In a simple circuit, where does the energy flow

8/8/2019 In a simple circuit, where does the energy flow

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ENCE HOBBYIST: Flowing Electrical Energy http://amasci.com/elect/poynt/poynt.htm

8 31/03/2008 05:33 p.m.

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IN A SIMPLE CIRCUIT, WHERE DOES THE ENERGY

FLOW?

A Collection of Diagrams

William Beaty

Electronics students commonly assume that electrical energy flows inside metal wires. Physics students

know differently! Normally the electrical energy doesn't flow inside of metals. In fact, the electrical

energy being sent out by batteries and generators is located in empty space: it takes the form of

electromagnetic fields surrounding the wires. The diagrams below will show us the details.

While coils will store energy as a magnetic field outside the windings, and while capacitors will store

energy as an electric field in the insulating layer between the metal plates, an electric circuit handles

energy a bit differently. An electric circuit as a whole does both at once: it's both coil and capacitor. The

energy which flows across a circuit is not moving through the interior of the metal wires. Instead it

flows through the space surrounding the metal parts of the circuit. For example, whenever a battery

powers a light bulb, the battery spews electrical energy into space! The electrical energy is then grabbed

firmly by the wires and guided by them. The energy flows parallel to the wires, and eventually it dives

into the light bulb filament. There it drives the metal's charges against the resisting force of electrical

"friction," and the electrical energy gets converted into thermal energy. An electric circuit is like a duct

for electrical energy, but this duct has no walls.

Fig. 1 A SIMPLE CIRCUITA battery is connected to a resistor such as a light bulb. The battery converts

its chemical fuel into waste products, and the resistor gets hot.

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8 31/03/2008 05:33 p.m.

Fig. 2 THE CONDUCTIVE PATH: CURRENT

All conductive materials contain movable charges. The resistor and the

battery's electrolyte both are conductive. When we include them with the

wires, we can see that an electric circuit is a complete circle which is full of

"fluid" charge. It acts like a liquid flywheel; a flywheel hidden inside a

closed ring of pipe.

Fig. 3 THE MAGNETIC FIELD CAUSED BY THE CURRENT

LOOP

A circular electric current is an electromagnet. The magnetic field-lines

form rings around the conductors. Note that I've slightly tilted the circles tomake them visible. In reality, we should be looking at them edge-on. (Also:

note that the physics name for the magnetic field is "B-field".)




8 31/03/2008 05:33 p.m.

Fig. 3A THE MAGNETIC FIELD CAUSED BY THE CURRENT

LOOP

Here's a better view of the above circuit... the three-dimensional oblique

view.

To be more accurate, we need to draw more than just two patterns. Betweenthe two patterns above, draw a third. Then between each of those draw

more and more. The end result looks like "tubes" of magnetic flux

surrounding the wires.

Fig. 4 TWO CHARGED CONDUCTORS: VOLTAGE

Everything connected to one battery terminal acquires the same electrical

potential (voltage.) The circuit acts like two separate conductors, one with a

positive charge imbalance and one with negative.




8 31/03/2008 05:33 p.m.

Fig. 5 THE ELECTRIC FIELD CAUSED BY THE OPPOSITE

CHARGES

The two charged wires act like the plates of a capacitor. "Force lines" of

e-field spew out of one charged conductor and dive into the other. This is a

side view of the e-field in the plane of the circuit. In a full 3-D view we'd

see the lines spreading outwards in radial star-shapes from each wire.

Fig. 5A THE ELECTRIC FIELD CAUSED BY THE OPPOSITE

CHARGES

Again, here's a 3D oblique view. The two halves of the circuit act as

opposite-charged wires with e-field flux connecting them. As with figure

3A we need to draw a third pattern between the two above, then draw morebetween those until the whole wire is covered with bent sheets of

electrostatic flux which arcs between the wires.




8 31/03/2008 05:33 p.m.

Fig. 6 E-FIELD AND B-FIELD TOGETHER

Fig. 6A E-FIELD AND B-FIELD TOGETHER

The 3D oblique view of the two fields. Add more and more patterns

between the two shown above, until empty space is packed full of "hair."

Note that most of the flowing energy lies between the two wires... but quite

a bit also surrounds the "cable pair" as a whole. Also note that the E and B

flux lines are always at 90 degrees to each other. When we say that E and B

in light waves are always perpendicular, the above diagram shows what

such a thing looks like.




8 31/03/2008 05:33 p.m.

Fig. 7 THE ENERGY FLOW (POYNTING FIELD)

Electromagnetic energy flows out of the battery and into the empty space

around the circuit. It flows parallel to the connecting wires, then it dives into

the resistor. The field of energy flow is found by multiplying the e-field by

the b-field (E x B vector cross-product.)

Fig. 8 ENERGY FLOW FIELD WITH E-FIELD IN GRAY

Note that the energy always flows perpendicular to the lines of e-field




8 31/03/2008 05:33 p.m.

Fig. 9 ENERGY FLOW WITH B-FIELD IN GRAYNote that the energy always flows perpendicular to the lines of b-field too.

Fig. 10 A SIMPLE CIRCUIT?

When all the separate invisible phenomena are displayed together, you can

see why "electricity" might be a bit hard to understand. And this diagram

only shows a two-dimensional slice; a sort of side view of the fields. The

real fields are 3D and volume-filling, so an accurate drawing would look

like a black glob of hairs.

SEE ALSO:

mit.edu, TEAL animated field diagrams:

Magnetic fields

e-fieldsFaraday induction: dynamic magnetic fields

EM and waves

Guided tour

Right-angle circuitry (bill b)

'Electricity' is not energy (bill b)

Understanding Electricity and Circuits: What the Text Books Don't Tell You (pdf)

Unifying electrostatics and electric circuits Chabay & Sherwood 1999 (.PDF)

Roy McCammon's GIFs of electric and magnetic fields in EM waves:

http://www.armory.com/~rstevew/Public/Tutor/RoyMcC_Waves/index.html

How can longwave EM pass through tiny holes? (bill b)

Poynting-flow diagrams are *extremely* rare in physics texts, and the majority of physicsinstructors seem unaware that they exist. Perhaps the reason is that, as children, all physicists

were taught that energy flows *inside* the wires. Childhood science misconceptions are

extremely difficult to cure. They frequently remain unexamined, and often persist well into

adulthood. For example, Feynman mentions the Poynting-flow concept in "The Feynman

Lectures," Chapter 27, and performs EM-field energy flow analysis on capacitors and resistors,

but he doesn't analyze 2-wire transmission lines, nor does he link the components together into a

continuous system as with my figure 7 above. Worse, at one point he bad-mouths the whole

concept, and asserts that we should not change our original viewpoint, but instead suggests that

we continue to assume that the energy flows inside the copper! Feynman? Counsiling

dishonesty rather than harnessing this "alternate toolkit?" Amazing. (And ...doesn't he know that

the speed of light inside solid copper, the speed which causes Skin Effect phenomena, is down

in the meters per second range?) If the misconception that "energy flows inside wires" had such

a deleterious effect on an honest free-thinker like Feynman, think how much trouble any more

conventional minds would have with it.

Here's another version of my figure 7: page 417, Fig 10-19, found in:




ELECTROMAGNETICS 2nd Ed., John D. Kraus & Keither R. Carver, McGraw-Hill

1973

This is interesting, because it shows one place where poynting vector energy flow is a crucial

idea: Antenna Design! Kraus Electromagnetics is essentially an antenna design book aimed at

physics students.

http://amasci.com/elect poynt/poynt.html

Created and maintained by Bill Beaty. Mail me at: .

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In a simple circuit, where does the energy flow