# When would a circuit be nonohmic ?

1. Jun 14, 2009

### Jules18

When would a circuit be "nonohmic"?

When would a circuit be "nonohmic"? I always thought Ohm's law was universal.

2. Jun 14, 2009

### Staff: Mentor

Re: Nonohmic??

Have you seen diode in action?

3. Jun 14, 2009

### Civilized

Re: Nonohmic??

Ohm's law is not at all universal, it just comes from the empirical observation that the current density J which arises in a conductor due to the prescence an external electric field E will usually itself be proportional to the applied electric field:

$$\vec{J} = \sigma \vec{E}$$

Where $\sigma$ is called the 'conductivity' of that particular conductor, and this conductivity is 'universl' for the particuar material e.g. copper (at a certan temperature). Two generalizations are possible: sometimes the response of the material is not linear, and so the relationship is not simple proportionality; in a diode, it is possile to apply an electric field below a certain threshold strength and in a particular direction and have the output current be zero. Another possible generalization is that the electric field and the current desity do not have to be parallel, for example in crystal that has its own preferred directions for conduction channels.

4. Jun 14, 2009

### Born2bwire

Re: Nonohmic??

Heh, another fun example is take a capacitor rated for say 5 V or a resistor rated for 0.25 mW and hook them up to a voltage source. Now slowly increase the voltage and see what happens. Well... maybe I shouldn't recommend that you intentionally see what happens but I'm sure you can guess how it is a one time demonstration of non-ohmic properties.