Placeholder Image

Subtitles section Play video

  • This is two amp fuse wire

  • it's copper, tinned copper wire which used to be used in old fashioned fuses

  • and George has this long bit of fuse wire, which is hung between two electrodes.

  • You probably can't see so we put some paper on this wire

  • and you see the line across there and then it comes along here

  • Well this is a wire which is jittering a bit because ... I don't know why ..

  • you do just leave me ...

  • Allright it's hanging there in a shape which doesn't seem to be particularly special.

  • Now I am going to turn the current on

  • This is volts going up and this is the current in amperes.

  • So it goes all the way up and this goes up to 1.8 amps roughly, 1.75

  • So this is below the point where the fuse wire will melt.

  • So now, I am going to do the experiment. I am going to turn the current on

  • and here it goes up to 1.8 amps

  • and now I am going to turn it off.

  • on

  • off

  • It's reversible: it goes up, it goes down.

  • on

  • off

  • on

  • off

  • When the current's on, it goes through the fuse wire

  • and because of the resistance of the fuse wire

  • it heats up - that's what a fuse wire is for

  • it's meant to heat up and melt when you get up to about 200°C

  • this gets up somewhere near there.

  • so we are not melting this

  • We are just heating it up quite a bit

  • and as we heat it up it expands

  • and you normally can't see the expansion of a metal when you heat it up

  • but here you can

  • So this sagging in the middle is due to the fact that the wire has got

  • a little bit longer. Less than 1% increase in length will cause it to sag by that amount

  • So this is a measure of the current going through

  • That's the first thing, current leads to heat, changes the temperature

  • leads to expansion, leads to sagging.

  • Now I've got another magic box

  • This is a magic box with an N on the top because that's probably something to do with north

  • and something else on the bottom - south

  • I am not meant to take this out. George tells me that if I take it out

  • it will then suck itself into anything and cause great dammage

  • oh it's a magnet, allright, here it is

  • another one of these neodymium magnets

  • it's very, very strong and I am very clumsy with these things

  • you put near a piece of metal it will disappear into it.

  • So now I've got a magnet

  • if I put this horizontally nothing much happens

  • but if I put it this way

  • as I move the magnet towards the wire

  • you can see it's repelled quite strongly

  • If I turn it rount the other way

  • so now the north pole is at the bottom

  • it should pull in the other way, can you see that Brady?

  • yeah

  • it comes and hits the magnet

  • What's going on?

  • And if I put it horizontally not much happens, well

  • there is a magnetic field going this way

  • and when a current goes down the wire this way

  • that leads to a force on the wire

  • in a ... not in this direction, not in that direction but in the third direction, perpendicular to it.

  • And if I turn the magnetic field round I change the direction of the force

  • so this is the force between a moving charge and a magnetic field

  • which acts in a funny, funny way. Which you normally can't see

  • but here it's very graphic cause I got such a strong magnetic field you can actually see it beeing repelled

  • If I were to be a magician in the 19th century (8, 20th century?)

  • and I wanted to go on stage and fool people

  • and I set this up it would look as though I had magic powers, pushing it away

  • oh, float my beauty

  • If I now turn the current off

  • you have a wire without any current

  • and you have no effect at all, it is proportional to the current

  • this is sagging a little bit

  • it would move if it could but it doesn't want to because there's no current.

  • Pretty cool. Yeah I love this because I mean

  • thermal expansion used to be one experiment and you could never measure it

  • and this thing, the force on a moving current, you could never see it, it's so weak

  • and now I can see both, cause the magnet is so big

  • So this is, this is, George again to the rescue, this is wonderful, thank you George.

  • We're talking about the rotation of the earth or other objects and the effects of small forces that can disturb that

This is two amp fuse wire

Subtitles and vocabulary

Operation of videos Adjust the video here to display the subtitles

B1 wire current fuse magnet magnetic field george

Current and Magnets - Sixty Symbols

  • 0 0
    林宜悉 posted on 2020/03/30
Video vocabulary