A Crash Course in Basic Circuitry: How Flashlights Work

This post is all about the flashlight and uses it as an introduction to circuit terminology and some common conventions. For full comprehension, I recommend reading the whole article, but in the spirit of keeping it brief, here is how a flashlight works:

Cut to the Chase

When you put a battery or two into the end of your flashlight, you are completing a circuit that contains a switch, the lightbulb, and the batteries themselves. The energy in the batteries goes almost exclusively to the heating of the tungsten filament (in incandescent light bulbs) or mercury vapor (in fluorescent light bulbs); the purpose of the wire connecting all of the elements is to use up as little of the energy in the battery (or batteries) as possible. The switch and the battery are quite common to circuit drawings, called 'schematics', and have their own exclusive symbols (battery at left, switch at right). A light bulb falls under the umbrella term of 'resistor', which is represented by the squiggle in the middle, but since a light bulb is a very common form of resistor it also has its own symbol (the loop enclosed in a circle in the center).

Sources: http://www.quia.com/jg/2234746list.html ; http://lcamtuf.coredump.cx/electronics/

Sources: http://www.quia.com/jg/2234746list.html ; http://lcamtuf.coredump.cx/electronics/

When the flash light is off, the switch has not connected the two wire ends, which means the circuit is 'open' (the symbol above depicts a switch in the open position, as it's disconnecting the lines). When the switch is flipped, it connects the two ends of the wire, completing the circuit and lighting the lightbulb!

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Digging Deeper

It doesn't do much good to know what's going on with inventions on the outside if we don't understand what's happening on the inside. Believe it or not, we can explain the functionality of a flashlight by analyzing one simple 'loop', or circuit. There are three basic electronic pieces (also called 'elements') that make up a flashlight circuit: the light bulb, the battery (or batteries), and the switch. 

These are all very common items to find in many different electric circuits. Because they are so common, a group of simple symbols was established to represent them so that circuit drawings (often called 'schematics') could be done more quickly and ideas communicated much faster.

Sources: http://www.quia.com/jg/2234746list.html ; http://lcamtuf.coredump.cx/electronics/

Sources: http://www.quia.com/jg/2234746list.html ; http://lcamtuf.coredump.cx/electronics/

A combination of these three circuit elements connected by lines representing electrical wires gives us the basic circuit schematic for a flashlight:

Source: http://bingo.cdyn.com/techno/readschem/flashlight.html

Some En-light-ening Elaboration: 

The Battery

At left, we have the symbol for a battery. Batteries might appear in any given circuit in more than one place, but in basic circuitry there will always be at least one. The longer line denotes the positively charged end of the battery, therefore the shorter line represents the negative end. Batteries are one of the most common ways that energy will flow through a circuit. In other words, if you have a flashlight, phone, laptop, car, etc. that doesn't have a battery inside it, it won't turn on. This is because energy that powers circuits is stored within batteries. The energy itself comes from a combination of chemical compounds or ions; when the reactions that occur within the battery reach equilibrium, the battery becomes 'dead' and must be switched out. (Check out this link for more information on different battery types!) Batteries can be thought of like a pump; they take electrons whose energy has been depleted from going through a circuit and replenish their energy to send them through again, just like a water pump shoots the same water repeatedly through a fountain.

If batteries are the pump in our analogy, then current is like water. The energy in the electrons that constitute the current is transferred to the light bulb when the electrons flow through the metal filament in the middle, leaving them in a low energy state before the battery 'pumps' them up again and they repeat the cycle. This can be thought of like a pump raising water above a water wheel, which then falls on the paddles to spin the wheel. Since the water has lost its height, it can't turn the water wheel, and must be again raised by the pump to repeat the cycle.

A convention among engineers mandates that current flows out of the positive end (called a 'terminal') of the battery, through all of the elements of a circuit, and back into the negative terminal; this is how the direction of the current in the diagram below was selected.

This represents a flashlight that is turned on, seen by the switch connecting both ends of the orange wire.

The Resistor

In the middle, we have the symbol for a resistor. The purpose of a resistor is to dissipate the energy flowing through the circuit in a useful way. The light bulb is an example of a resistor; it uses energy from the electrons to heat a tungsten filament (or mercury vapor, in fluorescent bulbs) which then releases the energy in heat  and light. The light bulb is a very common resistor and is very useful, which is why it has its own symbol. Other examples of resistors include stove-top elements, the glowing elements in a toaster, and hair curling irons. As a general rule: if an appliance gets warm when you plug it in, a resistor is at work dissipating energy as heat.

The Switch

On the right, we have the symbol for a switch. The pictured symbol is more specifically an open switch; a closed switch would simply connect the two dots in a straight line. The purpose of the switch is to control when the circuit is on ('live' or 'closed loop') or off ('dead' or 'open loop'). Current cannot flow through wires unless there is a continuous path, called a 'closed loop'. In the case of a flashlight: if the switch is open the circuit is called an 'open loop'; nothing will happen until the switch closes and completes the circuit. This is why your flashlight only turns on when you flip the switch a certain direction. When you flip it back, you're disconnecting the wires, creating an open loop, and turning off the circuit.

Another way to turn flashlights on and off is to rotate the part of the flashlight that contains the light bulb. This in itself is a switch! As you turn it to the left, the threads in the flashlight act like loosening a screw and the light bulb stops touching the battery, which creates an open loop and turns your flashlight off. Conversely, when you turn it clockwise, you are decreasing the distance between the light bulb and the battery--eventually they will touch and form a closed-loop circuit, which turns the flashlight on.
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Thank you for reading! If you have any questions or comments about what I've covered here (or any ideas as to what I should cover next), feel free to post a comment below! Alternatively, send me an email at nghagler@ucdavis.edu and include 'Blog' in the subject line to let me know what you think. Thanks!