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

Ions in a flame

Demonstration

A candle flame is electrically conductive. An EHT will visibly separate its free charges, causing a current to flow in an external circuit.

Apparatus and materials

Light source, compact

Metal plates with insulating handles, 2

Power supply, EHT, 0-5 kV (with internal safety resistor)

Candle

Screen

Variable voltage supply, 0-12 V, 8 A

Health & Safety and Technical notes


See guidance note on Managing radioactive materials in schools

A school EHT supply is limited to a maximum current of 5 mA, which is regarded as safe. For use with a spark counter, the 50 MΩ safety resistor can be left in circuit so reducing the maximum shock current to less than 0.1 mA. 
Although the school EHT supply is safe, shocks can make the demonstrator jump. It is therefore wise to see that there are no bare high voltage conductors; use female 4 mm connectors where required. 

Read our standard health & safety guidance

A Van de Graaff generator or a Wimshurst machine can be used instead of the EHT power supply. 

A 'flexcam' and display screen could be used instead of the light source and screen. 

Procedure


Metal plates and power supply Photograph courtesy of Mike Vetterlein

Fix the plates in vertical planes parallel to each other and five to ten centimetres apart by means of their handles held in retort stands.

b Light the candle and place it so that its flame is lit a little below the plates.  

c Set up the small bright light source so that a shadow of the plates falls on the screen beyond. Both the light source and screen should be at least a metre from the candle, and the source preferably more. 
 
d Apply a high potential from the EHT power supply to the plates. The flame divides into two parts, one towards the positive plate and one to the negative. This can be seen clearly on the screen.  

Apparatus set-up2 Photograph courtesy of Mike Vetterlein

Teaching notes


1 The flame divides into two. This shows that there are two streams of particles moving in opposite directions. One stream is luminous, probably with carbon particles, and the other is not so luminous. 

2 The two streams will not look equal. One is a stream of negatively-charged particles, moving to the positive plate. The other is of positively-charged particles, moving to the negative plate. 
 
This experiment was safety-tested in August 2007

Related guidance


Electric charge and current - a short history

Related experiments


Experiments with a Van de Graaff generator