self-inductance, back-emf In self-inductance calculations, why the back emf is in negative sign? 
For example: it is given that the emf induced in the solemoid is 5.0V in the question.
E=-L(dI/dt)
-5.0=-L(dI/dt)…
Something like this. Why there is a negative sign in front of 5.0V ?
Thank you.
 A: There may be some confusion due to failing to distinguish between the emf $\mathcal{E}$ and the potential difference $v_L$ across the inductor.
For example, if a battery of voltage $V$ is connected across an inductor, you will measure a voltage $V$ across the inductor and, in the case of an ideal inductor, you will find that the rate of change of inductor current $i_L$ is (assuming the passive sign convention)
$$\frac{di_L}{dt} = \frac{V}{L}$$
since, for an ideal inductor, we have
$$v_L = L \frac{di_L}{dt}$$
The associated emf $\mathcal{E}$ is of the opposite sign since it must 'oppose' the applied voltage if the inductor current is to be finite and so, in the above case, we have
$$\mathcal{E} = - V = - L \frac{di_L}{dt}$$.
A: The self inductance is a characteristic property by which a current carrying coil generates some emf across it against the applied emf that makes a current flow through the coil. The principle is as follows.
When a current starts to flow through a coil, the magnetic flux linked with the coil changes and this happens only for a few seconds. This change in magnetic flux induces a curling electric field as given by Faraday's law. This electric field induces an emf on the coil in a direction opposite to the cause that causes a changing magnetic flux. The changing magnetic flux is the increase in current. So the induced emf opposes this applied emf to produce a current in the circuit. The energy required for this is stored in the magnetic field of the coil. So the induced emf has a polarity opposite to the applied field. This is why the back emf is represented by a negative sign.
A: Think of it as blowing up a balloon. When you release the balloon the flow is in the opposite direction to when it was inflated.The pressure (voltage) opposes the inflation.
