Fly zapper!

[Gareth56]

I've bought one of these electric fly zappers in the shape of a small tennis racquet, it runs off 2 AA batteries and virtually vapourizes small flies so what's the current of one of these things? I understand the voltage produced from the 2x1.5V batteries is 1500V.

 

G56

[ecoli]

not really without knowing the resistance, internally of the wires and components and also of the fly itself.

[Gareth56]

Presumably these items cannot deliver anything which would be considered dangerous or even lethal. I always thought it was the current that did the damage not the voltage. So presumably it doesn't take much current to fry a fly?

[ecoli]

I shouldn't expect so, no.

 

btw - I assume the bug zapper is on the order of 1mA

[Gareth56]

What's lethal to a human then?

[ecoli]

depends where you get shocked. A small current to the brain or heart can kill you. A larger current through the hand, and you might just lose the hand.

So for an average value, probably from 100-500 mA of DC (don't try this at home).

[Cap'n Refsmmat]

You can stop a heart with only a few milliamps, but you have to deliver the current directly to it. (A few milliamps to the skin won't work because of the resistance of your skin.)

[ecoli]

plus the nice saline solution on your skin means the current would rather flow on top of your skin than through it, for only a couple of mAmps.

[Janus]

What's lethal to a human then?

 

Between 100-300ma.

 

100ma being the lower limit of what could kill you and anything over 300ma definitely being lethal.

[swansont]

I just got one of these, and got my first confirmed kill the other day. It charges with the same kind of circuit that charges up a flash on a camera, known as a flyback or blocking oscillator.

 

http://en.wikipedia.org/wiki/Blocking_Oscillator

[Gareth56]

Gosh, i didn't know it took so little current to cause such damage. My 13amp electric kettle is pretty lethal then

[ecoli]

i think those are pretty safe though, I don't think current runs through the heating coil (or the water would be electrified too).

 

Or does it? Anyone want to clarify?

[Janus]

i think those are pretty safe though, I don't think current runs through the heating coil (or the water would be electrified too).

 

Or does it? Anyone want to clarify?

 

It does, but it is insulated. Basically, you have a nichrome wire coil that is encased in a ceramic casing which is in turn protected by an outer metal shell,

[Gareth56]

So if there's nothing connected to the 240V output socket what is the current that is "waiting" to get used by some appliance? Is it 13amps, which I suppose is quite a lot of electrons available for use?

[swansont]

So if there's nothing connected to the 240V output socket what is the current that is "waiting" to get used by some appliance? Is it 13amps, which I suppose is quite a lot of electrons available for use?

 

Your device will draw whatever it needs, based on that voltage and its resistance (I = V/R), but you most likely have a fuse or circuit breaker that limits the draw in case you effectively "short" the two connections with a low-resistance conductor that would have you draw too much current.

[Gareth56]

So why is a toaster cord shorter and thicker than a lamp cord?

[Janus]

So why is a toaster cord shorter and thicker than a lamp cord?

 

A lamp typically uses 100w, so at 120v, it draws 833 ma

A toaster can use 900w and will draw 7.5 A.

 

The thinner the cord, the greater the resistance per foot.

using the formula P=I²R. you can see how much wattage is used by the cord itself. Thus, the same cord as used by a lamp would use 81 times as many watts if used on a toaster.

 

So not only are you wasting power via the cord, but this wattage is lost by heating the cord, and the thinner the cord, the hotter it must become to shed the heat. A thin cord trying too carry to much current runs the risk of overheating and causing a fire. This is also why you need to be sure that when you use an extension cord that it is rated high enough to run whatever you plug into it. If the cord becomes overly warm to the touch while in use, it is too thin.

 

On an additional note, this is why you never replace a circuit breaker or fuse with a higher rated one than is already there. And you never, never, never replace a household fuse with a penny. Those fuses/circuit breakers are there to protect your household wiring from overheating and causing a fire hazard.

Edited July 5, 2008 by Janus

[Gareth56]

Many thanks

 

Wouldn't a thicker cord have a greater resistance per foot as there is more copper to resist the flow of the electrons?

 

I have an extension cord on a reel and on it it says that the maximium load while the reel is fully wound up is 720W and when fully unwound 2400W, why the differnece in wattage just becaause the cable is would up?

 

I've always wondered as to why the USA has a different voltage to the UK & Europe and what would happen if I came over for a holiday and plugged in my 1200W hairdryer as it would be drawing twice the current (10A vs 5A) would I blow a fuse or blow the hairdryer up

Edited July 5, 2008 by Gareth56

[iNow]

Wouldn't a thicker cord have a greater resistance per foot as there is more copper to resist the flow of the electrons?

 

No. Think about a water hose, like the type you would use to water your flowers or plants in a garden.

 

You want to transfer 50 gallons of water to your plants. What is going to offer the least resistance:

 

A really long drinking straw with a 0.7cm diameter, or a hose with a diameter of 4cm? The bigger hose has less resistance to flow, and will allow more water to be transferred to your garden more quickly.

 

While the analogy is not one to one (between the water hose and the wire), it's close enough to answer your question and get the idea across.

[Janus]

Many thanks

 

Wouldn't a thicker cord have a greater resistance per foot as there is more copper to resist the flow of the electrons?

 

No, there is more copper to conduct electrons. Think of it in terms of a multi-lane freeway vs. a single lane road. The wider freeway can carry more traffic easier.

 

I have an extension cord on a reel and on it it says that the maximium load while the reel is fully wound up is 720W and when fully unwound 2400W, why the differnece in wattage just becaause the cable is would up?

 

Wound up, the extension cord acts as an inductor. The electric field around the cord cuts through adjacent coils of the cord. Since we are dealing with AC, the polarity of these fields are always changing. Changing electric fields induce a current in conductors. This current will be in opposition to the regular current. This in effect causes the wound up cord to act as if it has a higher resistance, which reduces its loading ability (to be correct, the opposition to current due to the coil is called "reactance" and the combination of the reactance and normal resistance is called impedance.

 

I've always wondered as to why the USA has a different voltage to the UK & Europe and what would happen if I came over for a holiday and plugged in my 1200W hairdryer as it would be drawing twice the current (10A vs 5A) would I blow a fuse or blow the hairdryer up

 

The current drawn by the appliance depends on its resistance, which is fixed and the voltage applied. Thus if you take a US appliance to the UK, it will try to draw more current. For instance a 1200 watt US hair dryer will normally draw about 11 A, meaning it has a resistance of 10 ohms. Hook this up to a European 240 V outlet, and it will try to draw 24 A, You will most likely blow a fuse, and damage your dryer.

 

On the other hand, a 1200 W European dryer normally draws 5 A and has a resistance of 48 ohms. Plug this into an US outlet and it will draw only 2.3 A, and only use 252 W. It would perform weakly or not at all.

 

Luckily, it is not easy to accidentally plug a US appliance into a UK outlet or vice versa, as they also use different styles of plugs and outlets.

[Gareth56]

So why does the US (and some other countries) have a different voltage from the UK & Europe. Is it easier to produce 120V or cheaper? Or is it just down to safety, sticking your fingers into a 120V socket is less dangerous than sticking them into a 240V one?

[Janus]

So why does the US (and some other countries) have a different voltage from the UK & Europe. Is it easier to produce 120V or cheaper? Or is it just down to safety, sticking your fingers into a 120V socket is less dangerous than sticking them into a 240V one?

 

It does come down to a matter of economics, just not in the production of the electricity.

 

It works like this, the lower the voltage, the higher the current needed to run a comparable appliance (see my last post). The thinner the wire, the higher the resistance. I^2R determines how much wattage is used up by the wire.

 

Thus at a higher voltage, you can use a thinner wire to deliver the same power.

 

Now for the economics:

 

When electricity use began to come into general usage, copper and cotton were inexpensive in the US, So it was cheaper to use cotton in the insulator, which was a good insulator up to 120v, and use a thicker copper wire.

 

In the UK, copper was expensive, so it made more economic sense to use a higher voltage which allowed them to use thinner wires, The trade off was that they needed to use a better insulator (rubber for instance), but it was over-all still cheaper than using thicker wire.

[swansont]

And the bulk of power transmission is done at higher voltages — there are local transformers that step it down to mains voltage.