Electrostatics and electrical current Flashcards
Current
Rate of flow of charge
Q
I*t
What is current in solid metallic conductors?
The flow of negatively charged electrons
Conductors
Graphite, copper, tin and gold
Insulators
Plastic, Rubber, Glass and Wood
What do like charges do?
Repel
What do unlike charges do?
Attract
What happens when an uncharged plastic rod is rubbed with an uncharged cloth?
Electrons from the atoms of the rod transfer onto the cloth. There is now an unequal number of positive and negative charges on each. The rod has lost electrons so is positively charged and the cloth has gained electrons so is negatively charged
What happens as drops of paint emerge from a spray gun?
They become charged. As the drops all have the same charge, they repel and spread out forming a thin spray. The metal frame has a wire attached to it and an electrical supply giving the frame the opposite charge. The paint drops are therefore attracted to the surface of the frame. There is the added benefit that the paint is attracted into places such as corners, that would usually be hard to reach.
How do modern inkjet printers work?
They use electrostatic forces. Each spot of ink is given a charge so that as it falls between a pair of deflecting plates, electrostatic forces direct it to the correct position. The charges on the plates change hundreds of times a second so that each drop falls in a different position, forming pictures and words in the paper as required.
How do photocopiers work?
Positive charges are spray onto a turning drum whose surface is covered in a metal called selenium. A bright light is shone onto the sheet of paper to be copied. The white parts of the paper reflect light onto the drum; the dark or printed parts do not. In those places where light is reflected the selenium losses its charge but where no light is reflected the charge remains. A negatively charged carbon powder called toner is blown across the drum and sticks to just those parts of the drum that are charged. A sheet of paper is now pressed against the drum and picks up the pattern of carbon powder. The toner is then fixed in place by a heater.
How does an electrostatic precipitator work?
As smoke rises up the chimney, it passes through a mesh of wires that are highly charged. As they pass through the mesh, ash and dust particles become charged. Higher up the chimney, these charged particles are attracted to earthed metal plates. The cleaner smoke is then released into the atmosphere and the dust and ash collected.
Electrostatic charge can build up with friction between an object and the air, why is this dangerous?
It can result in electric shocks or sparks e.g. refuelling an aircraft. The aircraft is earthed after landing to prevent an explosion.
What happens to the charges in a wall when a statically charged balloon is brought closer to it?
As the negatively charged balloon is brought closer to the wall some of the negative electrons are repelled from the surface of the wall. The gives the surface of the wall a slight positive charge that attracts the balloon.
