Why does sticking a knife into an operating toaster give you a shock?

I understand why sticking a knife into a toaster produces an explosive phenomenon. It completes the conductive connection between the two holes in the electrical outlet and the 250 volts or so that separate them cause a substantial current to rush through an almost 0 resitance. In other words the knife creates a short circuit.

But I don't understand why the person would get a shock. To simplify the problem we may as well assume that the person is holding a piece of aluminium with two branches, one in each hole in the outlet.

At any given moment, the voltage between each outlet hole and the earth is presumably at most something of the same order of magnitude as the tension between the two outlet holes. So the current will travel through both possible paths with currents inversely proportional to the resistances, that is, 300-1000 Ohms for the human body and some $10^-7$ or $10^-8$ Ohms for the aluminium, so basically the current travelling through the person to the ground shouldn't be of any significance should it ?

If the piece of aluminium only sticks into one hole then entire current flows through the person but if is sticks into both it doesn't.

2 Answers
2

here is why you can kill yourself with the knife-and-toaster trick.

the AC power line into which the toaster is plugged consists of a hot line (black wire) that has 120VAC on it and a ground return line (white wire) which is at zero VAC or very close to it.

The toaster has a switch inside which feeds power to the bare resistance wire inside the slots when the lever is pressed down and turns it off when the toast is done. this looks like:

black wire (120VAC)------switch-----resistance wire-----white wire (0VAC)

On older toasters and older outlets, the power plug can be inserted into the power outlet in one of two ways: one where the hot wire leads to the switch (as shown above) and one where the hot wire is connected to the resistance wire. In this configuration, the resistance wire is at 120VAC even when the toaster is off.

In this case, if you touch the resistance wire with your knife your hand is connected to 120VAC which wants very badly to flow to ground potential. If the floor is concrete and just a tiny bit damp, your body completes the circuit and you get shocked badly. If the floor is dry linoleum, you may get a surprising tingle.

Note that this can happen EVEN WITH THE TOASTER OFF.

On modern toasters and modern electrical outlets, the plug and outlet are designed so you cannot stick the plug in "backwards": the hot side will always be connected to the switch, so that if the switch is off, the bare wire inside the toaster is at 0VAC and touching it with a knife will not shock you.

Now we treat the case where the toaster is ON and you jam a knife down the slot WHILE IT IS TOASTING, and touch the resistance wire inside. The end of the resistance wire that is closest to the switch is at 120VAC and the end that is closest to the white return wire is at ~0VAC. In the middle of the resistance wire length, the voltage is at ~60VAC.

If your knife touches the wire at a point along its length where the voltage present there just happens to be sufficient to establish current flow through your body, you will get shocked (see above).

If you are lucky enough to have a toaster with a 3-prong plug, then all the metal parts inside the toaster that are NOT touching the resistance wire are tied to a good electrical ground. this includes the thin wire guides that prevent the toast from touching the hot wires. in this case, if you jam a knife down the slot the chances are your knife will touch a guide wire BEFORE it makes contact with the hot wire and if the knife then touches the hot wire, the current will be shorted to ground via a better path than that provided by your body, and you will not get shocked.

If you stick a piece of aluminum in an outlet, it would be a short: the current will surge and the circuit breaker will trip or the fuse will blow. So this does not simplify the problem, but rather changes it to a different problem.

The wire used in a heating element of a toaster, unlike a piece of aluminum, has a relatively high resistance, so the voltage applied to it would be almost full AC voltage, as you've suggested. So, if a knife is stuck into a conventional toaster and its blade comes in contact with a heating element (red hot wire), it will likely be at a high voltage (not necessarily a full AC voltage, but I would not bet on it).

If the handle of the knife is not insulated from the blade, this voltage will be also applied to the person holding the knife. The effect on the person, as in other similar cases (like sticking a piece of aluminum in an appropriate hole of an AC outlet), would depend on the situation.

For instance, if the person is standing in a puddle of water or touching a grounded water pipe with the other hand, the effect could be very serious or even lethal. Otherwise, the person could experience a light shock, but I would not try it.

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