Constant Current in a series circuit

[NascentOxygen]

um i know what ur trying to say but why couldn't some of it be used up by the resistor?

It is, precisely. The voltage is all used up by the components in the series circuit. Every millivolt of it. By the time you get to the other terminal of the battery, the voltage is all gone.

there are voltage drops...i mean there is potential difference and this is due to different levels of electrical energy across a component which is actually the elctrical energy used up bby the component which is also tha voltage of the component right... (this is what my tuition notes define voltage as)

That's exactly how it is. It is so predictable, that it makes electronic design really easy.

 

[Celluhh]

omygosh after u confirmed it i read thru it again and i realize i get it now! thankyousomuch! i can tell a logical mind really helps. ;)

 

[NascentOxygen]

Are we finished already? I'm disappointed. I thought we might have managed to push this thread to at least 5 pages.

 

[Celluhh]

haha i think i might have more questions after this weekend! after all I'm attempting practice papers so i might have some questions I'm unsure of! btw, do you have tips for science exams?? i always panic and can't think straight.

 

[Celluhh]

i have a new question. how exactly does the battery take in electrons and give out electrons? and, does the current ever change its speed? based on what you have said, the speed of the current seems to be decided by the battery and is constant throughout the circuit in a series circuit. in a parallel circuit, when the current splits, does it have different speeds in different branches?

 

[NascentOxygen]

i have a new question. how exactly does the battery take in electrons and give out electrons?

That's the principle of the electrochemical cell or battery. Metal plates immersed in acid, and electrons literally fall over themselves to escape from one plate and get to the other. It's magic. We can harness this--free electricity! Pure magic! : : You don't even need 2 metal plates; one will do, and the other one can be made of carbon.

and, does the current ever change its speed?

I reckon current flows at probably much the same speed in all metals, but may be quite different in other materials, such as the silicon used in the semiconductor industry, or the ionised gas of some lights. But I'm just speculating.

based on what you have said, the speed of the current seems to be decided by the battery

All that the battery determines is the force it will apply to the electrons; whether they flow is controlled by the elements in the circuit connected. A really good alternative name for voltage is EMF (electromotive force). Try using that to impress your teacher!

in a parallel circuit, when the current splits, does it have different speeds in different branches?

Not in the circuits you have been talking about. But if you have a jar of salty water included as part of the circuit, for example, then time may be significant while ions migrate through the solution from one electrode to the other to pass current.

You need to be careful about what you call the speed of the current. Let me illustrate. Consider a long train comprising 30 carriages and a loco at each end. It is stationary inside a long tunnel through a mountain. The tip of of the front loco is not quite in sight at the mouth of the tunnel, and the tail of the push loco is just inside the tunnel's end. A frightening scenario: you see another identical loco speeding along on the same track behind them. Its driver sees the stationary train ahead and manages to slow down, but cannot stop and he cannons into the rear of the stationary train. You don't actually see the collision or what's happening inside the tunnel, and it's too far away for sound to quickly reach you. But in the blink of an eye a loco pops into view at the mouth of the tunnel. Had you been unaware that there was already a train inside the tunnel--and that the loco that emerged was not the same one that went in--you may have said to yourself at that moment, "Wow! That loco sure sped up in that tunnel!"

 

[Celluhh]

wow i think ur last explanation is really good! i never thought of it like that before.

but the problem is, current cannot be seen by the naked eye right? so how do we ever know its speed? hmmm so your saying that even with different resistance in branches in a parallel circuit, the current will still move at the "same speed"?

thanks so much for explaining! i really appreciate it.^^

 

[NascentOxygen]

but the problem is, current cannot be seen by the naked eye right? so how do we ever know its speed?

We can sense its presence. So, clicking a morse key at one city, and timing (using synchronised clocks) how long before a click was heard at the receiver a long distance away, via a long cable, would give you its speed of travel. Not exactly the blunt speed of current in plain copper, but the idea is close enough.

hmmm so your saying that even with different resistance in branches in a parallel circuit, the current will still move at the "same speed"?

Yes. Electric current differs from water current in that respect.

 

[Celluhh]

Yes. Electric current differs from water current in that respect.

thanks for the first explanation, i get it now!

umm for this statement, how exactly? water current does slow down when there is resistance right? for electric current, because the electrons are less free to move, won't they slow down too? even by a little bit?

 

[NascentOxygen]

water current does slow down when there is resistance right?

Not necessarily. If there is a narrowing in a pipe (a region of higher resistance), the water particles passing through that constriction actually speed up.

for electric current, because the electrons are less free to move, won't they slow down too?

I don't know for certain, but I doubt it. A thin piece of copper has a higher resistance than a thick piece of the same length, and I would expect current to have identical speeds in each. You might say, but if a lot of current flows through the thin piece, maybe the current will slow down? But if that were so, then the material would have a non-linear characteristic, leading to unpredictable signal distortion in high-speed circuits, and I've never heard that said of metal conductors.

 

[Celluhh]

Not necessarily. If there is a narrowing in a pipe (a region of higher resistance), the water particles passing through that constriction actually speed up.


I don't know for certain, but I doubt it. A thin piece of copper has a higher resistance than a thick piece of the same length, and I would expect current to have identical speeds in each. You might say, but if a lot of current flows through the thin piece, maybe the current will slow down? But if that were so, then the material would have a non-linear characteristic, leading to unpredictable signal distortion in high-speed circuits, and I've never heard that said of metal conductors.

what, the water particles speed up and don't slow down?? i don't get it!:(

Shouldn't the thick piece of the same length have more resistance cos there is less space for electrons to move?

Thanks for answering once again!;)

 

[NascentOxygen]

Shouldn't the thick piece of the same length have more resistance cos there is less space for electrons to move?

There are a lot more free electrons to participate, and there are more paths for them to follow. A thick conductor is the equivalent of having lots of thin conductors bundled together: lots of parallel circuits to lower that overall resistance.

 

[Celluhh]

oh ok!

hey i have another question.
why is it that in parallel circuits the overall resistance is lower? i mean i know there are more paths for the current to flow, so there is less R and more I. but then, the current actually gets split into diff branches cos there are more resistors, so it kind of contradicts right?

 

[PeterO]

oh ok!

hey i have another question.
why is it that in parallel circuits the overall resistance is lower? i mean i know there are more paths for the current to flow, so there is less R and more I. but then, the current actually gets split into diff branches cos there are more resistors, so it kind of contradicts right?

Not a contradiction.

If there was one resistor, a certain amount of current can flow. If another resistor is added in parallel, an extra branch open up with some current going through there - but still as much flows through the original resistor.

The current through the battery increases, but the current through each resistor is the same as if that resistor was connected alone.

It is like opening a second register in an over-crowded super-market. The first register continues to process at full speed, but now some of the waiting people will shift lines and an additional number of people will get through the checkouts.

 

[Celluhh]

Not a contradiction.

If there was one resistor, a certain amount of current can flow. If another resistor is added in parallel, an extra branch open up with some current going through there - but still as much flows through the original resistor.

The current through the battery increases, but the current through each resistor is the same as if that resistor was connected alone.

It is like opening a second register in an over-crowded super-market. The first register continues to process at full speed, but now some of the waiting people will shift lines and an additional number of people will get through the checkouts.

still as much current flows throught the resistor? are you saying the extra branch with resistor added does not affect the current flowong through the original branch?


so the battery takes in more electrons and gives out more electrons, so there's more current through the battery, but why does the current through the resistor remain the same??

i like ur analogy, but when i try to visualize the movement of th electrons in a circuit, its kinda different.>.<


thanks !

 

[NascentOxygen]

still as much current flows throught the resistor? are you saying the extra branch with resistor added does not affect the current flowong through the original branch?

That's what he's saying. Providing the battery doesn't change its voltage, then the current through one resistor it's powering does not affect the current in any other load that battery is also powering.

so the battery takes in more electrons and gives out more electrons, so there's more current through the battery,

Yes. Though if you put it that way most people will do a double take, even though you are perfectly precisely correct. Best just say "so the battery supplies more current ..." and save yourself the perennial contortions of thinking about electrons going in one direction but current in the other.

but why does the current through the resistor remain the same??

Same battery, same voltage, same resistor, same current.

i like ur analogy, but when i try to visualize the movement of th electrons in a circuit, its kinda different.

It sure is. If they were exactly the same, it wouldn't be called an electric current, it would be called a supermarket queue.

 

[Celluhh]

That's what he's saying. Providing the battery doesn't change its voltage, then the current through one resistor it's powering does not affect the current in any other load that battery is also powering.

Best just say "so the battery supplies more current ..." and save yourself the perennial contortions of thinking about electrons going in one direction but current in the other.


Same battery, same voltage, same resistor, same current.

but isn't the current split through the branches? but then again, in a parallel circuit we zoom in on the indiv series circuit, cos the voltage for each branch is the same...and yeah ur right the resistor is the same...so logically the current and brightness should be the same...

but then, why do we say then that: in a parallel circuit, the resistance decreases, and the current increases? this law has always been bugging me.

so when the battery takes in electrons(i.e. supply more current), from which terminal does it take in the elctrons?

 

[NascentOxygen]

then, why do we say then that: in a parallel circuit, the resistance decreases, and the current increases?

Because "we" haven't defined what resistance we are talking about. The resistance of each resistor doesn't change. But as far as the battery is concerned, adding more resistors (in a parallel arrangement) across its terminals reduces the total resistance the battery is powering. If you add a second light bulb, so the battery has to now power two bulbs, the effect as far as the battery is concerned is the same as if you replaced the original single bulb with a higher power bulb which draws double the current. Two low power bulbs in parallel, or one high power bulb alone, the battery is only concerned with how much current the total load is using. (A higher power bulb has lower resistance, of course.)

so when the battery takes in electrons(i.e. supply more current), from which terminal does it take in the elctrons?

Best not to think too often about this one. You should be mainly thinking "current", not electrons, or life will get just too complicated. The battery draws electrons in on its terminal marked (+) but keep quiet about it or you may confuse your teacher.

 

[PeterO]

but then, why do we say then that: in a parallel circuit, the resistance decreases, and the current increases? this law has always been bugging me.

The electric circuit is connected to the battery.

The circuit may be as simple as a single resistor. It may be slightly more complicated with 2 resistors connected in parallel or perhaps in series.
It may be a whole bunch of resistors, and capacitors, and inductors and any other thing you can think of, all connected together with parallel branches and Series branches in a complex , who-knows-what, way.

When you begin studying circuits, we start with simple connections, and the key to a couple of simple, two-resistor circuits is.

If the resistors are connected in series, the effective resistance is bigger than either one [they add together actually]. That higher resistance circuit means a lower current -COMPARED TO EITHER RESISTOR ON ITS OWN.

If the resistors are connected in parallel, the effective resistance is smaller than either one. That lower resistance means a higher current - COMPARED TO EITHER RESISTOR ON ITS OWN.

In both cases we mean the current through the battery.

 

[Celluhh]

Because "we" haven't defined what resistance we are talking about. The resistance of each resistor doesn't change. But as far as the battery is concerned, adding more resistors (in a parallel arrangement) across its terminals reduces the total resistance the battery is powering. If you add a second light bulb, so the battery has to now power two bulbs, the effect as far as the battery is concerned is the same as if you replaced the original single bulb with a higher power bulb which draws double the current. Two low power bulbs in parallel, or one high power bulb alone, the battery is only concerned with how much current the total load is using. (A higher power bulb has lower resistance, of course.)

why does a higher power bulb have lower resistance? hmm so your saying that, although the total resistance the battery is powering reduces when two bulbs are connected in parallel across the battery, they don't actually affect each other as their amount of individual resistance won't change and the electrical energy supplied by the battery to each resistor does not change either, hence the current flowing through each resistor is solely dependent on the individual resistance of that resistor alone?

 

[PeterO]

why does a higher power bulb have lower resistance? hmm so your saying that, although the total resistance the battery is powering reduces when two bulbs are connected in parallel across the battery, they don't actually affect each other as their amount of individual resistance won't change and the electrical energy supplied by the battery to each resistor does not change either, hence the current flowing through each resistor is solely dependent on the individual resistance of that resistor alone?

Let's scale this up.

The battery is your local power station, and the parallel resistors are your house, and your neighbour's houses. This means each of the resistors can be connected and disconnected from the circuit independently.
Inside your house, you turn on your lights, and/or Television.
When your neighbour decides to turn on their electrical appliances, it makes no difference to your house - HOWEVER, the power station has to supply a greater current.
Some of that current goes to your house - [ it did so before your neighbour "switched on", and the current through your house continues unaffected once they do "Switch on" ]- the rest of the current from the power station goes to your neighbours.

When everyone turns on a light, the current supplied gets quite high - characteristic of a low resistance.
A lot of houses connected in parallel behave as a lower resistance than one house on its own.

 

[PeterO]

why does a higher power bulb have lower resistance?

Look at it the other way.

A low resistance will allow a higher current to flow - more coulombs per second.

Each coulomb brings with it the same number of Joules [they may have come through a 6 V [6 Joules per Coulomb battery]

That means a higher current allows the transformation of more Joules per second.

But Joules per second is power.

So Lower resistance bulb allows high current which means high power output.

A low resistance bulb is a high power bulb.

Easier to explain than why a high power globe has a low resistance; but it is the same thing.

 

[Celluhh]

hmm so ur saying that i should view a parallel circuit as separate series circuits right? thank you petero for explaining it to me! but anyway, my main question still remains:

NascentOxygen said: If you add a second light bulb, so the battery has to now power two bulbs, the effect as far as the battery is concerned is the same as if you replaced the original single bulb with a higher power bulb which draws double the current. Two low power bulbs in parallel, or one high power bulb alone, the battery is only concerned with how much current the total load is using.


i understand the last sentence but my question is:how is replacing two low power bulbs with one high power bulb the same for the battery? (assuming voltage is constant)
i mean, the same voltage for a higher resistance in the case of the low power bulb
(one bulb in one branch, two branches in the parallel circuit)
and the same voltage for a lower resistance in the case of the high power bulb)

ps: pls correct me if i understood you wrongly, nascentoxygen.

 

[NascentOxygen]

why does a higher power bulb have lower resistance?

So that it will draw more current.

If it drew less current, then it would be a lower power bulb, and that means it would have been manufactured with a higher resistance filament, not lower.

hmm so your saying that, although the total resistance the battery is powering reduces when two bulbs are connected in parallel across the battery, they don't actually affect each other as their amount of individual resistance won't change and the electrical energy supplied by the battery to each resistor does not change either, hence the current flowing through each resistor is solely dependent on the individual resistance of that resistor alone?

Yes.

 

[NascentOxygen]

i understand the last sentence but my question is:how is replacing two low power bulbs with one high power bulb the same for the battery?

All that the battery "sees" is something draining current from its terminals. The battery goes flat at exactly the same rate whether it powers 2 bulbs each draining 1 amp, or 1 bulb draining that same 2 amps. Or 200 tiny LED lights (in parallel) each draining 0.01 amps.

 

[Celluhh]

um i get what you mean, but this is only if the bulbs are identical right?

 

[PeterO]

oh gosh i just realized i have a huge problem. current flows from positive terminal of battery, electrons flow from negative terminal of battery, so exactly what causes the bulbs to light up? the charges(current right)? then what are the electrons for? are they actually the same thing? my teach said current flows from the battery, but ain't there already charges in the wires? or is that just a term?


ARGHHHHHH I'MMA TOTAL IDIOT.

There are charges everywhere. The battery just makes them all move. They all start at the same time, they all flow at the same average rate.

As for what is flowing, I use the following aexample.

I have the whole class arrange their chairs in a circle, and put one extra chair in the ring.
I then instruct students as follows:
"If the chair to your right is empty - move into it"

Once everyone has shuffled around for a few minutes I ask them to stop, and ask" What did you just see?"

The most common answer [unanimous actually] is "Everyone shuffled clockwise around the circle"

I then respond "That's strange, all I saw was one empty chair moving anticlockwise around the circle"

Electrons traveling in one direction - conventional current traveling the other way - it really doesn't make much difference, and we generally choose to talk about the conventional current.
That is, we 'pretend" that a whole bunch of positive charges are flowing around the circuit, "coming out of the positive terminal - flowing around the circuit - then returning to the negative terminal"

 

[NascentOxygen]

um i get what you mean, but this is only if the bulbs are identical right?

Electrically identical, which means 'draw the same current'. (So that's a tautology.)

 

[Celluhh]

There are charges everywhere. The battery just makes them all move. They all start at the same time, they all flow at the same average rate.

As for what is flowing, I use the following aexample.

I have the whole class arrange their chairs in a circle, and put one extra chair in the ring.
I then instruct students as follows:
"If the chair to your right is empty - move into it"

Once everyone has shuffled around for a few minutes I ask them to stop, and ask" What did you just see?"

The most common answer [unanimous actually] is "Everyone shuffled clockwise around the circle"

I then respond "That's strange, all I saw was one empty chair moving anticlockwise around the circle"

Electrons traveling in one direction - conventional current traveling the other way - it really doesn't make much difference, and we generally choose to talk about the conventional current.
That is, we 'pretend" that a whole bunch of positive charges are flowing around the circuit, "coming out of the positive terminal - flowing around the circuit - then returning to the negative terminal"

i like ur analogy a lot! but when i tried to draw it out, the chair somehow moved clockwise too?=(

 

[Celluhh]

Electrically identical, which means 'draw the same current'. (So that's a tautology.)

so what happens if it isn't stated if the bulbs are electrically identical?

 

[Celluhh]

Energy in circuits
The potential difference between any two points in a circuit is the energy transferred to, or from, a given amount of charge as it passes between those points. In the circuit above, the charges gain energy in the cell, and then transfer that same amount of energy into light and heat in the lamp. That is why the potential difference across the cell is the same as the potential difference across the lamp; it is the same amount of energy.
( i found this on the net, i kind of understand it, but now I'm wondering, how do the charges transfer the same amt of energy into light and heat in the lamp? as in, there are charges everywhere in the circuit. all these charges have electrical energy right? so they have the same amt of electrical energy?if so, why? or does the energy actually transfer from charge to charge to reach the lamp?) oh btw, does this mean that all the charges transfer all of its electrical energy to the lamp the moment they pass through it? yes right?
Potential difference
A typical cell produces a potential difference of 1.5V. When two or more cells are connected in series in a circuit, the total potential difference is the sum of their potential differences. For example, if two 1.5V cells are connected in series in the same direction, the total potential difference is 3.0V. If two 1.5V cells are connected in series, but in opposite directions, the total potential difference is 0V, so no current will flow. ( can someone explain this more clearly pls?)
Current
When more cells are connected in series in a circuit, they produce a bigger potential difference across its components. More current flows through the components as a result. (how is a greater pd created?)

 

[PeterO]

i like ur analogy a lot! but when i tried to draw it out, the chair somehow moved clockwise too?=(

I think you had everyone shuffle with their seat!

People move to the vacant chair, leaving their previous seat behind.

Perhaps I should say "I saw an empty space move anticlockwise around the ring" The chairs themselves do not move, just the people - and the empty space.

 

[NascentOxygen]

so what happens if it isn't stated if the bulbs are electrically identical?

Nothing untoward happens, apart from the sum of the currents not being exactly double.

 

[NascentOxygen]

i'm wondering, how do the charges transfer the same amt of energy into light and heat in the lamp?

You could think of it as the energetic electrons bumping, jostling, pushing and shoving, as they try to force their way through a path that is resisting their progress.

does the energy actually transfer from charge to charge to reach the lamp?)

They pass their energy on to adjacent electrons. Each electron from the battery doesn't need to make its way all the way around the circuit, it just passes on its energy to others.

does this mean that all the charges transfer all of its electrical energy to the lamp the moment they pass through it? yes right?

Providing there is zero resistance in the remainder of the circuit, yes. But in practice, some of the energy is lost in the resistance of other wires, the switch contacts, bulb's pressure contacts, etc.

If two 1.5V cells are connected in series, but in opposite directions, the total potential difference is 0V, so no current will flow. ( can someone explain this more clearly pls?)

Voltages add, by simple addition. So they also subtract if a polarity is reversed: one cell wanting to push electrons in one direction, but opposed by the other wanting to push in the other direction. However, no two cells will ever be exactly identical, because of manufacturing differences in chemical concentrations, so while their voltages will subtract to almost zero, the result won't be exactly zero, e.g. 1.5123 - 1.5084

When more cells are connected in series in a circuit, they produce a bigger potential difference across its components. More current flows through the components as a result. (how is a greater pd created?)

By simple addition. Each cell gives the circuit's electrons a boost of energy of 1.5 volts, and it's not concerned where the electrons come from or are going.

So to be really accurate, a 1.5v battery causes the voltage on its (+) terminal to be +1.5 volts more than the voltage on its (-) terminal. It's not concerned with, nor can it have any idea of sensing, what the voltage on its (-) terminal is. Voltage is like height: it's all relative to some other point, an agreed or default reference. Floor level is a good reference level inside a house; sea level is a good reference for geographic features.

 

[Celluhh]

Nothing untoward happens, apart from the sum of the currents not being exactly double.

huh?? what do you mean?