Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Bicycle dynamo

Demonstration

Showing how the brightness of a lamp increases when a dynamo powering it turns faster.

Apparatus and materials

Bicycle 'dynamo' assembly

Demonstration meter with dial (2.5–0–2.5 mA DC)

Lamp, 1.5 V in lamp holder

Health & Safety and Technical notes


Read our standard health & safety guidance

The 'dynamo' assembly consists of a simple bicycle generator, mounted and geared so that it can be driven both at speed and slowly see the illustrations. For convenience, a lampholder may be fitted between the terminals. (Strictly this is not a 'dynamo' because that term is reserved for a d.c. generator.)

 

Procedure


Apparatus

a Connect the output from the generator to the demonstration meter. 

b Turn the handle with low speed gearing so that the deflection is clearly visible. Note the effect of increasing the speed of rotation. Note how, at higher speeds, the pointer merely vibrates over a very small range about the zero position, since the output is a.c. 
 
c Turn the dynamo at high speed using the other gears (with the meter disconnected). The output will be sufficient to light a 1.5 V lamp connected across it. 


Teaching notes


Turn dynamo

1 A bicycle dynamo is one of the simplest of generators and is easily available. It also has the advantage that the armature/coil is stationary and the field moves relative to it, in accordance with standard practice in heavy engineering. The field is normally produced by an 8-pole circular magnet rotating between two coils, generating alternating voltages. 

2 Turning the dynamo more quickly will increase the e.m.f. 
 
3 The use of a demonstration meter is suggested so that it is clearly visible to the whole class. Afterwards, you may want to leave the dynamo at the side of the class for students to use for themselves. If you do this, confine this experiment merely to lighting the lamp (a sensitive meter may be damaged if the dynamo is turned very fast). 
 
This experiment was safety-checked in April 2006
 

Related experiments


Bicycle dynamo and oscilloscope