- Slides: 30
L 25 Electricity & Magnetism  • static electricity – the van de Graff generator – electrostatic shielding • lightning • batteries and frogs legs
review – electric charge • Matter (stuff) has two basic properties • mass gravitational force • charge electric and magnetic forces – positive charge – negative charge • electric forces • like charges repel +/+ or - / • unlike charges attract + / - • charge is measured in Coulombs [C]
Where is the charge? • the charge is in atoms – positive protons – negative electrons • matter is usually electrically neutral it has the same amount of positive and negative charge • electrons can be transferred from one object to another by rubbing (friction)
Conductors and Non- Conductors • Metals (copper, aluminum, iron) are conductors of electricity that means that charge can move through them • Plastics, wood, ceramics, and glass are non-conductors (or insulators) they do not let electricity flow through them • You should not stick a metal fork into an electrical outlet! • You could stick a plastic fork into an outlet without electrocuting yourself, but don’t do it!
What makes conductors conduct? • Atoms have equal numbers of positive and negative charges, so that a chunk of stuff usually has no net charge the plusses and minuses cancel each other. • However, when you put a bunch of metal atoms (like copper) together an amazing thing happens one electron from each atom forgets which atom it belongs to. • All the homeless electrons are free to wander about inside the material
Current– charges moving about If I connect a battery to the ends of the copper bar the electrons in the copper will be pulled toward the positive side of the battery and will flow around. this is called current – flow of charge copper An electric circuit! Duracell +
Seeing and hearing electricity! The capacitor keeps charging until it reaches its limit. Charge storage device Capacitor Many Batteries
Fully loaded and ready to go! • The sudden discharging of the capacitor is accompanied with a big spark and a bang manmade lightning! • danger fully charged A spark occurs when there is enough energy released to cause the electrons in the air molecules to be ripped out of the molecules ionization
Non-conductors can be charged too! Even though non-conductors do not have free electrons meandering about, they can be charged by friction l When you move your comb through your hair, the friction (rubbing) between the comb and hair can pull some of the electrons out of your hair and onto the comb l as a result your comb ends up with a net negative charge and attracts your hair which is now positive. l
Charging by friction If you rub plastic with cat’s fur, electrons are rubbed onto the plastic making it negative l if you rub glass with silk, electrons are rubbed off the glass making it positive l the charge can be transferred to other objects. l
Attracting uncharged objects + + uncharged metal sphere • A negatively charged rod will push the electrons to the far side leaving the near side positive. • The force is attractive because the positive charges are closer to the rod than the negative charges
You can bend water with charge! The water molecule has a positive end a negative end. charged rod When a negative rod is brought near the stream of water, all the positive ends of the water molecules turn to the right and are attracted to the negative rod. stream of water
The Magic Wand 2 x 4 We can cause the 2 x 4 to move with electric forces
The charging process • an object is charged positive (has a net positive charge ) if electrons are removed from it • an object is charged negative (has a net negative charge) if electrons are transferred to it • charges can be transferred from conductors or non-conductors but they can only move through conductors.
Example • 10 Coulombs of negative charge are transferred from object A to object B. What is the net charge on each object? • ANSWER: – object A has a net charge of +10 C – object B has a net charge of -10 C. +10 C A -10 C B
One Coulomb is a HUGE charge • To get a charge of one Coulomb on an object we would have to remove 6, 250, 000, 000 electrons from it! • In the capacitor discharge demo, only 0. 01 C of charge were involved.
• Seeing the effects of charge: the electroscope is a simple device for observing the presence of electric charge • it consists of a small piece of metal foil (gold if possible) suspended from a rod with a metal ball at its top • If a negatively charged rod is placed near the ball, the electrons move away because of the repulsion. The two sides of the metal foil then separate.
Making Sparks: The Van de Graff Generator • The van de Graff generator is a device for building up a large electrical charge on a metal sphere. • The charge is generated by friction between a rubber belt and a roller. • the charge on the belt is transferred to the sphere by a brush.
Electric Potential voltage 9 Volt battery • The amount of charge on a charged sphere can be measured in terms of its electric potential or voltage • the more charge that is on the sphere, the higher its voltage • electric potential is measured in VOLTS • if I connect a 9 V battery to the sphere and the ground, it will have a potential of 9 V
Danger High Voltage ! • The van de Graff can charge the sphere to 50, 000 volts! • This is enough to cause discharges to the surrounding air • The sparks excite air molecules which give off light
Electrostatic shielding • The effect of the high voltage on the van de Graff generator stops on the outside of the metal cage Homer is SAFE! • Being inside your car during a lightning storm offers you some protection • radio signals cannot penetrate through a metal enclosure • the metal bars (rebar) that reinforce the concrete in walls can interfere also
Lightning- outdoor spark n n n causes 80 million dollars in damage each year in the US On average, kills 85 people a year in the US over in a thousandth of a second carries up to 200, 000 A causes the thunder!
development of a lightning bolt charge separation stepped leader & streamer leader meets streamer lightning bolt
applications of electrostatics • Xerox copiers use electrostatic attraction to put the ink droplets on the paper • electrostatic precipitators use the attraction of charged dust to remove dust particles from smoke. • can be used to hold balloons on your head
Removing soot particles Positive cylinder Chimney stack soot Charging units spray electrons on the soot particles
Frog's leg Batteries • in 18 th century Luigi Galvani a professor of anatomy at the University of Bologna found that a freshly dissected frog leg hung on a copper hook twitched when touched by an iron scalpel. • The two metals had to be different. • Galvani thought that he had discovered the secret life force
Alessandro Volta • Professor of Physics at the University of Pavia realized that the electricity was not in the frog’s leg but the twitching was the result of touching it with two different metals • Volta had discovered the first battery. • Lemon battery
Batteries use chemical energy to produce electricity • two dissimilar metals immersed in a conducting fluid (like an acid for example) cause a chemical reaction which can produce electric current. zinc electrode copper electrode acid
Inside a Duracell 1. 5 Volt battery Metal Cap plastic case + Carbon center electrode Electrolyte paste Zinc outer electrode - Bottom electrode