Brightness of bulbs in series and parallel

[Barclay]

Homework Statement

Bulbs in series are less bright than those in a parallel circuit. But are all the bulbs in the series circuit equally dim?

Homework Equations

The Attempt at a Solution

I think for the purpose of high school physics bulbs in series are considered to be equally dim in a series circuit because they are receiving equal amounts of current (coulombs per second). But in reality each subsequent bulb from +ve to -ve terminal (conventional current flow) is dimmer than the first in series because the current has less and less energy to give up as it passes through the circuit.

Which is right? Which to I use for school science?

Thanks

 

[siddharth23]

You're on the right track. Just answer these questions:

1) Why will the bulbs in series be dimmer than those in parallel?

2) Across every bulb, there is potential drop from +ve to -ve. But the potential difference across the entire series line will be the same. So why will the current flowing through each series bulb vary?

 

[CWatters]

I think for the purpose of high school physics bulbs in series are considered to be equally dim in a series circuit because they are receiving equal amounts of current (coulombs per second).

Correct.

But in reality each subsequent bulb from +ve to -ve terminal (conventional current flow) is dimmer than the first in series because the current has less and less energy to give up as it passes through the circuit.

Incorrect.

What equations can you write for the power dissipated in each bulb?

 

[haruspex]

I think for the purpose of high school physics bulbs in series are considered to be equally dim in a series circuit because they are receiving equal amounts of current (coulombs per second).

What physical quantity determines brightness? Are the bulbs known to be identical?

 

[Barclay]

What equations can you write for the power dissipated in each bulb?

P = IV

V = IR

I think I see what your're getting at with the equations. Using P = IV. 'P' is the power or light emitted and since the voltage drop across each bulb is the same. And the current is the same all along the series circuit then the 'P' should be equal for all bulbs. BUT I've been watching YouTube videos and I think they're suggesting that by the end of the circuit the charges are depleted of power and regain their potential energy in the battery. So the charges must be loosing coulombs along the way so each subsequent bulb in the series in dimmer.

 

[Barclay]

What physical quantity determines brightness? Are the bulbs known to be identical?

Yes assume the bulbs to be identical

 

[Barclay]

2) Across every bulb, there is potential drop from +ve to -ve. But the potential difference across the entire series line will be the same. So why will the current flowing through each series bulb vary?

Coulombs of charge are being lost as the electrons travel along the circuit?

 

[William White]

barclay

in a series circuit the current at any point is the same as any other point; If you have 1A leaving the source, then 1A goes through all the loads (the bulbs) and 1A goes back into the source. All the loads see 1A (in such small circuits with such low power its not worth worrying about current leakage, its virtually nil)In a parallel circuit, voltage is dropped across both the series components (the wires) and the parallel loads (the bulbs); so in a parallel circuit, the further you go from the source, the dimmer your bulbs. If you kept adding parallel components, eventually you would "run out" of volts.

This is why voltages on the power supply to your house are regulated, and the number of (parallel) connections are limited - to ensure customers at the end of the line get enough volts!

 

[CWatters]

P = IV

V = IR

I think I see what your're getting at with the equations. Using P = IV. 'P' is the power or light emitted and since the voltage drop across each bulb is the same. And the current is the same all along the series circuit then the 'P' should be equal for all bulbs. BUT I've been watching YouTube videos and I think they're suggesting that by the end of the circuit the charges are depleted of power and regain their potential energy in the battery. So the charges must be loosing coulombs along the way so each subsequent bulb in the series in dimmer.

No that's not correct. All that matters is the voltage dropped in that particular resistor. The order of the resistors in a series circuit does not effect the power that each dissipates.

 

[haruspex]

Coulombs of charge are being lost as the electrons travel along the circuit?

Coulombs measure charge, and charge is made up of electrons. If the coulombs in were not equal to coulombs out then electrons in would not be equal to electrons out. Either electrons would be leaking away or building up, creating an electrostatic charge at points in the circuit. I think you can ignore both possibilities.

 

[CWatters]

BUT I've been watching YouTube videos and I think they're suggesting that by the end of the circuit the charges are depleted of power and regain their potential energy in the battery.

That's not quite correct. They aren't depleted of power but they are depleted of voltage.

Voltage is sometimes called Potential. As an analogy... perhaps think of voltage like you do potential energy or height. For example you could think of a battery as raising the height of an electron to the top of a cliff. Then as the electron travels around the circuit it looses voltage/potential or "height" as it goes through each light bulb. If you add up the height lost in each bulb it will equal the height of the cliff.

If you fall 10m the potential energy you loose depends on the change in height. It doesn't matter if it's the first 10m or last 10m you fall.

 

[William White]

That's not quite correct. They aren't depleted of power but they are depleted of voltage.

he was talking about series circuits, so that's not correct either (and if you deplete the voltage you deplete the power dissipated: Power dissipated in a resistor is proportional to the square of the voltage across it = V^2 R)

If you have a series circuit of x number of indentical resistors of resistance R (bulbs); the current going through each resistor is the same; and the resitance is the same; so the volt drop across each resistor is the same. i (constant) x R (constant) = V = constant.Bulbs furthest from the source get dimmer in parallel circuits as you add more bulbs - because the resistance of the circuit increases by distance from the source. The series component "seen" by each resistor increases.
All bulbs get equally dim in series circuits as you add bulbs - the series component "seen" by each resistor is fixed.

 

[CWatters]

Indeed. I was trying to guess what was meant by a video I hadn't seen.

 

[Barclay]

Trying to summarise my understanding:

Identical bulbs in a series circuit will appear the same brightness all the way through the circuit. This is because it is assumed that there are equal numbers of charged electrons (with the same charge) available for each bulb. This charge has not been depleted along the path from positive to negative terminal (passing through the various bulbs) to the 1st bulb is just as bright and the last bulb in the series.

Bulbs in a series circuit will appear dimmer than the same number of bulbs in a parallel circuit.
This is because in a parallel circuit the bulbs will all be sitting at the same potential level (height of the hill) and there will be equal potential drop (a large drop to the bottom of the hill) as current passes through the bulbs (and light is emitted). The battery works harder in a parallel circuit because it needs to provide enough charged electrons per unit time for all the components at the same time.
In a series circuit the potential drop across each bulb is smaller than in a parallel circuit the same number of bulbs. This is because the electrons drop voltage down the same hill step-by-step (each step being a bulb) and reach the bottom.

 

[William White]

In a parallel circuit there is always a series component - which is the conductor connecting the load to the source. The resistance of the series component increases as the length of the conductor increases. Further away from the source, the more volt drop.

So, parallel loads do not all see the same voltage. Loads near the source see a greater voltage than loads far from the source.

When the number of loads connected is small, and their power rating is small, then you will not have too many problems with volt drop. As the number of loads increases, the volt drop increases, and the low voltage at the "end" of the parallel circuit can become unusable.