Electricty generation / MPG

englishpremier · 13 Mar 2011 at 16:32

Sorry, maybe a daft question, but can anyone tell me why people say that using more electronic devices in the car uses more fuel?

Isn't electricity generated by generated by the movement of a loop of copper wire, between the poles of a magnet?

The speed of the spinning is dictated by the speed of the engine.

With this in mind, how does it make a difference how many electronic devices the car uses?

Jokester · 13 Mar 2011 at 16:35

Because you're putting more electrical load on the alternator, so the engine has to do more work, even if it's running at a constant speed.

Tealc · 13 Mar 2011 at 16:53

If you put loads of electric devices on in a car you can sometimes hear the engine sound change as the alternator pulls more load to the battery and causes a few extra rpms. You can hear it at idle more clearly than at speed though.

thepeganator · 13 Mar 2011 at 17:11

Its the same concept of the AC drawing power away from the engine.

You would have to be using a lot of gadgets though to make as much as a difference s the AC!

I can feel the drop in power turning the AC on on the motorway, speed drops about 2mph from 70 when keeping my foot still on the accelerator!

ZG002 · 13 Mar 2011 at 17:13

Can hear it at idle in my car just turning my lights on lol

Minstadave · 13 Mar 2011 at 17:14

Conservation of energy innit, the generation of energy by the alternator means the axel is slowed so you have to increase engine output for a given speed.

Minstadave · 13 Mar 2011 at 17:16

thepeganator said:
Its the same concept of the AC drawing power away from the engine.

You would have to be using a lot of gadgets though to make as much as a difference s the AC!

I can feel the drop in power turning the AC on on the motorway, speed drops about 2mph from 70 when keeping my foot still on the accelerator!

Sightly different the AC is driven directly by the engine so it's obvious this uses energy, I think the op isn't aware that the generation of energy by the alternator uses energy, despite there being no contact between the wire and the magnet producing the magnetic field.

thepeganator · 13 Mar 2011 at 17:22

Minstadave said:
Sightly different the AC is driven directly by the engine so it's obvious this uses energy, the op isn't aware that the generation of energy by the alternator uses energy, despite there being no contact between the wire in the magnetic field.

All driven by the same belt are they not?

PhillyDee · 13 Mar 2011 at 17:24

thepeganator said:
All driven by the same belt are they not?

Yes, but the alternator is ALWAYS running. AC only when you enable it will it engage the motor.

Minstadave · 13 Mar 2011 at 17:38

thepeganator said:
All driven by the same belt are they not?

Yeah you're right, edited it to make it a bit clearer. The generation of energy by the alternator uses energy although the rotating magnet doesn't touch anything, it still creates a slowing force on the engine.

englishpremier · 13 Mar 2011 at 18:17

It still doesn't make any sense to me. Surely the altenator just spins around some magnets or spins a around a coil. And it spins as the engine is spinning.

This is my understanding of electricity generation. http://www.youtube.com/watch?v=MBMYdE5NQOQ

So imagining that the wand he has in that video moves up and down as the engine spins, then how does it make any difference as to how many devices are plugged in? The wand would always move at the same speed for a given RPM. e.g 100 movements at 1000rpm, 500 movements at 5,000 rpm.

Edit: ok so the alternator rotates a magnet that doesn't touch anything. how is there a slowing force on the engine when it magnet doens't touch anything, so no friction. Surely the pull from the magnet never changes.

Minstadave · 13 Mar 2011 at 18:41

When no current is being produced in the around the alternator (ie everything off) the magnet is free spinning and no electricity is produced.

When a current is being produced the rotating magnet induces an electromotive force in the stationary wire (it pushes the electrons, creating the current), so experiences a decelerative force. Kinetic energy is converted into electricity.

The decelerative force is not due to friction, it is due to the having to push a wire through the magnetic field effectively and creating the electromotive force (although in the alternator its the magnet that is pushed relative to the wire).

LoGaN_TnT · 13 Mar 2011 at 19:12

Is this why motorbike engines sound better

Mr^B · 13 Mar 2011 at 19:21

englishpremier said:
It still doesn't make any sense to me. Surely the altenator just spins around some magnets or spins a around a coil. And it spins as the engine is spinning.

This is my understanding of electricity generation. http://www.youtube.com/watch?v=MBMYdE5NQOQ

So imagining that the wand he has in that video moves up and down as the engine spins, then how does it make any difference as to how many devices are plugged in? The wand would always move at the same speed for a given RPM. e.g 100 movements at 1000rpm, 500 movements at 5,000 rpm.

Edit: ok so the alternator rotates a magnet that doesn't touch anything. how is there a slowing force on the engine when it magnet doens't touch anything, so no friction. Surely the pull from the magnet never changes.

Your assumption that there is no slowing force is incorrect. Just because there's no friction doesn't mean there aren't any forces acting which slow it down.

Isn't this covered in GCSE physics any more?

mattyprice4004 · 13 Mar 2011 at 19:26

The more load there is on the alternator, the more turning force is needed to keep it spinning at the same speed. This is why you need more fuel to keep it turning.

thepeganator · 13 Mar 2011 at 19:34

The alternator has magnets spinning in a coil.

The magnets have a magnetic field like this:

Even though the magnet is spinning not the wire, we can imagine it is the other way round.
For the wire to cut through the magnetic field lines (and hence create moving electrons and electricity) it takes energy, so that's why it creates a load on the engine.

Imagine trying to force two magnets (N and N or S and S) together, or pull S and N apart, it takes work.

Similar principle.

englishpremier · 13 Mar 2011 at 21:59

Fair enough. spinning the magnet in the demo generate at school was pretty easy and I didn't feel any resistance no matter how many light bulbs were attached. But I understand the theory. Cheers.

PhillyDee · 13 Mar 2011 at 23:30

Those little bulbs use very little amounts of power though!

starfighter · 14 Mar 2011 at 06:37

It's kinda like if you had a power station which basically have massive alternator/generator you couldn't run it from your 1.6 petrol engine and it aint just to do with friction.

You can't make or destory energy, only change it's state. GCSE science ***.

monkey_boy · 14 Mar 2011 at 07:07

Mr^B said:
Your assumption that there is no slowing force is incorrect. Just because there's no friction doesn't mean there aren't any forces acting which slow it down.

Isn't this covered in GCSE physics any more?

Not for a few years afaik. We only covered eddy currents at A level, and that was a few years ago now. Some kids today don't even do a GCSE physics course. :eek: