Maker Pro
Maker Pro

Effective Power and Load Behavior

Q

qude

Jan 1, 1970
0
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.

Pls. elaborate (also the physics part of it). Many thanks.

Qude
 
D

Dave

Jan 1, 1970
0
qude said:
Suppose you have voltage = 110 volts and current 2 Ampere


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.

Pls. elaborate (also the physics part of it). Many thanks.

Qude
no answer possible from what you are asking as your terms are not defined.
what do you mean 'affected more'?? what type of 'behavior' are you thinking
of?
 
Q

qude

Jan 1, 1970
0
Dave said:
no answer possible from what you are asking as your terms are not defined.
what do you mean 'affected more'?? what type of 'behavior' are you thinking
of?

For example, what creates brighter light bulbs. A 110V/2Ampere
bulb or a 12 Volts/18.3Ampere bulb, both produce 220 watts of power.
what's the occasion when you must choose the 110V/2A or 12V/18.3A
in designing loads.

qude
 
D

Dave

Jan 1, 1970
0
qude said:
For example, what creates brighter light bulbs. A 110V/2Ampere
bulb or a 12 Volts/18.3Ampere bulb, both produce 220 watts of power.
what's the occasion when you must choose the 110V/2A or 12V/18.3A
in designing loads.

qude

you pick the voltage based on other considerations... usually by what is
available for whatever you are designing. i.e. if you are designing for a
house or office you use 110v, if you are designing for a car you use 12v.
or from what the load you have to power needs. i.e. if the load is designed
for 110v you give it 110v, if its designed for 12v you give it 12v. from
that you calculate current needed and how big of a wire you need for it.
 
O

operator jay

Jan 1, 1970
0
qude said:
For example, what creates brighter light bulbs. A 110V/2Ampere
bulb or a 12 Volts/18.3Ampere bulb, both produce 220 watts of power.
what's the occasion when you must choose the 110V/2A or 12V/18.3A
in designing loads.

qude

You design it based upon what voltage the user will have available. If you
manufacture an amplifier for use in homes in North America, and design it so
it will work off of +-31Vdc, you're gonna have trouble selling it and anyone
who buys it will have trouble using it. If you make it so it works off of
120Vac or thereabouts that's a great idea. If you make an amplifier for use
in cars in North America, then you would probably design it to work off of
12Vdc or thereabouts.

If there is some situation whereby there is no voltage constraint like that,
then typically a moderate voltage and moderate current work well. High
voltages require lots of insulation. High currents require heavy conductors
and lose a lot of power to parasitic resistance.

j
 
S

sue jahn

Jan 1, 1970
0
Dave said:
no answer possible from what you are asking as your terms are not defined.
what do you mean 'affected more'?? what type of 'behavior' are you thinking
of?

I am so bad at maths I'll let you do
P = I^2 R
and see the resistance must be much lower for the 12 volt load
to consume the same power.

That also means a conductor like nichrome or tungsten will be fatter and
shorter than the 110 volt load. This can be a real mechanical advantage
for hot envelope halogen lamps because a long thin coil of wire can flop
around, shedding, fatiguing or actually striking the envelope.

This is why you don't see this type of lamp below ~100 watts in the
110 volt variety. Above ~300 watts the filiment can be short an
fat so doesn't require too much mechanical support.

That isn't quite what we call physics and I am guessing... lamps.

Sue...
 
Q

qude

Jan 1, 1970
0
operator said:
You design it based upon what voltage the user will have available. If you
manufacture an amplifier for use in homes in North America, and design it so
it will work off of +-31Vdc, you're gonna have trouble selling it and anyone
who buys it will have trouble using it. If you make it so it works off of
120Vac or thereabouts that's a great idea. If you make an amplifier for use
in cars in North America, then you would probably design it to work off of
12Vdc or thereabouts.

If there is some situation whereby there is no voltage constraint like that,
then typically a moderate voltage and moderate current work well. High
voltages require lots of insulation. High currents require heavy conductors
and lose a lot of power to parasitic resistance.

j


I'm just interested in the theoretical side of it.

Suppose my source can equally do a 220v/2A and 12V/18A. If I
get a light bulb for each. What would be brighter.

Or better yet. I'm interested in the physics side of what's
going on inside the wires. In the 12V/18A. More electrons
flow in the wire.. however the voltage is only 12V. In
the case of the 220v/2A. Less electrons flow in the wire. So
what is the function of the voltage or potential difference.
In the 220v/2A case. There are fewer electrons but bigger
potential difference. What is the effect of it on electron
flow compare to the larger 18A but lower voltage or potential
difference. Since electron flow has same speed in all ampere
rating. What is the function of the voltage in the behavior
of the current?

qude
 
O

operator jay

Jan 1, 1970
0
qude said:
I'm just interested in the theoretical side of it.

Suppose my source can equally do a 220v/2A and 12V/18A. If I
get a light bulb for each. What would be brighter.

Well, the bulbs will be different to suit the voltage and current to be
applied. These construction and materials differences would determine
brightness and other light characteristics more than the voltage and current
per se. In either case the voltage and current deliver energy at the same
rate to the filament.
Or better yet. I'm interested in the physics side of what's
going on inside the wires. In the 12V/18A. More electrons
flow in the wire.. however the voltage is only 12V. In
the case of the 220v/2A. Less electrons flow in the wire. So
what is the function of the voltage or potential difference.
In the 220v/2A case. There are fewer electrons but bigger
potential difference. What is the effect of it on electron
flow compare to the larger 18A but lower voltage or potential
difference. Since electron flow has same speed in all ampere
rating. What is the function of the voltage in the behavior
of the current?

Hmmm. I think the electron flow does not have the same speed in all ampere
ratings. I would think that

I = v * A * p

I current (C/s, or A)
v drift velocity of electrons (m/s)
A cross sectional area of conductor (m^2)
p density of free electrons in the material that the wire is made of (C/m^3,
or # free electrons/m^3 * 1.6E-19)

A and p would basically be fixed for a certain conductor. So drift velocity
should vary with current. Though I may be proven wrong. Thinking about
Ohm's law, this all seems to work out nicely, in my head.

Are you familiar with the difference between (a) the speed of light in a
conductor (on the order of 2*10^8m/s), (b) the instantaneous speed of an
electron as it pinballs its way down a conductor (on the order of 10^6m/s),
and (c) the "drift velocity" or overall average speed with which an electron
makes its way down a conductor (I'll say 10^-4m/s to give a feel of the
magnitude but I think it would vary hugely)?

j
 
D

Don Kelly

Jan 1, 1970
0
qude said:
I'm just interested in the theoretical side of it.

Suppose my source can equally do a 220v/2A and 12V/18A. If I
get a light bulb for each. What would be brighter.

Or better yet. I'm interested in the physics side of what's
going on inside the wires. In the 12V/18A. More electrons
flow in the wire.. however the voltage is only 12V. In
the case of the 220v/2A. Less electrons flow in the wire. So
what is the function of the voltage or potential difference.
In the 220v/2A case. There are fewer electrons but bigger
potential difference. What is the effect of it on electron
flow compare to the larger 18A but lower voltage or potential
difference. Since electron flow has same speed in all ampere
rating. What is the function of the voltage in the behavior
of the current?

qude
Don't get wound up in electron flow. The individual electrons just drift
along and, in case of AC, wobble around a bit.

Note that power depends on both current and voltage, not just current alone.
The theoretical answer is that both the 12V/18A and the 220V/2A lamps the
power and the brightness is the same assuming the same filament temperature
(-which depends also on non-electrical factors). What appears to be more
electrons passing a given point at 18A than at 2A really does not determine
this.

The voltage is the driving force behind the current-Your 12V lamp has a
resistance of 12/18 ohms while the 220V lamp has a resistance of 220/2 ohms.
If you applied 220V to your 12V lamp- it would be extremely bright
momentarily just before it blew up as the current would be 330A.

For any application there is an optimal voltage range. For automobiles 12V
was it but more recently moving to 48V has been considered because of
higher electrical loads. For homes, 12V is impractical and 120/240V is
better. For power distribution, this is too low, etc.
 
R

R. Steve Walz

Jan 1, 1970
0
qude said:
Suppose you have voltage = 110 volts and current 2 Ampere


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?
------------------------
There isn't one same load.
They are two different loads, their resistances differ wildly.

R = V/I

Pls. elaborate (also the physics part of it). Many thanks.

Qude

-Steve
 
R

R. Steve Walz

Jan 1, 1970
0
qude said:
I'm just interested in the theoretical side of it.

Suppose my source can equally do a 220v/2A and 12V/18A. If I
get a light bulb for each. What would be brighter.
------------------------
They would be two bulbs of different resistance, both the same
brightness.

Or better yet. I'm interested in the physics side of what's
going on inside the wires. In the 12V/18A. More electrons
flow in the wire.. however the voltage is only 12V. In
the case of the 220v/2A. Less electrons flow in the wire. So
what is the function of the voltage or potential difference.
------------------------
Two push their currents through two totally different resistances.

In the 220v/2A case. There are fewer electrons but bigger
potential difference. What is the effect of it on electron
flow compare to the larger 18A but lower voltage or potential
difference. Since electron flow has same speed in all ampere
rating.
--------------------
Oh no, that's totally wrong!! Electrons don't all "go at the speed
of light" or some such baloney. They travel at the drift velocity
for those circumstances. You need a physics course.

-Steve
 
C

CWatters

Jan 1, 1970
0
qude said:
Suppose you have voltage = 110 volts and current 2 Ampere


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.
 
Q

qude

Jan 1, 1970
0
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
 
J

Joe Soap

Jan 1, 1970
0
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.
It is 9 times greater. But current doesn't flow 'into' a wire, it flows
THROUGH it.
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.
A guy called Ohm explained this ages ago. I = V/R. Current is
proportional to voltage, and inversely proportional to resistance.
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. Yes.

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.
Voltage difference - otherwise known as EMF (electromotive force) is the
force that drives the electrons, resistance opposes that movemnt. So more
emf, or less resistance, both mean more current flow.

-- Joe Soap. JUNK is stuff that you keep for 20 years, then throw away a
week before you need it.
 
S

sue jahn

Jan 1, 1970
0
qude said:
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 byhttp://scienceworld.wolfram.com/physics/ElectricalConductivity.html

Sue...
 
Q

qude

Jan 1, 1970
0
CWatters said:
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
 
R

redbelly

Jan 1, 1970
0
qude said:
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
 
Q

qude

Jan 1, 1970
0
redbelly said:
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.


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.


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
 
R

redbelly

Jan 1, 1970
0
qude said:
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).
 
C

CWatters

Jan 1, 1970
0
qude said:
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
 
Top