incandescent light is basically a heater that gets white hot and gives off light. Lights don’t work for Ohm’s Law experiments because their resistance hot is around 10 times as high as their resistance cold. Many years ago I tried having students do Ohm’s Law calculations with lights – it just does not work. The readings were so far off, that the results did not make sense. I tried to put the difference down to measurement error, but the readings were always similar. Screw in cone heaters work better, but if you put enough current through them for them to get red hot, their resistance will increase and your readings will still be a little off. At least they are in the ballpark. It is something of a catch 22. To measure resistance an ohm meter puts a small amount of voltage across a resistance and measures the current, and then determines the resistance based on the amount of current that flows through the resistance. The problem is that the resistance changes when different amounts of current flow through the same device, based on the temperature change of the device. Remember that the resistance is turning the electricity into heat, and the heat changes the resistance of the device. Fortunately, the relatively small difference we see in electric strip heaters does not create a substantial problem for us, but it does raise questions when trying to demonstrate Ohm’s Law. If you want to reduce the amount of error introduced by the temperature change, just work with lower voltages and current levels. Use a typical electric strip heater, but only put 50 volts through it. The current and voltage are still measureable and the results will be much closer to what Ohm’s Law predicts. I know there are probably many skeptics reading this, so I challenge you to prove it to yourself. Take a heater, a variable voltage source, a voltmeter, and an ammeter. Read the current draw of the heater at several voltages ranging from just enough voltage to get a current reading to the full voltage the heater can take. Then calculate the resistance using Ohm’s Law by dividing the measured current into the measured voltage. You should see that the resistance is higher at higher voltages because the heater is getting hotter. Have Fun!