Theoretical AC/DC Circuits: 1.5V Light Bulb & Battery

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Discussion Overview

The discussion explores the theoretical implications of connecting a 1.5V AC light bulb to a DC circuit powered by a 1.5V battery, including the effects of reversing the battery connections. Participants express confusion regarding the behavior of light bulbs in AC versus DC circuits and the significance of battery polarity in everyday appliances.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that a light bulb behaves like a resistor, glowing steadily under DC and flickering under AC, with the flicker being imperceptible at higher frequencies.
  • Others argue that the direction of current flow does not affect the operation of a light bulb, suggesting that it should work regardless of how the battery is connected.
  • A later reply questions the practicality of this claim, noting that everyday appliances like flashlights require correct battery orientation to function properly.
  • Some participants clarify that while incandescent bulbs may not be sensitive to current direction, the design of battery cases and terminals is intended to prevent user error and ensure proper function.
  • There is mention of the potential for confusion if users were allowed to insert batteries in any orientation, leading to operational issues.

Areas of Agreement / Disagreement

Participants express differing views on the implications of battery orientation and the design of battery cases. There is no consensus on the necessity of polarity in practical applications, as some believe it is essential while others suggest it may not be critical for certain types of bulbs.

Contextual Notes

Participants discuss the limitations of their theoretical assertions, particularly regarding the behavior of different types of bulbs (incandescent vs. LED) and the mechanical interchangeability of battery terminals. The discussion remains open-ended with unresolved questions about practical applications.

Red_CCF
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Out of curiousity, I am wondering what happens, theoretically of course, if we place a 1.5V AC light bulb (which do not exist) onto a DC circuit with a 1.5V battery? What would happen if we connect this battery backwards on the circuit (positive to the negative terminal and vice versa). This question arises from my confusion of positive and negative voltages in AC and DC circuits.
 
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A light bulb is just a resistor that glows.

If you put 1.5 VDC across it, it will glow steadily. If you put 1.5 VAC across it, it will glow and then darken and then glow again, at the frequency of your AC. If this frequency is high enough (60 Hz is plenty), the variation is brightness will not be perceptible.

Like a resistor, it does not matter which way you connect your light bulb to your battery. Either way, the same amount of current will flow -- just in the opposite direction. The light bulb is not sensitive to the direction of current flow, though, so you won't see any difference.

- Warren
 
chroot said:
A light bulb is just a resistor that glows.

If you put 1.5 VDC across it, it will glow steadily. If you put 1.5 VAC across it, it will glow and then darken and then glow again, at the frequency of your AC. If this frequency is high enough (60 Hz is plenty), the variation is brightness will not be perceptible.

Like a resistor, it does not matter which way you connect your light bulb to your battery. Either way, the same amount of current will flow -- just in the opposite direction. The light bulb is not sensitive to the direction of current flow, though, so you won't see any difference.

- Warren

If a light bulb does not care which direction the current is coming from, then why must everyday appliances like a flashlight have positive and negative terminals that we must position our batteries to comply or else the device doesn't work?
 
Try it and find out if that's really true...

[note: don't try it on an LED flashlight!]
 
Red_CCF said:
If a light bulb does not care which direction the current is coming from, then why must everyday appliances like a flashlight have positive and negative terminals that we must position our batteries to comply or else the device doesn't work?

With a regular incandescent bulb, it doesn't matter which way you put the batteries in as long as they're both polarized correctly in relation to each other. Battery cases, however, are usually set for the negative (non-nippled) end of the battery to impinge upon a spring terminal.
 
russ_watters said:
[note: don't try it on an LED flashlight!]
No harm in reversing the batteries, it just won't work. Diode breakdown voltage is typically >20VDC. Application of a 60Hz AC (low voltage) source to the LED will cause it flash on/off, appearing to the eye as a lower than normal intensity.
 
Last edited:
Danger said:
With a regular incandescent bulb, it doesn't matter which way you put the batteries in as long as they're both polarized correctly in relation to each other. Battery cases, however, are usually set for the negative (non-nippled) end of the battery to impinge upon a spring terminal.

Oh okay; so if the cases don't exist then I can put a battery (assuming a flashlight with only one battery) whichever way I want and the flashlight will still work?

But if what I said is true then what's the advantage of adding the cases?
 
Red_CCF said:
But if what I said is true then what's the advantage of adding the cases?

Take a flashlight that requires two batteries.

Have the users put them in however they want.

Disconnect your technical support when case after case of unhappy customers are mad their batteries exploded on you.
 
Red_CCF said:
Oh okay; so if the cases don't exist then I can put a battery (assuming a flashlight with only one battery) whichever way I want and the flashlight will still work?

But if what I said is true then what's the advantage of adding the cases?

Yes, it will still work. What I meant by 'case' is simply the receptacle into which the batteries are inserted. Most have one solid contact and one spring-loaded one. The positive (nipple) end of the battery contacts the solid piece, and the negative (flat) end contacts the spring. In a side-by-side arrangement, the directions are usually reversed so that one goes in backwards in reference to the other. That's the easy way to put them in series without excess wiring. Most cases these days are also marked with '+' and '-' signs in the appropriate locations.
 
  • #10
mheslep said:
No harm in reversing the batteries, it just won't work.
I know, I just didn't want to confuse the OP.
 
  • #11
Tac-Tics said:
Take a flashlight that requires two batteries.

Have the users put them in however they want.

Disconnect your technical support when case after case of unhappy customers are mad their batteries exploded on you.
Also, as was noted earlier, the terminals may or may not be mechanically interchangeable and batteries are not all exactly the same shape.
 

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