Understanding Volts and Amps!!

[HappyHippyDad]

I'd like to understand a bit more about what actually gives you an electric shock. I think the more you understand something the safer you will be.

I have been taught that it is the current (amps) that gives you the shock, but this then confuses me with regard 12v equipment (lets say anything domestic - downlights for example using a transformer), as if the lights are 10W then:

Power = Voltage x Amps

10 = 12 x Amps

Amps = 10/12 = 0.83

If however the light were from the mains:

Amps = 10/230 = 0.04

So according to the maths a low voltage system uses more current and would be more dangerous as its amps that cause the shock. I know I'm missing something simple as this cant be right but I get a bit carried away with maths!

Could anyone explain this a little please?

 

[Silly Sausage]

You're right saying it's current that does the damage.

It's voltage that drives current through a given resistance, Ohm's Law.
So greater the voltage, greater the current, hence 12V is regarded as 'safe'.

In your lamp example, the 12V ones have a lower resistance than the 230V variety, hence larger current to give the same Power rating for both. P = IV.

 

[Chr!s]

I'd like to understand a bit more about what actually gives you an electric shock. I think the more you understand something the safer you will be.

I have been taught that it is the current (amps) that gives you the shock, but this then confuses me with regard 12v equipment (lets say anything domestic - downlights for example using a transformer), as if the lights are 10W then:

Power = Voltage x Amps

10 = 12 x Amps

Amps = 10/12 = 0.83

If however the light were from the mains:

Amps = 10/230 = 0.04

So according to the maths a low voltage system uses more current and would be more dangerous as its amps that cause the shock. I know I'm missing something simple as this cant be right but I get a bit carried away with maths!

Could anyone explain this a little please?

Human Body 1000 ohms

Voltage 11kv Current through body = 45.83 amp
Voltage 240v = 0.24 amps
Voltage 12v = 0.012 amps

 

[Brightspark2]

Think of it as the voltage is the size of the punch and the amps is how fast its coming at you...

 

[DanDobson]

My college teacher compared electricity to water flowing through a pipe.
The current is how much water there is and the voltage is the pressure behind it making it flow.
Just another simile type description to try and help you understand.

Dan

 

[Brightspark2]

My college teacher compared electricity to water flowing through a pipe.
The current is how much water there is and the voltage is the pressure behind it making it flow.
Just another simile type description to try and help you understand.

Dan

I was taught this also :smug:

 

[i=p/u]

adding on to this, are you saying its cheaper to run 12v/50w lamp ,than 230v/50w lamp...?????????????????:rockon2:

Human Body 1000 ohms
?????????????
Voltage 11kv Current through body = 45.83 amp
Voltage 240v = 0.24 amps
Voltage 12v = 0.012 amps

 

[HappyHippyDad]

I think I'm getting it! Its all to do with resistance!

At lower voltages your body has much greater resistance therefore less current will run through you.

Apparently at 25V the total body impedance of a hand to hand circuit for dry skin, large contact areas, 50 Hz AC currents would be roughly 3250 ohms.. so

current passing through you would be only 0.007A

Thankyou Chris and Archy for making me think of resistance! Brightspark2, wouldn't it be the other way around? Amps are the electrons (i.e the punch), volts the push, i.e how fast coming at you?

 

[Brightspark2]

adding on to this, are you saying its cheaper to run 12v/50w lamp ,than 230v/50w lamp...?????????????????:rockon2:

No, 12V would use more amperes...

Brightspark2, wouldn't it be the other way around? Amps are the electrons (i.e the punch), volts the push, i.e how fast coming at you?

Nope, volts size of punch, amps is how fast its coming at you. Remember its not a fact just a similie, dont look into it too much lol...

 

[i=p/u]

so cost more to run...................

No, 12V would use more amperes...



Nope, volts size of punch, amps is how fast its coming at you. Remember its not a fact just a similie, dont look into it too much lol...

 

[Brightspark2]

so cost more to run...................

No there both 50W...

You get charged per kWh...

 

[i=p/u]

yeah nothing more than a safety issue in using 12v downlighters..

 

[Brightspark2]

yeah nothing more than a safety issue in using 12v downlighters..

Yes and constantly changing lamp holders and transformers in them lol...

 

[Darkwood]

Low voltage lamp cost more to run but only fractional as the control gear 230/12 will have losses, this was arguably recouped by the extended lamp life of 12v as they are ramped up and down slower thus less thermal shock to the filament but all changed now that gu10 has interchangable energy saving options with controlled ramp up and down.

If a 50watts lamp is the constant here then ohms law dictates

50w / 230v =0.22amps
50w/ 12v =4.17amps

As already mentioned both lamps use 50watts so this will cost the same at both voltages with a little discrepency afforded by the transformer losses, do remember the filament resistance is changed by design to suit the voltage to give the same light output.

 

[i=p/u]

yeah clear as day now lads, men

 

[Darkwood]

It was much easier to learn in my day with good old 240v as it was devisable by 12v leaving 20 or vice versa...

50/240 = 0.21
50/12 = 4.17

so to confirm change the 12v back to 240 (240v/12v = 20)
then convert the current used at 12v to 240 (4.17amps/20 = 0.21amps)

No current demand difference when transformed to mains voltage.

 

[i=p/u]

i can do the maths its the theory of nothing changes just the filament i didnt know. cheeers big lad or ty man.

 

[Darkwood]

If i recall alot had issues getting their head around this at college me too but it was 25yrs ago ..the way he showed us was to work out the filament resistance of a 50w lamp at 12v lamp then keep the filament resistance the same and calculate what the current would be at 240v....Bang .....although the the voltages and wattages are different as we didnt have 50w diachroic lamps in our day it still made it stick in your mind.

Beer time now so i wont bother doing the sums as im sure you are quite capable :smug: