The devices thgat do not follow ohms law?

Answer 1

All devices follow Ohm's law. Ohm's law states that voltage is equal to current times resistance. This is always true. DC, AC, RF, whatever. Resistance, capacitance, or inductance, whatever. Its a matter of proper perspective, and its a matter of taking everything into account.

Some devices do not have a linear response to voltage or current.

Take the light bulb, for instance. If you measures its resistance you can calculate what you think its current and power would be at a certain voltage, and you would be wrong, unless you also considered temperature. This is because light bulbs have a very dramatic resistance to temperature coefficient. As an example, a 40W applicance bulb might have a cold resistance of 27 ohms. That translates to 4.4A or 533W, with a 120V source. The truth is that the 40W bulb has a hot resistance of 360 ohms, giving an on current of 0.333A and an on power of 40W, with a 120V source.

For another example, look at the diode. Initially, it appears to have high resistance. When the voltage across it reachs the forward bias voltage (or the reverse bias voltage for a zener diode) it starts to conduct and draw current. As you increase the current, you note that the voltage is relatively constant. That is certainly not a resistor - the diode's resistance is increasing as current increases so as to make the voltage be somewhat constant. You can still calculate voltage, current, resistance, and power, but only at each point of observation.

For the last example, though not really an example of non-linear resistance, consider the capacitor. In a DC circuit, the capacitor initially has zero ohms resistance, but it quickly rises to infinity ohms depending on current and capacitance. In an AC circuit, this a much more interesting and useful case - the capacitor stabilizes at a certain capacitive reactance, and the operation of the curcuit complies with Ohm's law, but lo and behold - you find that the current is no longer in phase with the voltage. While this complicates calculations, nothing changes the fact that voltage is current times resistance. That is an immutable given - your task is to learn how to measure it correctly.

In each of these cases, and in any other case, Ohm's law applies, but it applies in a certain way, under a certain set of circumstances. The bulb has 360 ohms of resistance at 120V, giving 0.333A and 40W. That is very clear. What you must maintain equally clear is that resistance is not always a constant - and you must consider that inconstancy every time you change the conditions of the circuit.

AnswerOhm's 'Law' is somewhat of a misnomer, because it is not really a universal law because it only applies to a small range of conductors.

Ohm's Law only applies when the ratio of voltage to current is constant for variations in voltage. Devices that follow Ohm's Law are called linear or ohmic; those that don't are called non-linear or non-ohmic. Most metal conductors are linear, but some are not. For example, tungsten does not obey Ohm's Law because its ratio of voltage to current changes as the voltage applied across it changes -in other words, tungsten is non-linear. Electronic devices, such as diodes, electrolytes, and gases are all non-linear and do not obey Ohm's Law.

Having said that, the ratio of voltage to current will always tell you what the resistance of a device happens to be for that particular ratio and, so, the equation R = V/I applies to all devices whether they are linear or non-linear. However, this equation is not derived from Ohm's Law, but from the definition of resistance.