Constant Current in a series circuit

[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.