Leaving Cert Physics (Electricity)...

JackKelly

Ok, i've tried to understand this but it still really doesnt make much sense. Regarding electricity, i still don't understand what voltage is. Its the energy lost in a circuit by 1 columb moving from one point to an other?wtf. So in a circuit , a battery has a certain voltage.What does this mean? I don't get how a potential difference is set up in the cell, and i don't get what effect it has on a circuit.And what does a high voltage actually mean, (danger wise or energy wise) Also, current is an easy one to understand, but i don't see how, when you have a low voltage, you have a high current etc? So can someone please give me a hand with this basic physics! Argh.

breadmonkey

tbh I never understood this sh!t, but the reason the current goes up when the voltage does is because v=r*i (ohm's law). So you can see that a big v makes a big i.

chump

i'm talking nonsense ...

snappieT

I was told that voltage was the "Pressure, pushing the current through the resistance"

Dunno if that helps

*Angel*

The way I see it is that there is a p.d/voltage between the two terminals of a battery and this causes a current to flow, when there is a circuit for it to flow through.

sephirosis

bear in mind im just doing LC physics too, so my explanation will probably be bad/inaccurate, given the atrocious amounts of stuff the people who set the course think we "wont want to know until college"

the best analogy i can give is that of mechanical potential energy. we have a reference point for potential energy, being the earths surface. at the earths surface, you have potential energy of 0. then if you are at any height above the earths surface, you have a potential energy equal to the amount of work needed to get you back to 0 potential energy.

similarly, the earth has an electrical potential of zero. all voltages on batteries or whatever are given in relation to this, ie if you have a six volt battery, it has the effect similar to passing charge from a point of 6 volt potential difference to earth. the charge, as such, "falls" from higher voltage to lower voltage, and like the gravity analogy, work is done in moving the charge from the point at 6v to the point at 0v (or from 12v to 6v, or 20v to 14v, or whatever).

therefore work done in moving object from one height to another = difference in potential energy, work done in moving charge from one electrical potentail to another = difference in electrical potential.

and the low voltage, high current thing is in relation to power. usually a certain power (watts) is being transferred, eg 60 watts are transferred to your 60w light bulb to make it light. by the formula power = voltage * current, a desired power can be transferred either at high voltage and low current, or high current and low voltage. usually in electricity cables, high voltage, low current is used as the higher the current, the more energy is lost due to resistance (think of joules law - heat produced = current^2 * resistance * time)

i hope that helps you a little at least. as i said im only doing LC too so im sure anyone with 3rd level physics/electricity can do a better job.

JackKelly

hmm, cool. I'm starting to understand it more. I know definitions and all that, but i just don't really understand it. Spend a while on wikipedia, that place helps a lot!
So how is a P.D. set up?

breadmonkey

sephirosis, I liked your analogy, you should be set for the leaving!

Ishmael

TimAy wrote:

Ok, i've tried to understand this but it still really doesnt make much sense. Regarding electricity, i still don't understand what voltage is..
Its the energy lost in a circuit by 1 columb moving from one point to an other?wtf. So in a circuit , a battery has a certain voltage.What does this mean? I don't get how a potential difference is set up in the cell, and i don't get what effect it has on a circuit.

An easier way to think of voltage is the amount of charge one point in space has to another. In a Battery for example, there is an abundance of negativly charged electrons on the negative pin and a lack of electrons(positive charge) on the positive pin. When there is nothing connecting the 2 pins there is a potential for the electrons to flow if something were connected between the 2 pins.
The difference in charge on the 2 pins is called the potential difference or the voltage on the battery.

A Voltage doesn't need a circuit to exist. A voltage can occur between 2 points that have different amounts of charge.

And what does a high voltage actually mean, (danger wise or energy wise) Also, current is an easy one to understand, but i don't see how, when you have a low voltage, you have a high current etc? So can someone please give me a hand with this basic physics! Argh.

High Voltage means that there is A huge charge difference between the 2 points in space. usually if something is referenced as high voltage 10000V this means that there is a 10000V difference between it and Ground which is 0V.

When you talk about currents, this is when you need to consider resistance.

If you think of an electron in a circuit as a person on the way to the cinema and a resistance as a crowd outside the cinema, the more people(higher the resistance) the harder it will be for the person to get through(more energy dissipated). Therefore there is an energy difference from before the crowd and after the crowd.

Current is a constant flow of electrons. so if you have a endless stream of people trying to get into the cinema and theres a bunch of scumbags hanging around blocking the door(resistance) then the people can only get through at a certain rate(Current).

The reason for low voltages and high currents is because there is a small resistance where the current is flowing. So if there is a small amount of scumbags at the door then the rate at which people get into the cinema will be high.

If follows that the High voltages and low currents are from High resistance.
And sephirosis is right, the reason for low currents is that heat energy is a waste of electricity and electricity is converted to heat according to that heat eqn which is related to I, so if I is small then H is small(relativly).


I hope that helped!

And yes people that block access of doors are scum.

JackKelly

wow.Thanks, i get the idea now. Bit of an odd one but it makes sense. Thanks for the help.