When looking at graphed waves of reactive power, whats going on when
you have negative current and positive voltage, or vice versa? what
would be negative wattage? wattage is always positive because of
product of same sign current and voltage, no power usage/infinite
resistance? power flowing back into the supply, with it voltage, or
When the current from the source is the same sign as the voltage from
the source, power is flowing from the source to the load. When the signs
are different, power is flowing from the load to the source. That will
happen with reactive loads. Energy will be stored in the reactive
element during part of the cycle and some will be returned during
another part of the cycle. This means that extra current flows without
actually delivering power that is dissipated by the load to do something
useful. That is why volt-amperes are not the same as watts.
When you're reading graphs, all the pos/neg excursions show you is
voltage and its relationship to whatever the reference may be, usually
ground. From that you get flow direction, etc.. All voltage values
have to be seen as positive.
Or I suppose you could look at it as showing a negative E and a neg
I, resulting in a positive.
Then again, keep in mind that you often don't work with the peak
voltages, but with the averages or rms, etc..
Gee, with negative wattage and a rectifier we could make some pretty
cheap air conditioners, couldn't we?
When the voltage is positive and the current negative the power is
negative. There is no problem. The graphs are of instantaneous power
-not the real and reactive power - which are averages over the cycle.
If the load is purely resistive, the product of instantaneous voltage
and current is always positive. The average power over a cycle will be
Vrms*Irms and this is what we really are looking at when considering
real or reactive concepts in steady state AC.
If the load is purely reactive (inductive or capacitive) , then for half
of the cycle, the power in is positive and for the other half it is
negative. That is energy is sent to the load for half the cycle and is
returned to the source for half a cycle- average energy and power over
the cycle is 0.
If the load is a mix, then for part of the cycle the instantaneous
power is positive and for part of it - it is negative. The power can be
considered as having a variable part superimposed on a constant part.
The constant part is the real or average power and the variable part.
The constant part will be less than for a purely resistive load. The
reactive Volt amps is a way of expressing a measure of the variable part.
So true. A way I found to explain the concept is working out the
energy flow direction, (using my 2 fingers and thumb representing
electric field, magnetic field and resultant energy flow AND a
whiteboard or blackboard) by way of the Poynting vector. It is then
clear which direction the energy is flowing, how much is flowing...and
when it is flowing. My arm usually aches a bit after I unknot it
But, in dealing with circuit theory, which is a quasi-static
approximation to field theory, you are only dealing with sources and
R,L, C elements and don't need to even consider Poynting or even the
electric and magnetic fields per se(replaced by the behaviour, in
electric terms, of L and C)