Electricity can be compared to water. In a battery, you have a certain amount of energy stored up. To use our water analogy, a battery is like a big water tank. Batteries come in different voltages, 3V, 6V, 9V. etc. This voltage is a measurement of how much potential energy is stored in the battery. A battery with a high voltage is like a water tank that's really tall, so the water is ready to come pouring out with a lot of force. A battery with a low voltage is like a long, low water tank - the water will come out slowly and will take a long time.

Resistance is a measure of how easy it is for the electricity to leave the battery - like the size of a pipe leaving the water tank. If it's a small pipe, it will be harder for the water to come out and the tank will empty more slowly - a large resistance! If it's a large pipe, then it will be easier for the water to leave and the tank will empty in a hurry - a small resistance!

Current is the rate at which electricity leaves the battery, i.e. how fast the water tank empties.

Voltage (V), resistance (R), and current (I) are all related by the equation V=IR. Consider our water example. A big pipe, (low resistance) means the water will leave fast (high current). Electricity and water don't just leap out of a battery or a water tank - they need a path to follow. Water uses pipes, and electricity uses wires. If you block a pipe or break a wire then the water or electricity cannot flow. This is called an open circuit. The opposite of an open circuit is a closed circuit. Closed circuits allow electricity to flow through them. A short circuit occurs when you attach a wire between the two terminals of the battery. What happens? The wire gets hot, and the battery goes dead. It's like removing the wall from your water tank - all the water leaves at once and you get a huge current.

• 1 piece of corrugated cardboard (about 5 cm x 5 cm)
• 2 elastic bands
• 1 metal paperclip
• 1 9 volt battery
• 1 9 volt battery connector
• 2 pieces of phone wire (30 cm)
• masking tape
• 1 9 volt electric buzzer (3V - 12V)
• wire strippers or scissors

All electronics parts are available at most Radio Shack locations and will cost only a few dollars.

1. Phone wire on the outside looks like a piece of wire with an outer coating of white plastic. When split open and pulled apart, it is revealed that the 'phone wire' is four copper wires, each coated in a different colour of plastic. Cut the phone wire (the white bundle) so that you have a 30 cm piece. Open the phone cord and remove the four 30 cm wires. Strip the wire; that is, remove 2-3 cm of plastic at one end and 5-6 cm at the other end on each of the four wires.
2. Arrange the longer exposed lengths of each wire parallel to one another, and close to one edge of the cardboard. Hold the wires in place by placing a piece of tape at the very ends of the exposed wire and another piece of tape closer to the opposite end of the cardboard.
3. Twist elastic bands parallel to the wires, around the cardboard, so that they are tight (about 3 times). One elastic needs to be on each side of the wires. This cardboard, wire, tape, and elastic portion is the beginnings of the switch.
4. To attach wires together, conducting material must be touching. Wires can be connected by twisting them together or by securing them in place on the desk with tape. Attach the black wire of the battery adapter to the black wire of the buzzer.
5. This leaves a red wire on the battery adapter and a red wire on the buzzer. If these were connected and the battery were connected, it would ring indefinitely. We need to add our switch to prevent this. The red wire from the battery adapter must be attached to one wire from the switch (the shorter exposed end), and the red wire from the buzzer must be attached to the other wire from the switch (the longer exposed end).
6. It appears we almost have a complete circuit. However, we do not. If you place one finger on one exposed wire on the cardboard and follow the connections that this wire makes, you will end up with your finger on the other exposed wire. Thus we do not have a complete circuit. Brainstorm ways to complete the circuit.
7. Bend the paperclip until it forms an angle bigger than 90 degrees.
8. Insert the paperclip under the elastics and over the wires so that the elastics are holding the paperclip onto the wires, thus completing the circuit.

   

9. Now what is missing? The battery. Once the battery is attached to the battery adaptor, the buzzer will sound. Use the paper clip to act as a switch. When the paper clip is touching the wires, the alarm will sound. When the paper clip isn't touching the wires, you have an open circuit so the alarm doesn't sound.
10. Install the alarm system in your doorway so that when the door is closed it pushes on the exposed tip of the paperclip and holds it above the wires. (Circuit open, no buzzing.) When the door opens the paperclip is released, it falls onto the wires, closes the circuit, and the buzzer sounds.