Electromotive force when current is alternating

[mcastillo356]

Hello, let's imagine a generator as shown in image. I just don't understand alternative current.
-\epsilon shouldn't be always positive? Otherwise, how can it provide anything?
Thanks

 

[A.T.]

I just don't understand alternative current.

What exactly don't you understand? How it's generated or how it's useful?

 

[mcastillo356]

Sorry, I don't understand how it's useful, if sometimes is negative \epsilon, and, also, if the direction of the current changes direction; sometimes towards the consumer, and sometimes the opossite direction, towards the source

 

[A.T.]

Sorry, I don't understand how it's useful,

The light bulb will doesn't care which way the current flows, it will flicker tough when the direction changes and the current is zero. Other consumers might need to convert the current first.

 

[etotheipi]

Sorry, I don't understand how it's useful, if sometimes is negative \epsilon, and, also, if the direction of the current changes direction; sometimes towards the consumer, and sometimes the opossite direction, towards the source

The direction in which the physical charge carriers are moving doesn't change the power dissipated. Like @A.T. mentioned, you can connect a light bulb up with either end connected to either end of the battery and it will still turn on. In each case, the charge carriers are moving in the direction of decreasing potential energy and are transferring energy to the lightbulb.

 

[Dale]

Sorry, I don't understand how it's useful, if sometimes is negative \epsilon, and, also, if the direction of the current changes direction; sometimes towards the consumer, and sometimes the opossite direction, towards the source

The purpose of the electrical grid is to deliver electrical power. That power is what makes it useful.

We know that the power can be written $P=IV$ where $I$ is the current and $V$ is the voltage. If $I$ is negative and $V$ is also negative then $P$ is positive and therefore still provides useful power.

You cannot consider either the voltage or the current separately, you need to consider them together. When you do you find that the power is always positive

 

[mcastillo356]

Thanks, etotheipi, A.T, Dale!

 

[A.T.]

You cannot consider either the voltage or the current separately, you need to consider them together. When you do you find that the power is always positive

The consumed power (defined as Dale notes) is always positive at the consumer, but negative at the power source, like the generator.

 

[Delta2]

On why $\mathcal{E}$ becomes negative, there are two explanations

• one that uses Faraday's law of induction and is mostly mathematical,
• one that uses the Lorentz force law and it is a bit more qualitative-intuitive than mathematical

Which one do you want to hear?

just for reference
https://en.wikipedia.org/wiki/Faraday's_law_of_induction
https://en.wikipedia.org/wiki/Lorentz_force

 

[mcastillo356]

Sorry Dale, a condenser doesn't work that way. P and I are not in phase. How can I understand this?. In the case of a resistance, yes. But in the case of a condenser, P can be negative while I positive, or at the opposite

 

[Dale]

Sorry Dale, a condenser doesn't work that way.

Yes, everything works that way. $P=IV$ is a fundamental law in circuit theory and it always applies, 100% of the time, even for capacitors and inductors where $I$ and $V$ are not in phase.

For capacitors and inductors it means that during half of their cycle they are absorbing power from the circuit and during the other half of their cycle they are releasing power to the circuit. Over one full cycle their average power is 0 meaning that they do not dissipate power but just store it temporarily. The duration of the temporary storage can be used to tune circuits to specific frequencies.

 

[mcastillo356]

Thanks Dale!. I really appreciate your help. I've understood something difficult...No, not difficult, now that I've read your post.