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electricity question

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I need help on 16. Why is voltage constant and not current? How do I know which is the same and which isn't?

Thanks
Original post by krisshP
http://www.scribd.com/mobile/doc/28384284/html5
http://www.scribd.com/mobile/doc/57098823?width=768

I need help on 16. Why is voltage constant and not current? How do I know which is the same and which isn't?

Thanks

The power supply supply voltage is always assumed constant unless the question specifically states it is variable.

The reason is in almost all normal applications, power supplies are designed to work that way which is why they contain large reservoir capacitors so that a changing load current does not cause the supply voltage to drop.

Ultimately you have to go all the way back to the power station to see that the mains supply voltage is a function of the power station generator' speed of rotation, which is held constant.
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krisshP
OP
14
Original post by uberteknik
The power supply supply voltage is always assumed constant unless the question specifically states it is variable.

The reason is in almost all normal applications, power supplies are designed to work that way which is why they contain large reservoir capacitors so that a changing load current does not cause the supply voltage to drop.

Ultimately you have to go all the way back to the power station to see that the mains supply voltage is a function of the power station generator' speed of rotation, which is held constant.


I thought pd across a lamp relates to how bright it is? The more pd across a lamp, the brighter it is since voltage=work done/charge.
(edited 10 years ago)
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fayled
9
Original post by krisshP
I thought pd across a lamp relates to how bright it is? The more pd across a lamp, the brighter it is since voltage=work done/charge.


The lamps are in parallel so the voltage across them is the same.

Yes, voltage=work done/charge, but for how bright the lamp is, we need to rearrange this so that work done=voltage x charge. As you have mentioned, more work done means a brighter lamp. We can see that voltage is constant for both lamps because they are connected in parallel. Now we see that the only thing that separates the brightness of the two lamps from the equation is the charge. The lamp with the larger charge passing through it has more work done so is brighter. Current=charge/time so the lamp with the largest current is brightest...
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Avatar for krisshP
krisshP
OP
14
Original post by fayled
The lamps are in parallel so the voltage across them is the same.

Yes, voltage=work done/charge, but for how bright the lamp is, we need to rearrange this so that work done=voltage x charge. As you have mentioned, more work done means a brighter lamp. We can see that voltage is constant for both lamps because they are connected in parallel. Now we see that the only thing that separates the brightness of the two lamps from the equation is the charge. The lamp with the larger charge passing through it has more work done so is brighter. Current=charge/time so the lamp with the largest current is brightest...


Thanks :biggrin:

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