today's video covers static electricity which is the build-up of charge on insulating materials and can cause sparks when it's discharged now all materials contain charge because they contain positive protons and negative electrons normally though these charges cancel each other out which means that most materials are neutral overall if materials are rubbed together though like a bag and a hat then the friction between the two objects causes electrons to be rubbed off one of them and onto the other one in conducting materials like metal where the electrons are able to move the electrons just flow straight back again and so no charge ever builds up in insulating materials though like a polythene rod and a cloth the electrons can't flow back and so the transfer caused by the friction leaves a positive static charge on the material that lost the electrons in this case the cloth and a negative static charge on the one i gained them the polythene rod which way the electrons get transferred each time depends on the specific materials involved for example if we took a similar cloth and used it to rub an acetate rod instead then the electrons would get rubbed off the acetate rod leaving it positive and onto the cloth making it negative in any case the key point to remember is that it's only the negative electrons that are transferred the positive static charges occur because an object has lost negative charge and not because it's gained positive protons the next thing we need to look at is what happens if an object keeps on gaining negative electrons as more and more electrons are transferred and the size of the charge increases a potential difference will develop between the charged material and the earth or any earth object like us because remember earth objects are always at zero volts if the potential difference between the charged object and the earth object is large enough then electrons can jump across the gap and that jumping of electrons is what a spark is one thing we need to point out is that charges can sometimes build up on conductors too it's just much less common for example cars can build up charge because their metal frame is in contact with the wind which can transfer electrons you've probably experienced this yourself when you've touched a car and got an electric shock this is just a small spark from the car discharging to the earth through your body anyway that's everything for this video so if you enjoyed it then do give us a like and subscribe and we'll see you next time