1. The problem statement, all variables and given/known data This is not entirely a homework question but more a question about a homework assignment. We were asked to find out why a filament of a bulb heats up by talking about microscopic ideas. This is a link to a website that contains the relevant information under the heading 'higher tier only' http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/electricity/resistancerev4.shtml 2. Relevant equations None 3. The attempt at a solution I just wanted to ask if the electrons collide with metal IONS or metal ATOMS. I have found other sources that have said the electrons collide with the metal atoms and surely only one is correct. I thought a metal ion is when an atom has completely had an electron removed. I thought conductors just have electrons that are free to drift rather than electrons that have been completely removed?