Some really basic electronics questions.

[ldefeo1]

hi there I am a mech eng second year student and i have some electrical modules that i have to do though i just don't have much of a intuitive feel for it like i do for mechanical stuff. anyway some stuff i really should know but dont.

1) ac circuits. these as a whole confuse me, my understanding of electricity is that electrons have a a certain amount of potential (voltage) and they deliver it to loads unpon moving through it, at least in the case of direct current. though in ac circuits the current direction changes constantly. how they do electrons deliver their energy to the load? looking at it from an intuitive point of view it just seems like the electrons are oscillating about a fixed point not moving anywhere?

2) Q=IT ok i learned this is about 5 years ago but yeh current is rate of flow of charge. Now I am assuming the speed of electricity, or should i say the movement of electrons is a constant (the speed of light?) then how does this happen. I am trying to picture a wire and how move electrons can Physically move through it per second.

3) electrical power distribution/ the grid - ok so if i turn my kettle right now it draws current from the grid. so its like power is just available when its required. this is bit confusing as the power station obviously isn't just generating more electricity as I've turned my kettle on. so what's going on. there must be a surplus energy in the wire or something, the station generate slightly more than that is require or something but how can it be 'stored' or whatever in the grid. meh can someone help? maybe actually understanding ac theory might help with this.

cheers guys

 

[MATLABdude]

Welcome to PhysicsForums!

I find (as a non-MecE) that many water / wave analogies work with electricity (within reason). As with a water wave, it is not so much the individual water molecules as the bulk movement of the water as a whole. The individual water molecules (with a propagating transverse wave) don't move at nearly the speed of the wave, and so it is with electrons and current. The individual electrons have a very small drift velocity (physical electron speed as they move around from atom to atom) yet the electric field propagates at nearly the speed of light.

What helped me was to ignore the behaviour of individual electrons, and focus on the collective behaviour of 1.6E19 (or thereabouts) of them!

hi there I am a mech eng second year student and i have some electrical modules that i have to do though i just don't have much of a intuitive feel for it like i do for mechanical stuff. anyway some stuff i really should know but dont.

1) ac circuits. these as a whole confuse me, my understanding of electricity is that electrons have a a certain amount of potential (voltage) and they deliver it to loads unpon moving through it, at least in the case of direct current. though in ac circuits the current direction changes constantly. how they do electrons deliver their energy to the load? looking at it from an intuitive point of view it just seems like the electrons are oscillating about a fixed point not moving anywhere?

The current does change constantly, but instantaneously, there'll be a certain amount of current flowing through the load, and thus, you can work out an instantaneous power dissipation. Over time, you can figure out the total energy dissipated. Since there's no such thing as negative power, you get power dissipation on both the negative and the positive going cycles of the AC.

2) Q=IT ok i learned this is about 5 years ago but yeh current is rate of flow of charge. Now I am assuming the speed of electricity, or should i say the movement of electrons is a constant (the speed of light?) then how does this happen. I am trying to picture a wire and how move electrons can Physically move through it per second.

Refer to my comments at the top of this post. As well, the following page might be of help:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html

3) electrical power distribution/ the grid - ok so if i turn my kettle right now it draws current from the grid. so its like power is just available when its required. this is bit confusing as the power station obviously isn't just generating more electricity as I've turned my kettle on. so what's going on. there must be a surplus energy in the wire or something, the station generate slightly more than that is require or something but how can it be 'stored' or whatever in the grid. meh can someone help? maybe actually understanding ac theory might help with this.

It actually does generate more power as more load is applied. When you first start up a lot of high-powered equipment (say, when you're using a chop saw and trying to cut something big and tough), you'll often see the lights dim and flicker as you locally depress the voltage to draw the required startup current. As the system catches up, you go to normal operation.

I hope this helps!

 

[ldefeo1]

thanks for your response MATLAB dude. 1 and 2 were clear cheers but can you (or someone) clarify 3 for me. so your saying you turn on your appliance which draws current, what do you mean the systems catches up?

also how would a power station even generate more watts of electricity if it wanted to, wouldn't increasing the angular speed of a generator increase the frequency of the voltage...

one other thing, why is electricity generated 3 phase, why not 2 or 5?

sorry for what probably are pretty silly questions, my lecturers for electrical systems are pretty dire :)

 

[I_am_learning]

thanks for your response MATLAB dude. 1 and 2 were clear cheers but can you (or someone) clarify 3 for me. so your saying you turn on your appliance which draws current, what do you mean the systems catches up?

At first, the system instantly generate that extra power. So, to compensate, the voltage drops. But after some time the system starts generating that extra power, i.e. it catches up.

Also how would a power station even generate more watts of electricity if it wanted to, wouldn't increasing the angular speed of a generator increase the frequency of the voltage...

In synchronous generators extra power can be easily generated without increasing frequency of voltage by changing the excitation current.

Lots of theories hidden inside. But in general, they are cheaper in that less conductor is enough to carry same power to the load, at same voltage than 2 phase.
5 or more phases, I don't know.

 

[sophiecentaur]

I agree that electrons are totally irrelevant to getting most of elementary electrical theory. (Most of that stuff was sorted long before Chadwick's work).
Steer clear of analogies, get stuck into the very few simple formulae and believe their results. Electricity doesn't work by arm waving - it, very reliably, follows some very simple rules.
Remember:
1. Volts aren't a sort of force.
2. Resistance is not something that pushes against a current.
3. Power out is always less than Power in.

etc. etc.

 

[ldefeo1]

yeh sorry still not following the process of drawing current from the grid. your saying the system instantly generates more power? from where? by what means. where is this dormant power and how is it manifested. thanks. if everyone switched off their lights all of a sudden what happens to the power generated by the power station?

also what do you mean by increasing excitation current.

sorry for being so rubbish at this... :(

 

[sophiecentaur]

Left to themselves, the generators all over the grid would speed up and slow down (just like a car driving up and down hills) as the load varied. This is not acceptable because the 50 Hz needs to be held accurately. Regulators on the turbines allow more or less steam in, to keep the frequency constant (just as you vary your pressure on the accelerator pedal if you want to keep at a constant speed), and, in the slightly longer term, more or less fuel is supplied to the boilers.

If your little local system suddenly demands or supplies a bit of power, a minuscule (medium term) adjustment is made on the 'big suppliers' on the Grid. But the small amount that a single domestic user is responsible for is like someone jumping onto a moving train or pedalling a bit 'to help it along'. The difference is finite but small enough for the inertia of the system to ignore.

One way of instantly varying the volts produced by an alternator is to change the current supplied to the field windings. This system is used on your car alternator to keep the charging volts right, despite huge variations of the electrical load.
When everyone turns their heating off or on, the Grid Manager has to alter the power generated almost instantly and it can be a serious embarrassment. Nuclear Power stations (the reactors) take a long time to get going or turn down and special measures need to be taken to make sure that the electrical load is not suddenly removed or a real disaster could occur. They try to ensure, for instance, that there is more than one Grid line feeding a Nuclear station, in case one should be damaged.
P.S. Don't apologise. This stuff is not as intuitive as some people like to make out.