Static Electricity

Why do objects repel? Why do two objects attract? Why do they do nothing if you touch them first? Why does the repulsion or attraction only work if you rub them? What does rubbing them do? Why does rubbing them with different materials give different results? Which combinations of materials produce the best results?

Question One: A cloth is used to rub a thin polythene strip. This makes the polythene negatively charged. The same cloth is held near to the strip. Which two of these statements are correct? A. the strip lost positive electrons when it was rubbed B. the strip gained negative electrons when rubbed C. the cloth repels the strip D. the cloth gains electrons when it rubs the strip E. the cloth has become positively charged

Question Two: The diagram shows two metal balls, P and Q, hanging from the nylon threads. P is repelled by and Q is attracted to a negatively charged rod. Which two statements are true? A. P is negatively charged B. P is positively charged C. P is uncharged D. Q is negatively charged E. Q is positively charged

Question Three: A plastic ruler is rubbed on a duster. The ruler becomes negatively charged. Which two statements are true? A. the duster has also become negatively charged B. the duster had gained electrons C. the duster has lost electrons D. the ruler has gained electrons E. the ruler has lost electrons

There are only a few basic laws of Static Electricity: Like Charges Repel (--) or (++) Unlike Charges attract (- +) Charged and uncharged objects attract A neutral object has equal numbers of positive and negative charges An object becomes charged when there is NEGATIVE charge transferred ie some of the electrons move

How are objects charged? By Friction Electrons (-) only are able to travel

The Triboelectric series Paper Aluminium Silk Wool Perspex Glass Acetate Rabbit Fur Human Hands Cotton Steel Wood Amber Hard Rubber Polyester Cling film Polyethylene PVC Silicon Teflon Neutral These materials lose electrons These materials gain electrons

Why is this? Note electrons are on the outside of the atoms. Easier to get at electrons which move are known as ‘free electrons’ The positively charged nuclei don’t ‘flow’

So……. An excess of free electrons means an object is negatively (-ve) charged A lack of free electrons means an object is positively (+ve) charged An electric current is a flow of -ve charged free electrons ALTHOUGH conventional current is treated as a flow of +ve charge (!)

Van Der Graaf Generator How does it work? Rubber belt rubs against plastic roller Charge separation is caused Electrons move onto the belt Negative charge or electrons accumulate on the metal dome as the rubber belt touches the metal cover Negative charges pass onto the person touching the dome

─ ─ ─ ─ ─ ─ ─ ─ Rubber belt Roller ─ + ─ + ─ + ─ +

So what use is electrostatics? 1.Electrostatic precipitators

2.Photocopiers Light causes image to have +ve charge -ve charged toner sticks to -ve charged image The image is transferred to paper The final image is fixed by heating

3.Electrostatic paint sprayers Advantages?? Less waste as -ve particles of paint attracted to car More even coat as each particle repels its neighbour

How do we measure static charge? The GOLD LEAF ELECTROSCOPE

One last note : The Earth The Earth acts as a huge source of charge or a huge sink for charge Excess -ve charge will flow to the earth from a -ve charged body or up from the earth to neutralise a +ve charged object ie The Earth acts as a SINK and SOURCE of free electrons

Franklin: Flew a kite in a thunderstorm to ‘collect’ lightning (not recommended most who tried to repeat this died) Invented Bi focals, Catheters, Watertight bulkheads, Lightning Rods and odometers