Amperage not sensible

[Sarah]

I have tried a few times over the years to understand amps, but beyond
a superficial grasp, it eludes me.

1) In a river analogy, if voltage is like the action of gravity on
water resulting from the slope of the earth's surface, amperage is the
amount of current or water passing down the river, and resistance is
like a partial dam placed in the river. If gravity (voltage) is
increased, current (amperage) increases. If the river is damed to a
trickle, current DECREASES. So far so good for Ohm's law.

2) But, if you have an electrically-powered propeller-driven airplane,
or an electrically-powered automobile, the larger the propeller or the
larger the tires the MORE current is drawn through the system (up to
limits of the power source and impedance of the rest of the system).

A larger driven transducer device does more work, so must present
increased load and resistance.

So, is amperage (current) increased or decreased by resistance and/or
load?

Thanks, sorry for the dumb question.

 

[Chronic Philharmonic]

"Sarah" <sarah_sonderson@hotmail.com> wrote in message
news:1192081816.708851.172320@o80g2000hse.googlegroups.com...

I have tried a few times over the years to understand amps, but beyond
a superficial grasp, it eludes me.

1) In a river analogy, if voltage is like the action of gravity on
water resulting from the slope of the earth's surface, amperage is the
amount of current or water passing down the river, and resistance is
like a partial dam placed in the river. If gravity (voltage) is
increased, current (amperage) increases. If the river is damed to a
trickle, current DECREASES. So far so good for Ohm's law.

Yes, that is pretty much the case. And if you had a water wheel in the
stream and you added more resistance (closed the spillway), it would slow to
a stop.

2) But, if you have an electrically-powered propeller-driven airplane,
or an electrically-powered automobile, the larger the propeller or the
larger the tires the MORE current is drawn through the system (up to
limits of the power source and impedance of the rest of the system).

Yes...

A larger driven transducer device does more work, so must present
increased load and resistance.

No. If you increase the load on a motor, and /lowers/ its impedance (AC
resistance), and it draws more current.

So, is amperage (current) increased or decreased by resistance and/or
load?

Resistance and load are not the same thing. In fact, they are nearly the
opposite. Increasing the load reduces the resistance and increases the
current. Conversely, reducing the resistance increases the load and the
current.

 

[Eeyore]

Sarah wrote:

I have tried a few times over the years to understand amps, but beyond
a superficial grasp, it eludes me.

1) In a river analogy, if voltage is like the action of gravity on
water resulting from the slope of the earth's surface, amperage is the
amount of current or water passing down the river, and resistance is
like a partial dam placed in the river. If gravity (voltage) is
increased, current (amperage) increases. If the river is damed to a
trickle, current DECREASES. So far so good for Ohm's law.

It's helpful to think of voltage like water *pressure*.

2) But, if you have an electrically-powered propeller-driven airplane,
or an electrically-powered automobile, the larger the propeller or the
larger the tires the MORE current is drawn through the system (up to
limits of the power source and impedance of the rest of the system).

A larger driven transducer device does more work, so must present
increased load and resistance.

So, is amperage (current) increased or decreased by resistance and/or
load?

As the load (work or power) increases the current rises. A large hyroelectric
dam won't generate any power with no water flow will it ?

Ohm's law gives us V = I.R which re = arranged is I = V/R so if resistance
reduces, current increases for a given V

Graham

 

[Doug Miller]

In article <1192081816.708851.172320@o80g2000hse.googlegroups.com>, Sarah <sarah_sonderson@hotmail.com> wrote:

I have tried a few times over the years to understand amps, but beyond
a superficial grasp, it eludes me.

1) In a river analogy, if voltage is like the action of gravity on
water resulting from the slope of the earth's surface, amperage is the
amount of current or water passing down the river, and resistance is
like a partial dam placed in the river. If gravity (voltage) is
increased, current (amperage) increases. If the river is damed to a
trickle, current DECREASES. So far so good for Ohm's law.

The river analogy is a useful one. Think of voltage as the speed of the river,
and current as the amount of water flowing, and it may help you visualize it.
In this model:
Low voltage, low current: a small stream in relatively flat terrain.
Low voltage, high current: the Mississippi River.
High voltage, low current: a mountain stream.
High voltage, high current: the Niagara River.
Very high voltage, very low current: a fire hose.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.

 

[DJ Delorie]

Think of a load like a waterwheel.

The work done by the waterwheel (V*I) depends on the amount of water
passing it, *and* the height difference (drop) of the water. A
fast-running but rather flat river can't be used for much work, nor
can a waterfall at a trickle. A fast waterfall gives the most work.

So, consider the waterwheel. Putting the wheel in the water will slow
it down but also cause the water level before it to rise, and the
level after it to fall. To get the most work, you want to balance the
change in water level with the reduction in speed of the water.

Now, a small waterwheel doesn't slow down the water as much, but
neither does it change the levels much. A larger waterwheel slows the
water down a lot, but also changes the level a lot. It's producing
more work for less water speed.

A motor-type electrical load is like that. When lightly loaded, it
has less effective resistance[*] so the current is higher, but the
voltage drop is less also, so less net energy is used. A higher load
increases resistance, reduces current, but increases the voltage drop
- net result, more energy used.

[*] An unloaded motor is like an inductor, not a resistor. The
impedance is all imaginary, so no real energy is used. Putting a
load on it changes the impedance angle, making more of the
impedance real and thus increasing real energy used and real work
done.

This is again like a waterwheel - a big wheel with no load will
just "go with the flow" and not slow down the river much at all,
but as soon as you engage the wheel (say, to grind flour), it
starts slowing down the river and letting the water pile up in
order to extract useful work from it.

 

[Robert Roland]

On Wed, 10 Oct 2007 22:50:16 -0700, Sarah
<sarah_sonderson@hotmail.com> wrote:

A larger driven transducer device does more work, so must present
increased load and resistance.

No. Increased load means less resistance.

So, is amperage (current) increased or decreased by resistance and/or
load?

Current increases when the resistance is reduced. Resistance is
reduced when load is increased.
--
RoRo

 

[Eeyore]

Robert Roland wrote:

Sarah wrote:

A larger driven transducer device does more work, so must present
increased load and resistance.

No. Increased load means less resistance.

And this is where making 'folksy science' comparisons falls down of course.


Graham

 

[Tim Wescott]

Sarah wrote:

I have tried a few times over the years to understand amps, but beyond
a superficial grasp, it eludes me.

1) In a river analogy, if voltage is like the action of gravity on
water resulting from the slope of the earth's surface, amperage is the
amount of current or water passing down the river, and resistance is
like a partial dam placed in the river. If gravity (voltage) is
increased, current (amperage) increases. If the river is damed to a
trickle, current DECREASES. So far so good for Ohm's law.

2) But, if you have an electrically-powered propeller-driven airplane,
or an electrically-powered automobile, the larger the propeller or the
larger the tires the MORE current is drawn through the system (up to
limits of the power source and impedance of the rest of the system).

A larger driven transducer device does more work, so must present
increased load and resistance.

So, is amperage (current) increased or decreased by resistance and/or
load?

Thanks, sorry for the dumb question.

Current flow _is_ sensible.

1. Correct.

2. If you have a simple waterwheel in the river (think undershot, if
you're that familiar with waterwheels) with no load, the paddles will
dip into the water but won't disturb the flow. Now put a big load on
the wheel (like a propellor). Now the wheel will go slower than the
river, and the paddle _will_ disturb the flow.

At some point you'll find that you've stretched your analogy to
breaking. When that happens it's time to dispense with it and just pay
attention to the actual current and voltage.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Robert Roland]

On Thu, 11 Oct 2007 19:08:26 +0100, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:

No. Increased load means less resistance.

And this is where making 'folksy science' comparisons falls down of course.

Yes. Although the water stream analogy works well to visualize
electric current, the waterwheel does not provide a good analog to the
electric motor.

Maybe it helps to emphasize the difference between electrical
resistance and mechanical resistance:

When the mechanical resistance on the motor's shaft increases, the
motor's electrical resistance decreases.
--
RoRo

 

[John C. Polasek]

On Wed, 10 Oct 2007 22:50:16 -0700, Sarah
<sarah_sonderson@hotmail.com> wrote:

I have tried a few times over the years to understand amps, but beyond
a superficial grasp, it eludes me.

1) In a river analogy, if voltage is like the action of gravity on
water resulting from the slope of the earth's surface, amperage is the
amount of current or water passing down the river, and resistance is
like a partial dam placed in the river. If gravity (voltage) is
increased, current (amperage) increases. If the river is damed to a
trickle, current DECREASES. So far so good for Ohm's law.

2) But, if you have an electrically-powered propeller-driven airplane,
or an electrically-powered automobile, the larger the propeller or the
larger the tires the MORE current is drawn through the system (up to
limits of the power source and impedance of the rest of the system).

A larger driven transducer device does more work, so must present
increased load and resistance.

So, is amperage (current) increased or decreased by resistance and/or
load?

Thanks, sorry for the dumb question.
An electric motor has a back voltage (EMF) proportional to speed. If

you put 100 volts to a motor and let it run load-free at high speed,
the back EMF could be 95 volts, so only 5 volts net is left for the
motor.
With a load that would drag it to half speed, the back voltage would
be only 50 volts and now there are 50 volts left for the motor. With
50 volts the motor will draw much more current than with 5.
John Polasek

 

[Don Kelly]

----------------------------
"John C. Polasek" <jpolasek@cfl.rr.com> wrote in message
news:sdpah39mpu78ahptrocgsdha0d8kn2skb1@4ax.com...

On Wed, 10 Oct 2007 22:50:16 -0700, Sarah
sarah_sonderson@hotmail.com> wrote:

I have tried a few times over the years to understand amps, but beyond
a superficial grasp, it eludes me.

1) In a river analogy, if voltage is like the action of gravity on
water resulting from the slope of the earth's surface, amperage is the
amount of current or water passing down the river, and resistance is
like a partial dam placed in the river. If gravity (voltage) is
increased, current (amperage) increases. If the river is damed to a
trickle, current DECREASES. So far so good for Ohm's law.

2) But, if you have an electrically-powered propeller-driven airplane,
or an electrically-powered automobile, the larger the propeller or the
larger the tires the MORE current is drawn through the system (up to
limits of the power source and impedance of the rest of the system).

A larger driven transducer device does more work, so must present
increased load and resistance.

So, is amperage (current) increased or decreased by resistance and/or
load?

Thanks, sorry for the dumb question.
An electric motor has a back voltage (EMF) proportional to speed. If
you put 100 volts to a motor and let it run load-free at high speed,
the back EMF could be 95 volts, so only 5 volts net is left for the
motor.
With a load that would drag it to half speed, the back voltage would
be only 50 volts and now there are 50 volts left for the motor. With
50 volts the motor will draw much more current than with 5.
John Polasek
----------------------

Sorry, John, I wrote the following before seeing your reply so take this as
an addition to what you have said rather than a contradiction.

In the case of an electric motor, as in the airplane or automobile, Ohm's
Law does not apply- a motor is an active device which includes a voltage
source as well as an Ohmic resistance.

as an example for a DC motor:

V=R*I +E where V is the applied voltage and E is a back emf dependent on
speed. I is the current and R is the resistance.
The power converted to mechanical power is E*I =Torque*angular velocity
(or force*velocity)
In such a motor the torque is proportional to I and the speed proportional
to E. Actual mechanical uoutput is lower because of mechanical losses
The input power is V*I =R*I^2 (electrical loss) +E*I and if R*I^2 is
greater than E*I you have a lousy and inefficient motor.

Another part of the problem is that increased resistance doesn't necessarily
mean an increased load. In discussion of motors, power output (as determined
by the mechanical conditions) is the "load"


--

Don Kelly dhky@shawcross.ca
remove the X to answer