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jahn
science forum Guru

Joined: 08 May 2005
Posts: 821

Posted: Sun Jun 19, 2005 9:46 am    Post subject: Re: Effective Power and Load Behavior

"qude" <qmdynamics@yahoo.com> wrote in message news:1119172222.190468.82400@f14g2000cwb.googlegroups.com...
 Quote: Thanks for the answers but I want understand the physics side. When 18A is said to flow into a wire. What is the difference of it in terms of electron flow compared to lets say 2A. What does the voltage do to the current. It drives them you will say. But how come you can drive a 18A current with only 12 volts whereas in a 110 volts, the current is only 2A. It may have to do with the load. So in loads with lower resistance. More electrons can pass thru it, right. In the case of the 18A/12 volts supply, more electrons pass thru the load. In the case of the 2A/110 volts. Only few electrons pass thru it. Now here's the problem. That means using higher voltage didn't mean pushing greater amount of electrons. So what is the function of voltage and current in terms of electron movement?? Let's focus on the physics side of it. qude Clicking around these pages should get it on your own terms:

<< A "flow" defined as the rate of a quantity passing a given
point per unit time. When used without qualification, "current"
generally refers to electric current, which is given by:
http://scienceworld.wolfram.com/physics/Current.html

where is the resistivity. For a body of length L, resistance R,
and cross sectional area A, the electrical conductivity is given by
 Quote: http://scienceworld.wolfram.com/physics/ElectricalConductivity.html

Sue...
qude
science forum beginner

Joined: 18 Jun 2005
Posts: 17

Posted: Sun Jun 19, 2005 10:06 am    Post subject: Re: Effective Power and Load Behavior

CWatters wrote:
 Quote: "qude" wrote in message news:1119070735.052785.12830@g14g2000cwa.googlegroups.com... Suppose you have voltage = 110 volts and current 2 Ampere From P = V I Power = 220 Watts Suppose you have voltage = 12 volts and current 18.3 Ampere Power = 220 Watts What is the behavior of the load in each case? They both have the same 220 watts power yet the second one with a 18.3 Ampere current should be affected more.. yet if the voltage in the first is 110 volts.. the power is similar... What is the behavior of loads with more voltage or more current. Which can it take more. What part is affected more, etc. The effective resistance of each load is different for the two cases you describe... 1) 110/2 = 55 Ohms 2) 12/18.3 = 0.66 Ohms If these were light bulbs the wire filament needed to make the 12V lamp (0.66 Ohms) would be much thicker than the 110V lamp (55 Ohms). Therefore the 12V lamp would be more robust and resistant to the thermal shock that occurs when it's switched on/off. Indeed low voltage Halogen downlight bulbs do last longer than mains voltage lamps.

After thinking it over. I got the main idea. So the load is
related to it all. But you said lower resistant load requires
thicker material.. What's the physical basis? Is it because when
the material is thicker, there are more spaces for the electrons to
pass through that's why resistance is less. While in high
resistance load, it needs thinner material so the electrons
would have to struggle to pass thru it and hence the entire
electron line is adjusted to be lesser (lower current)??

If right. Well. How do you select material where thicker
means more electrons can pass thru and less resistance
yet have same lighting power to the thinner wire with
more resistance? Can you use same material to build
thicker or thinner parts or does it depends on material
properties for each (higher or lower resistance) application?

I want to be able to visualize it all and not just memorizing
formulas and equations. Thanks.

qude
redbelly
science forum beginner

Joined: 19 Jun 2005
Posts: 20

Posted: Sun Jun 19, 2005 1:20 pm    Post subject: Re: Effective Power and Load Behavior

qude wrote:
 Quote: After thinking it over. I got the main idea. So the load is related to it all. But you said lower resistant load requires thicker material.. What's the physical basis? Is it because when the material is thicker, there are more spaces for the electrons to pass through that's why resistance is less. While in high resistance load, it needs thinner material so the electrons would have to struggle to pass thru it and hence the entire electron line is adjusted to be lesser (lower current)??

That is pretty much what is happening. Instead of "more spaces",
people usually think in term of more area (cross-section), but
you have the right idea.

 Quote: If right. Well. How do you select material where thicker means more electrons can pass thru and less resistance yet have same lighting power to the thinner wire with more resistance? Can you use same material to build thicker or thinner parts or does it depends on material properties for each (higher or lower resistance) application?

The material selected is the one that can withstand the highest
temperature and conduct electricity. Of all known materials,
tungsten is the one that fits this description. As far as I
tungsten, regardless of power or voltage or other requirements.

Then the wire's length and thickness must be chosen.
This can be done by knowing the necessary
resistance and operating temperature of the wire. The resistance
can be calculated from the available line voltage and the power.

Operating temperature is usually chosen to be around 2200 to
2800 C for tungsten. Hotter temperatures result in unreasonably
short burn-out lifetime. Lower temperatures result in less
efficient operation (less light produced per Watt of electricity).

Even though many length & thickness combinations will give the
necessary wire resistance, only one combination of length and
thickness will give BOTH the desired resistance AND operating
temperature for the wire. A wire of that length and thickness will
only "work" for a given supply voltage. At lower voltages, the
wire will run less efficiently. At higher voltages, the
light will be more efficient BUT will burn out faster.

 Quote: I want to be able to visualize it all and not just memorizing formulas and equations. Thanks.

For resistance, visualize that a longer path length OR smaller
cross-section area will increase the resistance:

Resistance is proportional to ( Length / diameter^2 )

For temperature, a smaller outside surface area will increase the
temperature, since the power leaves the wire mainly by radiating
from this surface.

Temperature depends (approximately) on ( Diameter / Length^2 )

Mark
qude
science forum beginner

Joined: 18 Jun 2005
Posts: 17

Posted: Sun Jun 19, 2005 7:09 pm    Post subject: Re: Effective Power and Load Behavior

redbelly wrote:
 Quote: qude wrote: After thinking it over. I got the main idea. So the load is related to it all. But you said lower resistant load requires thicker material.. What's the physical basis? Is it because when the material is thicker, there are more spaces for the electrons to pass through that's why resistance is less. While in high resistance load, it needs thinner material so the electrons would have to struggle to pass thru it and hence the entire electron line is adjusted to be lesser (lower current)?? That is pretty much what is happening. Instead of "more spaces", people usually think in term of more area (cross-section), but you have the right idea. If right. Well. How do you select material where thicker means more electrons can pass thru and less resistance yet have same lighting power to the thinner wire with more resistance? Can you use same material to build thicker or thinner parts or does it depends on material properties for each (higher or lower resistance) application? The material selected is the one that can withstand the highest temperature and conduct electricity. Of all known materials, tungsten is the one that fits this description. As far as I know, all light bulb filaments made these days are made from tungsten, regardless of power or voltage or other requirements. Then the wire's length and thickness must be chosen. This can be done by knowing the necessary resistance and operating temperature of the wire. The resistance can be calculated from the available line voltage and the power. Operating temperature is usually chosen to be around 2200 to 2800 C for tungsten. Hotter temperatures result in unreasonably short burn-out lifetime. Lower temperatures result in less efficient operation (less light produced per Watt of electricity). Even though many length & thickness combinations will give the necessary wire resistance, only one combination of length and thickness will give BOTH the desired resistance AND operating temperature for the wire. A wire of that length and thickness will only "work" for a given supply voltage. At lower voltages, the wire will run less efficiently. At higher voltages, the light will be more efficient BUT will burn out faster. I want to be able to visualize it all and not just memorizing formulas and equations. Thanks. For resistance, visualize that a longer path length OR smaller cross-section area will increase the resistance: Resistance is proportional to ( Length / diameter^2 ) For temperature, a smaller outside surface area will increase the temperature, since the power leaves the wire mainly by radiating from this surface. Temperature depends (approximately) on ( Diameter / Length^2 ) Mark

It has to do with more electron collisions in resistors
to produce the increased temperature or heat (and hence
light). But something eludes me. If you make the resistance
of the tungsten load higher, meaning thinner... there is
more electron collisions to the lattice hence more friction
and light. Now in lower resistance, the area is larger so
less frictions. Isn't one wasting electrons in this latter
since more electrons pass thru the lower resistance and
didn't strike much lattice but some just passing thru
the space in between??

qude
redbelly
science forum beginner

Joined: 19 Jun 2005
Posts: 20

Posted: Mon Jun 20, 2005 2:19 am    Post subject: Re: Effective Power and Load Behavior

qude wrote:

 Quote: It has to do with more electron collisions in resistors to produce the increased temperature or heat (and hence light). But something eludes me. If you make the resistance of the tungsten load higher, meaning thinner... there is more electron collisions to the lattice hence more friction and light. Now in lower resistance, the area is larger so less frictions. Isn't one wasting electrons in this latter since more electrons pass thru the lower resistance and didn't strike much lattice but some just passing thru the space in between?? qude

You can think of a lower resistance meaning that there are fewer
collisons between electrons and the atoms in the lattice, BUT:
If the wire is connected to a constant VOLTAGE source, then
each collison will be more energetic (the electron gains more
energy in between collisions than it would in a higher resistance
material). This higher energy per collision actually results in
more power dissapated, so more heat and/or light is generated.

If the wire is connected to a constant CURRENT source, then the
lower resistance results in less voltage pushing on the electrons.
The net result is less power (and heat, and/or light).
CWatters
science forum Guru Wannabe

Joined: 01 May 2005
Posts: 120

Posted: Mon Jun 20, 2005 5:42 am    Post subject: Re: Effective Power and Load Behavior

"qude" <qmdynamics@yahoo.com> wrote in message
 Quote: Thanks for the answers but I want understand the physics side. So what is the function of voltage and current in terms of electron movement?? Let's focus on the physics side of it.

An electron has charge and current is defined as the number of unit charge
(Q) flowing per unit Time (T)

I = Q/T

Ohms law says

I = V/R

so

Q/T = V/R

or

Q = T * V/R

eg Increase V and more units of charge flow past in a given time.
or
Increase R and fewer units of charge flow past in a given time

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