Can a supermarket shopping trolley really build up a static charge? I can see the insulated wheels having chance, but would have thought the very large surface area of metal would discharge this to the atmophere quickly enough so that a shock from the trolley would be very unlikely. I would have thought any shock from a trolley would have been a discharge of static built up on the person.
Yes, a trolley can charge up. Air is a very good insulator and does not allow charge on the trolley to escape easily unless quite high voltages (thousands of volts) are reached. The main paths that charge can leak away are through the tyres and floor, both of which can often be highly insulating, or through the person touching the trolley, through their shoes and the floor.
There are at least 3 ways in which a person could get a shock when they touch a trolley - either the person is charged, or the trolley is charged, or both are charged.
You may be interested to read my on-line articles
Static shocks and how to avoid them
Why static builds up on people