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Physics - electricity problem

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chapz
Posts: 10182 - Karma: 66
hey guys, so i have an assignment for my engineering class and i need some help!

the question is
If you had an old car that needed a 6V battery to drive the starter motor, could you use a 12V battery with a transformer to get the necessary output voltage?
Mar 28 2010 9:10PM
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Shadows
Posts: 8068 - Karma: 42
depends on what kind of starter you got? what kind of numbers does it draw
Mar 28 2010 9:12PM
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Bepo
Posts: 8195 - Karma: 17
no, transformers only work with AC. A battery is DC
Mar 28 2010 9:13PM
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chapz
Posts: 10182 - Karma: 66
gah i knew it had to do with this. i found it weird how were studying AC and question dealing with batteries comes up.... thanks a lot!
Mar 28 2010 9:15PM
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Shadows
Posts: 8068 - Karma: 42
i got to ask... what grade is this?
i made myself feel dumb as shit
Mar 28 2010 9:18PM
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chapz
Posts: 10182 - Karma: 66
hahahah
well i live in quebec so we have cegep here.

we have
5 years of high school
2 years of cegep
4+years of university .... and so on

this class is one of 3 final science classes in cegep (2nd year, 4th semester) so its the same level of difficulty as a university class, so dont feel too dumb! haha
Mar 28 2010 9:22PM
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prontocuts.com
Posts: 5244 - Karma: 42
rectifier?
Mar 28 2010 9:25PM
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Bepo
Posts: 8195 - Karma: 17
a rectifier converts AC to DC, the batter is 12V DC and he needs 6V DC.
Mar 28 2010 9:35PM
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prontocuts.com
Posts: 5244 - Karma: 42
ah sorry i mean inverter
Mar 28 2010 10:29PM
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AlexF
Posts: 271 - Karma: 206
You could put a resistor in series with the stater motor. If the resistor and motor have the same resistance then the voltage will be split between the two (i.e. 6V p.d over each). This is assuming that the starter motor has a (roughly) constant resistance in startup. A potential problem is that resistors are normally small (so have a small thermal mass) but there will be quite a large current going through at start up so it will heat up very quickly (and maybe burn out).

Alternatively; if you assignment allows you to be practical/use common sense, go and get a 6V motor bike battery (not as common as 12V but definitely available).

You can make/buy a DC-DC inverter/transformer combo that will step up/down DC voltage to DC voltage but they normally wouldn't be capable of driving a load like a stater motor as they can't handle much current.

By the way if the car is an old 6V system it probably has a positive earthing, while modern car systems are negative earthed.
Mar 29 2010 1:45AM
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pmills
Posts: 7162 - Karma: 59
i have no knowledge on this at all; but i know in a pinch, you can tape AA to D's or C's etc. and transfer a charge -- would that be possible here?
Mar 29 2010 8:06AM
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Mike.Steezze
Posts: 407 - Karma: 17
Right on the money. If its a university class this WILL be the answer
Mar 31 2010 2:47PM
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