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# Physics watch

1. Resistance, resists the flow of electrons(current). For an ohmic device, resistance doesn't vary much with current or pd (R=V/I). So as current increases, resistance decreases. Why? Is it because the flow of electrons outweigh the resistor? My next question goes onto resistance temperature. "V and I for a filament lamp: resistance increases as the current increases(the filament gets hotter)" don't these contradict one another? or am i missing something here.

Thanks in advance.
2. anyone?
3. Resistance, resists the flow of electrons(current). For an ohmic device, resistance doesn't vary much with current or pd (R=V/I). So as current increases, resistance decreases. Why?
I don't follow what you are saying here.

For most metallic conductors, kept at constant temperature, the resistance stays constant; it doesn't depend on the current.
Current depends on pd. That's Ohm's Law.
If you allow the temperature to increase, then, for metals, the resistance increases.
4. (Original post by Stonebridge)
I don't follow what you are saying here.

For most metallic conductors, kept at constant temperature, the resistance stays constant; it doesn't depend on the current.
Current depends on pd. That's Ohm's Law.
If you allow the temperature to increase, then, for metals, the resistance increases.
I am talking about a graph of pd against current, for an ohmic device the resistance won't vary in terms of the data plotted, it will be constant difference because the graph is a straight line from the origin. That's what the book means i believe. If you increase the temperature how would that increase the resistance? is it because more electrons are colliding together hence less flow of electrons(current)?

Also, for semi-conductors, what NTC and PTC mean?
5. (Original post by boromir9111)
If you increase the temperature how would that increase the resistance?
As the temperature increases, the atoms in the conductor vibrate more, making it harder for the electrons to get through.
6. (Original post by boromir9111)
Also, for semi-conductors, what NTC and PTC mean?
A quick search on google told me they are negative temperatur coefficient and positive temperature coefficient (of resistance)

PTC - as temp. increases, resistance increases
NTC - temp. increases, R decreases

http://www.allaboutcircuits.com/vol_1/chpt_12/6.html
7. (Original post by starofale)
A quick search on google told me they are negative temperatur coefficient and positive temperature coefficient (of resistance)

PTC - as temp. increases, resistance increases
NTC - temp. increases, R decreases

http://www.allaboutcircuits.com/vol_1/chpt_12/6.html
Yeah, i know the definition, thought someone could shed some extra light on it but i will refer back to notes.... just wanted a fresh pairs of eyes on it to something i may have missed. Thanks mate for your help
8. (Original post by boromir9111)
Resistance, resists the flow of electrons(current). For an ohmic device, resistance doesn't vary much with current or pd (R=V/I). So as current increases, resistance decreases. Why? Is it because the flow of electrons outweigh the resistor?
No, the resistance will remain the same regardless of the current moving through it.

If you increase the current, the voltage dropped across the resistor will increase, by the formula you gave, V=IR. This is because the resistance is essentially the difficulty of passing a charge through a resistor. A greater current means you are trying to pass more charges through and hence a large voltage is dropped.

(Original post by boromir9111)
My next question goes onto resistance temperature. "V and I for a filament lamp: resistance increases as the current increases(the filament gets hotter)" don't these contradict one another? or am i missing something here.

Thanks in advance.
This issue is more complex, and is adding to your confusion.

In this case, as the current flows through the resistor, the (constant) resistance is drawing away some of the voltage. This voltage is then given out in the form of heat.

This leads to a second effect: Increasing temperature increases resistance.

So yes, in this case increasing current will indirectly cause resistance to increase.

Hope this helps.
9. (Original post by El Doctoré de Mystéro)
No, the resistance will remain the same regardless of the current moving through it.

If you increase the current, the voltage dropped across the resistor will increase, by the formula you gave, V=IR. This is because the resistance is essentially the difficulty of passing a charge through a resistor. A greater current means you are trying to pass more charges through and hence a large voltage is dropped.

This issue is more complex, and is adding to your confusion.

In this case, as the current flows through the resistor, the (constant) resistance is drawing away some of the voltage. This voltage is then given out in the form of heat.

This leads to a second effect: Increasing temperature increases resistance.

So yes, in this case increasing current will indirectly cause resistance to increase.

Hope this helps.

Okay thanks for you message. Let me see if i got this right, pd is of course energy acquired when 1 coulomb of charge passes through a pd of 1V (E=QV). So if there is a high pd that means there will be a high current as well cause more electrons are able to flow. But as current increases there is more interaction of electrons hence this decreases the pd and will increase the resistance cause less electrons are flowing through. Correct?

Okay so resistance is constant here? this is for metals i assume, it would have to be. So to keep the resistance constant, a lot of voltage is required to reduce the amount of electrons passing through.... this requires work done and this in turn causes heating to the environment. So as temperature increase, resistance will increase because of the energy required(pd) to keep the resistance constant. Correct?
10. well, is it?
11. (Original post by boromir9111)
Okay thanks for you message. Let me see if i got this right, pd is of course energy acquired when 1 coulomb of charge passes through a pd of 1V (E=QV). So if there is a high pd that means there will be a high current as well cause more electrons are able to flow. But as current increases there is more interaction of electrons hence this decreases the pd and will increase the resistance cause less electrons are flowing through. Correct?

Okay so resistance is constant here? this is for metals i assume, it would have to be. So to keep the resistance constant, a lot of voltage is required to reduce the amount of electrons passing through.... this requires work done and this in turn causes heating to the environment. So as temperature increase, resistance will increase because of the energy required(pd) to keep the resistance constant. Correct?
You're over complicating things.

A high pd means a high current will flow, according to V=IR. That's it. Assuming temperature change is negligible (which it often is), resistance will not change. That's why when you get a resistor it says the resistance is 10Ohms for example. It doesn't change when you put different currents through it.

Also a point to note is that in an example like this, you don't generally decide how much current to put through a resistor, you chose the resistance of the resistor and the voltage to put across it. The current that flows through will correspond to this. For example putting a pd of 5V across a 10Ohm resistor will result in a current flowing of 0.5A. (I=V/R)

I don't really understand what you're trying to say in the second bit. The heating is caused by collisions of electrons with the metal lattice. The heating is proportional to the current flowing through the resistor, which is itself determined by the voltage set across it.
12. (Original post by El Doctoré de Mystéro)
You're over complicating things.

A high pd means a high current will flow, according to V=IR. That's it. Assuming temperature change is negligible (which it often is), resistance will not change. That's why when you get a resistor it says the resistance is 10Ohms for example. It doesn't change when you put different currents through it.

Also a point to note is that in an example like this, you don't generally decide how much current to put through a resistor, you chose the resistance of the resistor and the voltage to put across it. The current that flows through will correspond to this. For example putting a pd of 5V across a 10Ohm resistor will result in a current flowing of 0.5A. (I=V/R)

I don't really understand what you're trying to say in the second bit. The heating is caused by collisions of electrons with the metal lattice. The heating is proportional to the current flowing through the resistor, which is itself determined by the voltage set across it.
Yeah, i agree with you. This is really easy and i am over complicating it. Thanks mate for your help, i understand it now.
13. (Original post by boromir9111)
Yeah, i agree with you. This is really easy and i am over complicating it. Thanks mate for your help, i understand it now.
Don't worry. It's simple, but certainly not easy, takes a while to get your head around it. You just have to get used to a certain way of thinking, once you get that it all becomes clear. You can actually start to appreciate the beauty of the simplicity!

I assume you've got an exam coming up. Good luck with it.
14. (Original post by El Doctoré de Mystéro)
Don't worry. It's simple, but certainly not easy, takes a while to get your head around it. You just have to get used to a certain way of thinking, once you get that it all becomes clear. You can actually start to appreciate the beauty of the simplicity!

I assume you've got an exam coming up. Good luck with it.
Yeah i pretty much think i get it now. The amount of current that passes through will depend on the pd of the power supply. The higher the pd the higher the rate of flow of electrons, the current. If current is decreased that means resistance has increased. That's the relationship they want you to realise it's because the current has decreased resistance has increased due to less flow of electrons i think.

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