As is the case with the "Lincoln Cent Project", electricity is another good example of science being part of our everyday lives. Look around you. Your television, your clock radio, the computer you are using and many other electrical appliances are all utilizing electrical power.
To explain things as briefly as possible, electricity is a flow of electrons. Substances that allow electrons to flow freely are called conductors and those that don't are called insulators.
This is all well and good but there are 2 drawbacks to this circuit 1) the light always stays on and 2) the power is constantly being used. How can we turn the light bulb 'off'? Well, we could unscrew the bulb from the socket but in the real world this is very inconvenient. (Light bulbs are inside fixtures, on ceilings and so on). Perhaps we could disconnect the power at the source. This too is very inconvenient. You would have to go down to your basement to shut the power off. Or - much more dangerous - you would have to disconnect the electrical supply wire before it reaches the light socket.
Is there a safe way to interrupt the electron flow without physically touching the wire? Sure. It is called a SWITCH !!!
The inside of a typical household wall switch has a strip of metal (B), making contact with point 'A', completing the circuit and thereby conducting electricity to the light. This would obviously be the 'ON' position. When the insulated lever is moved down to the 'OFF' position, it pushes the metal strip away from point 'A', breaking the circuit and turning the light 'OFF'.
This type of switch (having a lever which "flips" it on and off) is called a toggle switch.
Finally, let's talk about that dotted line in Diagram 1. Now what would happen if point A and point B were to touch OR if they were connected with a wire or other conductor? Well, the light bulb would turn 'off', the wires and the battery would get very warm very fast and the electrons would simply travel from the battery to point A to point B and then back to the battery. Notice that in this new circuit , the electrons are traveling a path (or circuit) that is shorter than the original one. Hence you have just learned what a "short circuit" is and how its name is derived! Short circuits are dangerous. They cause wires to heat, circuit breakers to 'trip' and can even start fires.
There are many different types of switches: toggle, rotary, pushbutton, "rocker", "pull-chain", slide, magnetic, mercury, timer, voice-activated, "touch-sensitive", and many others. Heck, even the Clapper? is another type of switch !
The "knife switch" (rarely seen nowadays) is the type that most easily demonstrates the functioning of a switch. Old sci-fi movies ("Frankenstein (1931)" or "Young Frankenstein (1974)", for example), made extensive use of these switches in the laboratory scenes.
As it is, this circuit alone could be your science project. A variation could be substituting a push-button switch and putting a 'buzzer' or 'doorbell' where the light is. Now you have a good demonstration of how a doorbell is wired. Pushbutton switches are usually "momentary on". That is to say the connection is made only when you press the button. There are "momentary off" pushbutton switches, but using one in a doorbell circuit would mean the bell would be constantly on until someone pressed the button. Impractical don't you think? (The comedian Tim Conway joked that his father wired a doorbell in just this way. When there was silence someone would say "Hey somebody's at the door").
A practical use of the momentary off switch is the "mute button" on your telephone. If a momentary on switch were used, it would be very annoying to press the button constantly as you talked and released it only for muting. This shows how each type of switch has its specific applications.
The next type of switch (no diagram) is the Double Pole Single Throw (DPST). These switches are used when there are 2 'live' lines to switch but can only turn on or off (single throw). These switches are not used much and are usually found in 240 volt applications.
If you are using the SPDT knife switch, you have a "center off" position, which an ordinary wall switch would NOT have in which case you will need to add an SPST switch for shutting this circuit off. (In electronics work, many SPDT switches have a middle position in which the electricity is turned off to BOTH circuits. It is an SPDT center off switch. Also, some electronic SPDT switches have a "center on" position. The best example of this type of switch is the "pickup" selector on an electric guitar which can choose the rhythm, treble or both pickups for 3 varieties of sounds).
Don't you think this switching arrangement would make a great science project?
A simple way to think of this switch is imagining 2 SPDT switches side by side with the 'handles' attached to each other. Perhaps the most popular use for this switch is 'phase or polarity reversal'. So, how does the DPDT switch accomplish this? First, you have to wire the 2 'top' and 2 'bottom' terminals in a 'criss-cross' fashion. The top 2 terminals become the input and the middle two terminals become the ouput. Now, referring to the bottom left diagram, the switch is in the 'up' position, W & Y are connected, as are X & Z. The polarity is maintained because the input and output are directly connected. No problem seeing that right?
Another important (though not very common) use is to put this switch between 3-way switches so that the same light may be switched from many different locations. Referring to Diagram 4, if A & B and E & F were connected, the bulb would be off. But now think of the wires going from A to D and C to F. If their connections were reversed, ( A to F, C to D), the light bulb would turn on again. So, how would we be able to reverse the polarity of these 2 wires? By using the polarity reversing switch ! (See Diagram 5 below).
Also,you don't have to limit yourself to using just one 4-way switch. If you were to attach a second 4-way switch from the Y and Z terminals of the first switch to the W2 and X2 terminals of the second switch, you could have the same light switched from a 4th location. (See Diagram 6).
Good luck with the project !!!
Diagram 4 (below) depicts what is probably the most common use for the SPDT switch - the 3 way light switching circuit. Electricians incorrectly call the SPDT switch a "3 way switch". The proper terminology should be "three terminal switch". However the term 3-way switch has stuck and it's a misnomer we'll just have to live with.
联系客服