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The exposed filament from a 100-watt incandescent light bulb is wired in series with a flashlight bulb and a 9-volt battery. Blow on the filament and the flashlight bulb gets brighter.
Connect the light sockets and battery:
Prepare and connect the bulbs:
Blow gently on the exposed filament and notice any change in the brightness of the flashlight bulb.
Before you read the explanation below, try to figure out why blowing on the filament affects the brightness of the flashlight bulb.
Unscrew the flashlight bulb until it goes off, and let the exposed filament cool off for several seconds. Then screw the flashlight bulb in again and observe its brightness very closely for the first second or two after it lights up.
When you blow on the exposed filament, you actually cool it off because the air current carries away a fair amount of heat energy. As the temperature of the filament decreases, its electrical resistance decreases as well. This is because the atoms making up the filament vibrate less at lower temperatures, making collisions between the atoms and the electrons moving through the filament less likely. With fewer collisions, the electrons move more freely through the filament—in other words, they encounter less resistance.
Lowering the resistance of the exposed filament lowers the resistance of the complete circuit, allowing the flow of current in the circuit to increase. Since the flashlight bulb is part of the complete circuit, current through it also increases, making it glow more brightly.
When you screw in the flashlight bulb, its tiny filament heats and glows almost instantaneously, but it takes the large exposed filament a second or two to reach maximum temperature. For the short amount of time that the large exposed filament is relatively cool and has low resistance, the flashlight bulb glows very brightly; but once the exposed filament heats up and its resistance increases, the current in the complete circuit is reduced and the flashlight bulb dims.
When you turn on an incandescent lamp, the filament starts out at room temperature. While the filament is relatively cold it has a low resistance; it draws a large pulse of electric current at first, then settles down to a lower constant current. The initial burst of current can be ten times greater than the constant current. That's why incandescent light bulbs tend to burn out when they're first turned on: The initial large rush of current causes stress in the filament.
Heating and Cooling
Use other means to change the temperature of the exposed filament (a match flame, a butane lighter, ice cubes, a hair dryer, etc.), and observe any change in the brightness of the flashlight bulb. Can you explain the changes that occurred? Caution: the exposed filament can get uncomfortably hot, and is very fragile.
Illegal Incandescents
Traditional incandescent light bulbs were notably inefficient, with more than 90% of their energy going to heat and less than 10% to light. The Energy Independence and Security Act of 2007 called for the phase-out of traditional incandescent light bulbs over a period of time. Production of traditional 100-watt incandescents stopped in 2012; 75-watt production stopped in 2013 and 60-watt and 40-watt production in 2014. In all cases, it was legal for merchants to sell and consumers to buy remaining inventories. Alternatives to traditional incandescent bulbs include CFL (compact fluorescent), halogen and LED (light emitting diode) bulbs.
An exception to the ban on incandescent bulbs is the rough service bulb. It differs from a traditional incandescent bulb by having extra support for the filament, so that vibration and rough usage is less likely to cause the filament to break. It is primarily intended for industrial use. Its energy consumption and lighting properties are generally comparable to the old bulbs, but it is notably more expensive.
We first saw this activity done by the Galileo Group of Japanese science teachers, and we would like to acknowledge their contribution.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Attribution: Exploratorium Teacher Institute