Random motion of particles when speed is altered (7)

I believe the answer is:

pedesis

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'random motion of particles' is the definition.
I can't judge whether this defines the answer.

'speed is altered' is the wordplay.
'altered' indicates an anagram.
'speed'+'is'='speedis'
'speedis' anagrammed gives 'PEDESIS'.

'when' is the link.

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I've seen this clue in The Guardian.

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