Question about ohms law and non-ohmic behavior.

In summary, Ohm's law, which defines the relationship between voltage, current, and resistance, may not always hold true due to factors such as temperature and the use of different components, particularly those based on semiconductors. While some components may exhibit a linear relationship between voltage and current, others, like the tunnel diode, may display non-ohmic behavior. Resistance is usually considered in terms of a specific temperature and can be seen as the variation of current density not being linearly related to the applied electric field.
  • #1
mrspeedybob
869
65
I recently learned that ohms law is not always obeyed. I found this surprising because I thought ohms were defined in terms of volt and amperes by the equation R=V/I. I did a little googling and found the following definition...

1.The SI unit of electrical resistance, expressing the resistance in a circuit transmitting a current of one ampere when subjected to a potential difference of one volt.

This clarified things a little in that the definition of an ohm specifies exactly 1 ampere, 1 volt, and 1 ohm. So if I have a wire with one ohm of resistance I know that if I apply 1 volt of potential, 1 ampere of current will flow. If however I apply 10 volts more current will flow, the wire will heat up, and its resistance will increase, so less then 10 amperes will flow.

My question is, if other conditions (such as temperature) are artificially kept constant as voltage and current vary, does ohms law hold more true or absolutely true?

Consider a spark plug. If I try to measure its resistance using ohms law while applying 100,000 volts will I get the same answer as if I ionize the air between the electrodes with another heat source and measure the resistance with 1 volt?
 
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  • #2
What is meant by the statement is that whilst the ratio of voltage to current is always called the resistance (and measured in ohms), this ratio may vary with voltage or current.

So if we plot a graph of current sgainst voltage we may get a straight line of constant slope = ratio of voltage to current. Such a component is called a resistor.

However many components, particularly those based on semiconductors, display a different current v voltage curve.

A particularly stark example is the tunnel diode which has a negative slope at certain parts of its I-V curve.

http://en.wikipedia.org/wiki/Tunnel_diode
 
  • #3
Resistance is usually meant "for a specific temperature". An ideal resistor has a linear relation voltage<->current, if you keep its temperature and other conditions constant. Many metals are good approximations of ideal resistors (in a reasonable range), even if their resistance depends on the temperature.
 
  • #4
May be it will be better to say that non-ohmic behaviour should be thought as variation of current density is not linearly related to electric field applied.
 
  • #5


I can confirm that Ohm's Law is a fundamental principle in electricity and is typically valid for most conductors under certain conditions. However, there are some materials and scenarios where Ohm's Law does not hold true, and these are referred to as non-Ohmic behaviors.

One example is when the temperature of a conductor changes. As you mentioned, as the temperature of a wire increases, its resistance also increases, causing a deviation from Ohm's Law. In this case, if the temperature is kept constant, then Ohm's Law will hold true.

In the case of a spark plug, the air between the electrodes behaves as an insulator until it is ionized by the high voltage, creating a plasma and changing the resistance of the air. This non-Ohmic behavior is not a violation of Ohm's Law, but rather a result of the changing conditions of the material.

In conclusion, Ohm's Law is a valuable tool for understanding and predicting the behavior of electrical circuits, but it is important to recognize that it is not always applicable in all scenarios. Other factors such as temperature, material composition, and external influences can affect the resistance of a conductor and lead to non-Ohmic behaviors.
 

1. What is Ohm's Law?

Ohm's Law is a fundamental principle in physics that describes the relationship between voltage, current, and resistance in an electric circuit. It states that the current through a conductor is directly proportional to the voltage across it, and inversely proportional to the resistance of the conductor.

2. How is Ohm's Law expressed mathematically?

Ohm's Law is expressed as V = IR, where V is voltage in volts, I is current in amperes, and R is resistance in ohms. This equation can also be rearranged to solve for other quantities, such as I = V/R or R = V/I.

3. What is the difference between ohmic and non-ohmic behavior?

Ohmic behavior refers to a material or circuit component that follows Ohm's Law, meaning that its resistance remains constant regardless of the applied voltage. Non-ohmic behavior, on the other hand, describes a material or component whose resistance changes as the voltage changes, deviating from the linear relationship described by Ohm's Law.

4. What are some examples of non-ohmic behavior?

Some examples of non-ohmic behavior include diodes, transistors, and thermistors. These components have a non-linear relationship between voltage and current, meaning that their resistance changes with the applied voltage.

5. Why is understanding non-ohmic behavior important?

Understanding non-ohmic behavior is important for designing and troubleshooting electronic circuits. It allows us to predict how different components will behave under different voltage conditions and ensure that the circuit functions properly. It also helps us to identify faulty components and diagnose issues in a circuit.

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