Science -- Your Future, Scotland's Future
SCI-FUN Roadshow Exhibits -- Human Battery
SCI-FUN Roadshow Exhibits -- Human Battery
In this exhibit you use your own hands and metal plates to learn how batteries work.

A battery consists of two different metals, separated by a liquid solution. The metals are called electrodes, and the liquid solution is called an electrolyte.

Your hands are coated in a thin layer of sweat, which is mostly water but also contains salt. When you make a human battery, the sweat acts as the electrolyte, with the metal plates acting as electrodes.

The effectiveness of the human battery depends on the kinds of metals used, and on the amount of electrolyte present. That means, if you sweat more, you'll make more electricity.

What's happening is that electrons are moving between the electrolyte and the electrodes. Electrons move from the liquid into one of the metals, they move through the circuit to the other metal, then go back into the liquid.

The direction the electrons move in depends on what the metals are. If electrons always move from metal A to metal B, we say that metal A is less electronegative than metal B. The types of metal also affect the amount of electricity they produce: if there is a big difference between the electronegativity of the two metals, they will produce more electricity.

After a while, the electrolyte in a battery runs out. We can recharge the battery by forcing a current through in the opposite direction. This send all the electrons back the other way, and reverses all the chemical changes that happened to the electrodes.

All batteries work this way, including mobile phone batteries and car batteries. The car battery is needed to give that little spark to get the petrol burning. Once the petrol is burning, the engine is running which recharges the battery for the next time, and powers the electrics in the car. That's why using your car headlamps while driving won't use up the battery, but leaving them on overnight will.

1 A battery is made of two different _______ separated by a liquid ________.
2 In the human battery, what acts as the electrolyte?
3 What is moving around in an electric circuit?
4 How do we recharge batteries?
5 If electrons move from metal A to metal B, which metal is more electronegative?
6 You can't just replace a car battery when it's flat (it's expensive and dangerous). Do you know what you might do?

1 You can make batteries out of various fruits and vegetables in the same way. Cut a nick into a lemon, then tuck a penny or 2p into the nick. On the opposite side of the lemon, stick in a nail. The nail and coin will form your electrodes. Now use a multimeter to measure the current and voltage between the two electrodes. The juice of the fruit acts in the same way as your sweat in the human battery. Try out different kinds of fruit and veg: other citrus fruits work well, as do apples and potatoes.
2 Compare how well different batteries work. Get a number of different brand new batteries. Put a battery into a torch, switch it on and time how long it takes for the light to go out (probably several hours). Try out all of your different batteries in the same torch. Compare the time the battery lasted to the price of the battery, which ones are the best value for money?