Richard Harrison wrote:

Keith wrote:
"The last is true, but p(t) = v(t)*i(t); volts and amps must be present
simultaneously for there to be power."

By the same token, a-c flow is discontinuous at all zero crossings! I
don`t think so.

There is certainly no power at the zero crossings. This variation in
the rate of energy flow is why the power dudes really prefer 3 phase;
energy flow is constant.

....Keith

wrote:
There is certainly no power at the zero crossings.

There is no NET power at the zero crossings. There exists
equal amounts of power flowing in opposite directions as
represented by the forward Poynting vector and the reflected
Poynting vector. Those two power flow vectors simply cancel
at the NET power equal zero point.
--
73, Cecil, W5DXP

W5DXP wrote:

wrote:
There is certainly no power at the zero crossings.

There is no NET power at the zero crossings.

I think you mean there's no instantaneous power at the zero crossings.

73, ac6xg

Jim Kelley wrote:

W5DXP wrote:
There is no NET power at the zero crossings.

I think you mean there's no instantaneous power at the zero crossings.

Well, since the NET voltage is always zero at a voltage node when
the forward power and reflected power are equal, the instantaneous
voltage is always zero, i.e. the steady-state voltage is always
zero. If we have equal power flow vectors in opposite directions,
the NET power is zero at all points up and down the line.
--
73, Cecil, W5DXP

W5DXP wrote:

Jim Kelley wrote:

W5DXP wrote:
There is no NET power at the zero crossings.

I think you mean there's no instantaneous power at the zero crossings.

Well, since the NET voltage is always zero at a voltage node when
the forward power and reflected power are equal, the instantaneous
voltage is always zero, i.e. the steady-state voltage is always
zero.

I've never actually seen the voltage at a node in a standing wave
pattern referred to as an instantaneous voltage - especially considering
that it doesn't vary with time. Instantaneous usually means the
solution to a function f(t) at time t (not f(x) and position x.) Nodes
and zero crossings aren't necessarily the same thing.

73, Jim AC6XG

Jim Kelley wrote:
Nodes and zero crossings aren't necessarily the same thing.

They are for standing waves on lossless unterminated lines,
by definition.
--
73, Cecil http://www.qsl.net/w5dxp



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W5DXP wrote:

Jim Kelley wrote:
Nodes and zero crossings aren't necessarily the same thing.

They are for standing waves on lossless unterminated lines,
by definition.

I think not. Standing waves are spatial. At certain points
on the line the (NET) voltage is always zero: nodes.
At other points on the line, the (NET) voltage is sinusoidal
and has 2 zero crossings per cycle. The amplitude of these
sinusoids varies spatially along the line resulting in the
standing wave.

In an ideal line terminated by Zo, no matter where you attach
your oscillograph to the line, you will observe a sinusoid
of the same amplitude. This sinusoid will have zero crossings
and power at the time of these zero crossings will be zero;
no energy will be flowing at the time of the zero crossing.

And going back to the comment that started this sub-thread,
it is this cyclical variation in energy flow which prompted
the power dudes to invent three phase lines in which the
energy flow does not vary cyclically; power is constant.

....Keith