Try this: Shocking capacity

  Nail through lid
 

Insert the nail through the middle of the container lid
   
  Brush
 

Bend the thin wires in half to form the brush.
   
  Thick wire
 

Bend the thick wire into a zig-zag shape.
   
  Aluminium foil
 

Wrap aluminium foil around the container.

 
  Charging the capacitor.
 

Now charge your capacitor!

Warning: This activity shows how to make a small capacitor. Bigger capacitors can hold large amounts of charge and produce a dangerous shock. Take care to follow the instructions.

You will need

  • Film canister

  • Large nail

  • Aluminium foil

  • Salt water

  • 3 x 15cm lengths of fine copper wire

  • 10cm length of thick copper wire

  • Sticky tape

  • Scissors

  • Wire cutters

  • Blu-tac

  • Cotton material

  • PVC pipe

What to do

  1. Take the 3 lengths of fine copper wire and bundle them together so their ends line up. Fold the bundle perfectly in half and twist the bend several times to hold it in place. This forms the ‘brush’.

  2. Pierce the plastic lid of the container with the nail so it pokes down into the container, leaving the head of the nail poking out of the lid by about 1cm. (Note: If you can’t find a film canister, you can use any small, non-metal container with a plastic lid)

  3. Cut a rectangle of aluminium foil to neatly cover the outside of the container. Tape it into place.

  4. Fill the container with salty water.

  5. Bend the brush to form an ‘L’ shape. Hang this on the edge of the container so the twisted end is in the saltwater and the brush end is pointing over the rim, out and away from the container.

  6. Close the container, leaving the brush poking out.

  7. Bend the thick wire so it can be taped to the side of the container while pointing at the nail. Tape the wire onto the side of the container so that one end points at the head of the nail with a tiny gap of about 0.5 mm between them, and the other end is touching the aluminium foil.

To charge your capacitor:

  1. Use blu-tac to secure the container to a surface. This will help stop it moving too much if you bump it.

  2. Holding the cotton material in one hand, rub the PVC pipe back and forth quickly with the other hand, making sure it runs over the brush. Continue this for about 30 seconds.

  3. Watch the gap between the thick copper wire and the nail.

  4. If you don’t see a spark, bring the end of the thick copper wire closer to the nail.

What’s happening?

This activity works best on cool, dry days. For a great explanation of static charges, see this Science by Email activity.
You’ve just made a Leyden jar. Leyden jars are a type of capacitor, operating much like a small battery. While batteries produce electricity through electrochemical reactions, capacitors store an electrical charge that you put into it.
In this case, the negative electrical charge is being pulled from the cloth by the PVC pipe and transferred onto the copper wire brush.
Salt is made up of two charged parts – a positive ion (always a metal, such as sodium or copper) and a negative ion (such as chloride, or sometimes a compound like sulphate). Water splits it up, making a soup of positive and negative ions called electrolyte. The ions carry the charge between the wire brush and the nail.
As the charge builds up on the wire brush, we can say that there is a difference in ‘voltage potential’ between it and the thick copper wire/aluminium foil. This produces an electric field in the gap between the thick copper wire and the nail. At a certain point, the difference is enough to cause create a current of electricity, which you see as a spark.

Applications

Capacitors are in nearly every electrical device you can think of. They are usually made of two conductive plates separated by either a vacuum or a material that is less conductive than the plates.
As a charge builds up on one plate it is stored until it reaches the right voltage and then jumps across the gap. This is especially useful when the voltage from a power supply fluctuates. It can also be used for a range of other things from tuning radio frequencies to filtering different currents.
A super capacitor is used in CSIRO’s UltraBattery, which will be used in hybrid cars and elsewhere. The capacitor allows high power discharge of electrical energy (and rapid storage when required), while the battery provides a storage capacity with a long, low-cost life.
The amount of charge a capacitor will hold is measured in ‘farads’. Electrical items such as microwaves, computer monitors and television sets will often have large capacitors that hold dangerously large amounts of charge, even when it is switched off. As tempting as it is to open a broken piece of electrical equipment to look inside, it’s always best left to the professionals.