Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Annesci1 on 14/02/2014 13:13:02
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I was demonstrating the use of circuit toys e.g. 'Energy Balls' (http://www.amazon.co.uk/Safari-652116-Energy-Cosmic-Ball/dp/B000OU9RMS) to some primary school teachers. The ball has two metal contacts on the outside- inside is a miniature circuit with light and sound. When you complete the circuit with skin- it flashes and makes a noise. However, a teacher asked me- why is it that you can complete the circuit on the energy ball with your skin and sometimes a whole class of children holding hands and yet if you put your hand in a circuit made out of croc clips, batteries, bulb etc, the bulb won't light up?? Would be grateful for any comments. Was thinking about resistance of the croc clip circuit being high, but then surely a circuit of 20 children holding hands would also be high??
Thank you!
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In some touch-sensitive light switches a minute current passes through the person when they touch the switch, this tiny current toggles the state of the electronic switch which controls the flow of mains-electricity to the light : mains-electricity does not flow through the person touching the switch , they are not “completing the circuit” providing power to the light. The electrical current passing through the person is not the current powering the light-bulb.
Sounds like the "energy ball" contains this type of switch ...
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Ftalkingelectronics.com%2Fprojects%2F50%2520-%2520555%2520Circuits%2Fimages%2FTouch-ON-OFF.gif&hash=8e98d7cbf33c6e44d5037fe8d98bf1cd)
(https://www.bareconductive.com/file/touch-giff-gif)
https://www.bareconductive.com/555-touch-circuit
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If you use your hand as part of the circuit, some electricity will go through the bulb, but not enough to light it or even heat it up recognisably, but if you replace the bulb with a sensitive meter of some kind (I don't know which type would be best), a tiny current could then be detected and that could be used to trigger a bulb to be lit via a separate circuit.
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It's probably like RD said, a momentary current. In the AC DC war between Tesla and Edison, Edison demonstrated that DC was safer then AC by electrocuting animals with DC and AC. When the animal was hooked up to AC the animal was contentiously electrocuted But when DC was used then only a momentary jolt was felt by the animal.
Try this for example, get about 7 9volt batteries and attach them in series to get 63 volts wet your fingers and touch the 2 electrodes you'll feel a barely noticeable shock for a split moment then you'll feel nothing.
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Try this for example, get about 7 9volt batteries and attach them in series to get 63 volts wet your fingers and touch the 2 electrodes you'll feel a barely noticeable shock for a split moment then you'll feel nothing.
Momentary electric shock with DC, yes, but DC is not harmless ...
With alternating current [AC], there is a feeling of electric shock as long as contact is made. In contrast, with direct current [DC], there is only a feeling of shock when the circuit is made or broken. While the contact is maintained, there is no sensation of shock. Below 300 mA DC rms, there is no let-go phenomenon because the hand is not involuntarily clamped. There is a feeling of warmth while the current travels through the arm. Making or breaking the circuit leads to painful unpleasant shocks. Above 300 mA, letting go may be impossible. The threshold for ventricular fibrillation (https://en.wikipedia.org/wiki/Ventricular_fibrillation) for direct current shocks longer than 2 seconds is 150 mA as compared with 50 mA for 60-Hz shocks; for shocks shorter than 0.2 seconds, the threshold is the same as that for 60-HZ shocks, that is, approximately 500 mA ...
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2763825/
i.e. DC is less efficient than AC at torturing & killing people but can still do both.
Subjecting primary-school children to "7 9volt batteries" so they experience an electric shock will probably loose you your job as a teachers assistant.
[ The touch-sensitive-switches made with 555 chips (http://www.thenakedscientists.com/forum/index.php?topic=50428.msg430846#msg430846) do not deliver any electric shock sensation ]
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To make a touch switch even safer, you can make a high-frequency oscillator; the signals from this can be sensed by another electric circuit even if the electrical contacts are covered by plastic film. This completely isolates the humans from the electrical circuit. Unlike residential AC power at 50-60Hz, your muscles cannot respond to high-frequency signals > 1kHz, so this is much safer. (You just need to ensure that the oscillator does not cause interference to radio stations, or drain the batteries too quickly while it is sitting idly on the shelf.)
If you make a circuit with a traditional 1 Watt/3V incandescent torch bulb, you need a current of about 300mA to turn it on fully.
LED lamps are far more efficient, and will give a visible glow with just 10mA flowing.
However, dry human skin has a breakdown voltage of about 30V, so almost no current will flow from a 3V battery and through the light to turn it on. Even wet human skin will have trouble turning on a LED, let alone an incandescent lamp.
7x9V DC batteries = 63V. This exceeds the breakdown voltage of dry human skin, so it will cause a shock. Special warning signs and insulation are required to prevent the public coming into contact with voltages > 60V DC or 25Vrms AC (http://en.wikipedia.org/wiki/Extra-low_voltage).
Even 1x9V DC battery will make you jump if it is applied to your wet, salty tongue.
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Thanks for the explanations- I'll keep with the 'Energy Ball' and won't attempt any circuits with children and batteries! I can explain to the other teacher now what is happening.
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Annie
I know this Q&A is a year old, but I came onto the forum looking for the same explanation. After reading the helpful replies, I was still not confident of being able to explain it to someone else, so I asked Dr Simon Foster, a physicist, and this was his reply:
"When you touch the ball you don’t make a circuit which connects the battery to the light bulb (i.e. you become the wire) instead you complete the circuit on a tiny touch switch which then turns on the light. When you connect these 2 sides of the touch switch together (by lots of people holding hands) then the switch will be turned on and so the bulb lights up. Turning this switch on may only require a tiny DC current hence you won’t feel it. Essentially the switch is very sensitive hence only needs a tiny current to turn it on. If you were to try and complete the circuit in a simple circuit with just a battery and bulb, with people being the wire, then the current required to light up the bulb would be very high so you’d get a shock"
I needed to fully understand this as I am producing free online CPD courses for science teachers at primary level, (www.reachoutcpd.com) and we have filmed a group of children demonstrating a circuit with a similar gadget - I think they're great!