Tuesday, 14 September 2010

Are high resistance bulbs brighter?


Energy is dissipated quicker in high resistance bulbs because the current has to try harder to get through - so theyre brighter, right?

Wrong - though it can be a seductive argument. One of the problems is the deeply ingrained idea that current flow is sequential. The incorrect story is that the electrons move happily through the low resistance wires but when they come to the bulb filament they find it difficult to squeeze through and all the jostling transfers energy to the filament, which we experience as heat and light. The higher the resistance, the more jostling and the brighter the bulb.

This is also an example of what you might call local reasoning. With circuits you have to think about what happens to every element in the circuit all at the same time: it normally causes problems if you focus in on one part and then another.

In fact what happens is that a higher resistance bulb decreases the current everywhere in the circuit. The slower moving charges transfer energy to the bulb at a lower rate and so the bulb is dimmer.

One other subtlety is that in a circuit energy is transferred quickest in the places where the resistance is RELATIVELY high. But the higher the actual (rather than the relative) resistance, the slower the overall energy transfer and the dimmer the bulb.

You can check out an animation explaining what happens when a very low resistance bulb is connected first to 12V made from joining small cells in series and then connected to 12V from a car battery.

14 comments:

  1. Can you tell me if 60 watt and 100 watt bulb are connected in series, which one will be brighter?

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    1. the 100 watt bulb as, since the current is the same in both, then the voltage in the 100 watt bulb will be higher, so it would in return be brighter.

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  2. than why filament of higher resistance is used in bulb?

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  3. HELP! please!!
    Ok I am doing a phyics GCSE ISA, about -"the greater the current the greater the resistance of a filament lamp" researching my context:12 V car headlamps getting confused
    Is this correct?-
    I found that car headlamps use filament bulbs, which I have investigated in my experiment. From my research and knowledge I have found that when a filament bulb in a house (which has a much greater current) is first switched on it is more likely to break, it involves a large surge of electrical current flowing through a cold filament. When you first switch on the bulb the resistance is small, so you get a current surge which can break the filament. Car headlamps require a smaller current, which reduces the likelihood of blowing, causing them to last longer, the greater the current of the headlamps, the greater the resistance and the brighter the bulb.
    There are laws concerning the maximum brightness of car headlamps, this 12V voltage causes a low current and resistance maintaining a sufficient brightness.

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    Replies
    1. headlamps run on banta not filament bulbs

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    2. Nah Dan Chappers

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    3. Nah chris you don't even banta

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    4. doth thou even hoist bretheren?

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    5. Carlos brenn theought ??????

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    6. what does tha even mean!?

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  4. can someone please tell me the method on resistance bulbs i need it for my GCSE ICA

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  5. Hi can someone help tell me the answer for this q
    x and y are of same material filaments in lamps
    filament of lamp x is thicker and shorter than that of lamp y
    when connected to mains and switched on which is brighter lamp and which lamp has larger resistance?

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    Replies
    1. For series
      P = I2R So, P directly Prop to R
      So 60watt Bulb will be brighter...

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