Constant Current in a series circuit

[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?

 

[Celluhh]

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.

um, i had the people move and the empty space as well but both went clockwise!=(

 

[Celluhh]

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.

energy can be lost?? hmmm what is there is resistance in the remainder of the circuit?

 

[NascentOxygen]

energy can be lost?? hmmm what is there is resistance in the remainder of the circuit?

There is resistance in every conductor in your circuit, so you lose some potential even before current reaches the bulb.

 

[Celluhh]

no as in, how do the charges ensure that they have enough electrical energy for the other resistances in the circuit as well?

 

[NascentOxygen]

no as in, how do the charges ensure that they have enough electrical energy for the other resistances in the circuit as well?

The battery voltage sets how much energy an electron will be given. Whatever energy the electrons have is then shared out, proportionally, between the various resistances around the circuit, according to the resistance they encounter. Current obeys Ohms law, even when the current comprises just a single electron going for an afternoon stroll.

 

[Celluhh]

ah so in a way the battery sees the resistance in the circuit then decides how much energy to give to each resistance??( for a series circuit) why did u suddenly talk about current?

 

[NascentOxygen]

ah so in a way the battery sees the resistance in the circuit then decides how much energy to give to each resistance??

Not really. The battery always gives each moving electron the same energy, this is set by the battery's voltage. The resistance determines how many electrons with that energy can flow. If not many are allowed to flow, then it must be a high resistance, so it won't get very warm. If resistance is low, then many electrons with that energy can go around the circuit, delivering a lot of energy to the resistance/s, which then get hot.

( for a series circuit) why did u suddenly talk about current?

I prefer to talk about current, because most electronics equations involve current.

 

[Celluhh]

Current obeys Ohms law, even when the current comprises just a single electron going for an afternoon stroll.

this sentence seems similar to what petero's analogy, but i don't really see how his energy depicts electrons and current flow in a circuit, and this sentence of yours seems similar to his analogy. would you mind elaborating it further in simple terms?

 

[Celluhh]

The resistance determines how many electrons with that energy can flow. .

ah, so the resistor resists the electron flow and the electrons which manage to get to the resistor will transfer its energy to the resistor. um wait, then what about current flow??

 

[NascentOxygen]

this sentence seems similar to what petero's analogy, but i don't really see how his energy depicts electrons and current flow in a circuit, and this sentence of yours seems similar to his analogy. would you mind elaborating it further in simple terms?

The actual charge carrier and related direction is almost always unimportant. So a stream of negative charge carriers moving from right to left is equivalent to a stream of positive charge carriers moving from left to right. They each transfer energy.

It remains a real shame that early scientists guessed that electricity in wires comprises positive charges moving from + to - , while we now know it's really negative charges moving from - to +. But as I said, either model works just as well, and the equations have by now been constructed around current.

 

[Celluhh]

yeah i know, but my teacher keeps emphasising on how current flows from positive terminal, electrons flow from negative terminal, as if it will affect our performance in the science paper.=(

 

[PeterO]

um, i had the people move and the empty space as well but both went clockwise!=(

I can only suggest you get hold of 6 friends, seven chairs and watch.

 

[NascentOxygen]

yeah i know, but my teacher keeps emphasising on how current flows from positive terminal, electrons flow from negative terminal, as if it will affect our performance in the science paper.=(

Well I think you should find that question a breeze.

 

[Celluhh]

ok so , electron flow and current flow is basically one and the same?i know its not really the same just cos scientists thought charges flow from positive terminal that's why we have the conventional current flow. but actually there's only one real flow- that's the flow of electrons from negative to positive terminal, but when we analyse circuits, if need be, we use the current flow. right? i think maybe that's why my teachers kept repeating it.