# Why the induced emf is always negative?

1. Jan 16, 2008

### Physicsissuef

Last edited by a moderator: May 3, 2017
2. Jan 16, 2008

### Ronnin

Lenz law supplies that answer. It basically states that the EMF that is induced will be in such a way to create an opposing magnetic field to the one inducing the EMF. This law stems from the conservation of energy.

3. Jan 16, 2008

### Physicsissuef

Conservation of energy of what?
Btw- Look at this http://www.youtube.com/watch?v=stUDqGzpev8". The voltage is not negative, always.

Last edited by a moderator: Apr 23, 2017
4. Jan 16, 2008

### rohanprabhu

Simply put, if the induced emf was in the direction of the emf of the circuit, then the inductor would increase the voltage in the circuit. Which would increase the current flowing across the inductor. As such, it would further increase the induced emf [they are proportional] as $B = \mu_o NI$. This increase would further increase the the emf and as such, the voltage should rise exponentially. Which would mean, we could derive infinite energy from the system.

as we know that this is not possible, the induced emf cannot be in the same direction. Although, this cannot be used to prove Lentz law, but is helpful for understanding the situation.

5. Jan 16, 2008

### Staff: Mentor

The negative sign in Faraday's law (Lenz's law) does not mean that the EMF (or current) always points in some "negative" direction. It means that the current always flows in a way to oppose the change in flux, which is nicely illustrated in that video clip.

Last edited by a moderator: Apr 23, 2017