Materials
- 6 Volt lantern battery or DC power supply
- alligator clip wires (3)
- small 6V light bulb
- small light bulb holder
- pencil
- razor blade
Procedure
- Take a new pencil and shave half of the pencil off the long way so that you expose the graphite center most of the length of the pencil.
- Set up a series circuit containing the battery, the light bulb, and the pencil as shown in the diagram below.
- Move the end of the pencil along the graphite and the brightness of the light bulb will change.
Explanation
As the length of a conductor increases, the resistance increases. Since the pencil and the light bulb are connected in series. Increasing the resistance of the graphite in the pencil will increase the resistance of the whole circuit. As the resistance through the pencil increases, more voltage is used there and the potential drop across the light bulb decreases. Less voltage means less power and less brightness.
Reinforcement Activities
The above demonstration is a good activity to do before talking about rheostats, or having the students use a Wheatstone Bridge apparatus.
Students should know about the effect of length on resistance in a conductor. Have students investigate this further using mounted resistance coils. Different lengths, gauges (thickness) and types of wire are given to students for them to investigate. Students can easily design their own experiments to test the effects of these three characteristics on the resistance of a wire.
Related Products
- Mounted Resistance Coils
- Leads with Alligator Clips
- Incandescent Lamps with Minature Screw Base
- Receptacle for Small Bulbs
- Seat of Eight Resistance Coils
- Slidewire Wheatstone Bridge
- Complete Wheatstone Slidewire Bridge Set
- Laws of Resistance Board
- Covered Rheostat
- 10 Ohm Battery Rheostat
- Mounted Rheostat
