Why was a screw hot after removal from a wall?

Question

I was mounting a tv and midway I removed the bolt so I could grab the mount. When I removed the mount the tip of the bolt it was hot. I’m wondering if this is because of friction or if I’ve hit a wire. I got a little freaked out so I haven’t touched it since. Is it okay to keep going?

Answer 1

It's friction. If you hit a power wire, you'll know immediately as a circuit breaker will pop and there will be a very loud sound. Removing or installing a long screw in wood with a screwgun can result in it getting very hot.

Answer 2

If you shorted a circuit, check your circuit breaker to see if any of the breakers have been unexpectedly tripped.

It's certainly possible you hit a wire, but if you hit a live wire, you would've likely heard a pop/buzz noise, and the tip of your bolt would likely be blackened. I hit one just a week ago. =( The gold-colored screw I was using had the tip blackened from the sudden sparks of electricity that jumped across it.

TVs tend to be mounted higher up on walls. While wires sometimes do go higher up walls, they more often congregate on the lower half of walls, wandering between any height from slightly above lightswitch height (4 ft high) to outlet heights (1 foot high). Probabilistically, you're less likely to hit a wire if you're in the upper 3ft of the wall than if you're in the lower 5ft.

On the other hand, screwing/unscrewing bolts and screws heats them up quite substantially. It's like rubbing two sticks together to start a fire: friction generates heat. A lot of heat.

The number of times I've unscrewed a screw and suddenly had it burning hot is... well, in the hundreds if not thousands of screws. And I'm not even a contractor, just someone who works on my own stuff.

Answer 3

Think about all of the energy it took to un-screw it - probably enough to warm you up by a degree or so - do quite a few and you will start to sweat. Now compare the mass of your body with the mass of the screw - a difference of maybe a thousand to one?

Now, bear in mind that the energy that you are putting in to turning the screw has to go somewhere - in this case it will be largely equal to the friction holding back the screw.

Wood is a pretty good insulator, but all the same, lets assume that half of the friction acts on the wood, and half on the tiny mass of the screw...

Answer 4

If you hit a powerline, you either short it (connecting "hot" to "neutral" or "ground") so it will make a big bang and breaker will trip, or you just make the screw (electrically) "hot" - meaning it is connected to the powerplant. Or you are plainly lucky and hit the ground wire or neutral only. Only hitting the ground wire might get unnoticed by GFCI breaker, which trips whenever the net current through main "hot" and "neutral" is not zero (That means, some current is "leaking out"), or the threshold current is exceeded (That means, there is a short-circuit).

Such accident will cause only GFCI trip the circuit and you might get struck, because you will be a part of an electrical circuit for a moment.

It won't heat the screw, unless the whole circuit is screwed in your house.

On the other hand, if you screw something in or out and the threads are not machined to have loose space (like UNC screws and nuts), there will be a significant friction force applied to the whole thread engaged.

In other words all the force (actually, it is the work) you apply to screw/unscrew is transformed into heat. As wood is generally poor heat conductor all the heat is "used" to warm up the bolt and near surounding.

The bolt, on the other hand, has much better conductivity so even it is getting hot inside, the whole bolt gets hotter.

The faster you screw the shorter time you leave for the bolt to cool down and the hotter it will be. The harder you screw, the more heat is produced and the hotter the bolt gets.