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Couple of questions

by alexmath
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alexmath
#1
Jan16-13, 09:04 AM
P: 31
1.assume i am riding a motorcycle at 100km/h, but i ride the motorcycle inside a very long train which is moving at also 100km/h. What speed do i have reported to the world? 200km/h?
2.what happens at a short circuit? practically there is no resistance so the value of the current is infinity? therefore 3v battery can generate infinite curent if the wires have 0 resistance?
3.is ot possible to have such a great resistance inside a circuit in order to stop a circuit from working? same as an open circuit?

Thank you!
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mfb
#2
Jan16-13, 09:48 AM
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1.assume i am riding a motorcycle at 100km/h, but i ride the motorcycle inside a very long train which is moving at also 100km/h. What speed do i have reported to the world? 200km/h?
If you ride in the same direction, measure ground speed and neglect special relativity, yes.

2.what happens at a short circuit? practically there is no resistance so the value of the current is infinity? therefore 3v battery can generate infinite curent if the wires have 0 resistance?
The battery has an internal resistance, which limits current. Without resistance and in a static (time-independent) setup, you cannot have a voltage.
3.is ot possible to have such a great resistance inside a circuit in order to stop a circuit from working? same as an open circuit?
An open circuit is just a huge resistance at some point.
alexmath
#3
Jan16-13, 01:28 PM
P: 31
What means at some point? When the circuit is open... there is no current, but when there is a resistence, the current is present.

4) If we have 3 in series... why every bulb gets the same voltage? Why not the first one takes more, second one less, and last one even less?

BruceW
#4
Jan16-13, 03:34 PM
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Couple of questions

when the circuit is 'open', it means that there is air separating two ends of the wire. But it is possible to run a current across the air (for example, think of lightning, well technically it might turn the air into plasma first) But the point is that there is no such thing as a truly closed circuit, because we can think of the air as 'closing' the circuit, but since the resistance of air is so great, we can for all practical purposes say that that the circuit is 'open'.

the rule about parallel voltage... there are several ways to 'reason it'. One way is to think about the potential at each point in the circuit. the change in potential around any closed loop must be zero (otherwise, it wouldn't make sense, because we assign the potential to have a particular value at each point in space, so it cannot be defined to have two different values at the same point!)
CWatters
#5
Jan17-13, 10:33 AM
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If we have 3 in series... why every bulb gets the same voltage? Why not the first one takes more, second one less, and last one even less?
That only happens if the bulbs have the same resistance.

The current flowing through all the bulbs is the same because they are in series. At each node there is nowhere for the current to go except through the next bulb. If the bulbs have the same resistance and current then the voltage drop must be the same due to Ohms law (V=IR).
Low-Q
#6
Jan17-13, 11:19 AM
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"1.assume i am riding a motorcycle at 100km/h, but i ride the motorcycle inside a very long train which is moving at also 100km/h. What speed do i have reported to the world? 200km/h?"

I have a question too regarding this:
If we use enough decimals, would really the bike ride at 200km/h? Say that the train is moving at 99.9999999999% of the speed of light. Would the bike ride at 99.9999999999% of the speed of light + 100km/h?

Vidar
DaleSpam
#7
Jan17-13, 11:33 AM
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P: 17,330
Quote Quote by Low-Q View Post
"1.assume i am riding a motorcycle at 100km/h, but i ride the motorcycle inside a very long train which is moving at also 100km/h. What speed do i have reported to the world? 200km/h?"

I have a question too regarding this:
If we use enough decimals, would really the bike ride at 200km/h? Say that the train is moving at 99.9999999999% of the speed of light. Would the bike ride at 99.9999999999% of the speed of light + 100km/h?

Vidar
If you are going a significant fraction of c then you need to use the relativistic velocity addition formula:
http://hyperphysics.phy-astr.gsu.edu...v/einvel2.html

Note, for low velocities this expression is approximately the same as simple addition of velocities.
mfb
#8
Jan17-13, 11:44 AM
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About 199.999999999998285 km/h in the example ;).
As you can see, relativistic effects are negligible for real trains and motorcycles.
alexmath
#9
Jan20-13, 04:40 AM
P: 31
thank you for answering! At question 3... if all battery have tiny resistance, that means if you wire a battery without any rezistor in the circuit, the battery will eventualy discharge. Right?
Drakkith
#10
Jan20-13, 04:59 AM
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Quote Quote by alexmath View Post
thank you for answering! At question 3... if all battery have tiny resistance, that means if you wire a battery without any rezistor in the circuit, the battery will eventualy discharge. Right?
It will discharge pretty quickly, yes. You can actually burn out wires or cause the battery to explode if you don't have any resistance in your circuit, so don't connect a piece of wire between two battery terminals.
alexmath
#11
Jan20-13, 07:25 AM
P: 31
last question, i promise... if i want to see what voltage has a battery, and i keep it connected with the voltmeter, this as well will drain up the battery right? because the voltmeter has a very high resistence.
Drakkith
#12
Jan20-13, 07:54 AM
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Quote Quote by alexmath View Post
last question, i promise... if i want to see what voltage has a battery, and i keep it connected with the voltmeter, this as well will drain up the battery right? because the voltmeter has a very high resistence.
I believe so. It will just take much longer than wiring the terminals together with a piece of wire thanks to the far greater resistance. Wiki's article on voltmeters said they usually have a 10 megaohm resistor in them.


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