Here's the script i use
The humble flashlight has a lot of lessons in it.
Get yourself an old fashioned two cell metal one to use for a prop while you're speaking.
Get a cheap meter, too i use the $9.95 Walmart analog one.Draw the flashlight circuit on the board, two batteries and a switch and a bulb. Bulb on right, batteries left.
Remove one battery from your flashlight and hold it up . Tell the kids
"Every quantum of positive charge that comes out the positive end (tap the top) of this battery must get back to its negative end.(tap the bottom)
Or, if you prefer to think of charge as negative and carried by electrons, every bit of negative charge that comes out the bottom must get back to the top."
Then move to the drawing on the board and trace out the loop, saying
"If that charge must flow through the switch and the bulb and another battery to get there, that's what it will do."
Then reassemble the flashlight and turn it on, saying "That's Kirchoff's Current Law for DC. DC on this drawing always flow clockwise(gesture around your loop) for positive charge carriers which we call conventional current or counterclockwise for negative charge carriers which we call electron current."
Take a breath and see if they got it.
Continue:
"In more highly educated circles they always use conventional positive current but many trade schools still teach electron current. You should learn to think in positive current , that's how most textbooks are written, and you know , 'When in Rome' .
You will however need to be conversant in both and i assure you they both work out to give same answers."
Take a breather
continue
"Notice we have not even mentioned the terms Earth, Ground, Hot or Neutral.
What do you think would happen if i measured the voltage between this flashlight and Earth ground? Would any current flow to deflect the meter needle?
(draw a meter on the board from battery negative to ground, maybe the frame of your chalkboard.)
Ask class. In my class half the kids always say 'Yeah current wants to flow to ground'
Ask class "How does going to ground help that charge find its way back to other end of the battery from which it came ?"
Connect your real meter from flashlight metal to something in the room that's grounded (I had already verified that my chalkboard frame was grounded) and have a kid run through all the ranges to show no current flows.
"Now we have demonstrated Kirchoff's Current Law which simply says "Current only wants to flow back to wherever it came from." That's why it flows in closed loops and if ground isn't in that loop current won't go there."
That'll generate some consternation.
Explain that lightning is obeying KCL because charge was carried aloft with water vapor that evaporated from the ground ahead of the thunderstorm, lightning is just that charge getting back to the Earth where it came from a little while ago
Kirchoff will hold an IOU temporarily. Or maybe i should say he gives out rain checks.
Anyhow lightning imprints us early with the misconception that 'electricity for some reason likes ground' when it's really just cashing in a Kirchoff's Rain Check.
Next i say
"So, what would happen if i connected my flashlight's frame to Earth ground? (i set mine on the metal chalk rail which i knew from prior test was grounded) Would current flow to ground? No, we already checked for that with the meter."
Then i move to the drawing on the board and draw a line from negative down to a ground symbol.
"If i measure from ground to flashight body i'll measure zero volts
If i measure from ground to positive i'll measure positive three volts
if i measure from ground to junction of the two batteries i'll measure positive one and a half volts."
Then i erase my ground wire and ask "What if instead of grounding the flashlight body i had instead reached inside and grounded the junction of the two batteries?" and redraw my ground wire there. Gesturing as i speak, continue
"Now from ground to flashlight body i'll measure minus one and a half volts.(write it on board)
From ground to positive of top battery i'll read plus one and a half volts.(write it on board)
From ground to battery junction i read of course zero.
From flashlight body to top of batteries i'll read positive three volts.(write it on board too)
You see, voltage is a potential difference between two points and nobody said either one of those two points has to be ground.
And that's Kirchoff's voltage law, voltages add up."
"Okay boys, now you have the basic nomenclature and concept.
The only difference between DC and AC is this -
DC always flows the same direction around the loop either clockwise or counterclockwise .
AC reverses its direction 50 or 60 times a second depending what country you're in , so it shuttles back and forth around the loop. (Gesture wildly, you want to be the next Doc Brown)
But when you train your brain to freeze-frame your thinking, at any instant AC is DC .
Kirchoff still applies.
AC current is usually delivered to us by a transformer winding not a battery.
But it's still as simple as our two cell flashlight. (draw another flashlight circuit with two transformer windings instead of batteries, no ground)
AC current wants only to get back to the far end of the transformer winding from whence it sprang
and if but only if ground will take it there is it happy to flow that way .
If the transformer winding isn't grounded it cannot send any current to ground. It's no different from that flashlight sitting over there.
Most of the transformer windings you'll encounter outside of industry have two 120 volt windings in series. That makes for 240 volts but for some reason it's called 230. That's a leftover from the old days when voltages were a little bit lower."
Continue
"
In the UK they ground one end of that winding (draw it on the board) so the 'hot' end will read 230 or 240 volts to 'ground'.
The wire on transformer's hot end has a circuit breaker in it and is said to carry current to the load,
The end that's grounded they call 'neutral' , and the wire on that end is said to return current to the transformer
but that's strictly true only while current is flowing clockwise (assuming your drawing has load on right side of transformer) be aware current is cycling at power line frequency." (gesture back & forth)
erase ground
"In the US they ground the junction of the two windings and call that point neutral (draw it in)
so the two transformer ends will both read 120 volts AC to ground and both are called 'hot' (write 120 and 120 beside them)
but freeze framing, at the instant when one is +120 the other is -120 so there's 240 between them (add signs to drawing)
your 120 volt appliances like lights and TV sets are connected between one hot and neutral, and the outlets in your house are divvied up among the two hots in hopes of somewhat balancing the load on the two windings,
your 230 volt appliances like water heater and furnace are connected between the two hots and might not even have a neutral wire...
and that's house wiring at its simplest."
whew!
Sorry for the long long essay
but in an hour you can plant basics that'll last the boys a lifetime
and will prevent much wayward thinking.
Then go on to "that's why letting a hot wire accidentally touch ground gives such impressive sparks. Current can get back to its transformer by going through ground. That's because we ground transformers to keep things orderly, but it makes people think electricity magically 'likes ground' which isn't so.
When messing with house wiring in the US always remember 'The Black wire is the one with smoke in it.
It always goes to the short slot in an outlet and to the center pin of a lamp socket."Read IEEE Green Book for an excellent introduction to grounding. Google finds it.
whew - sorry , but i think this is important.
Good luck !
old jim
(edited for typos and dumb mistakes )
