L 25 Electricity & Magnetism [2] static electricity the charging process the van de Graff generator electrostatic shielding lightning batteries and frogs legs electric circuits

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] all charge is a multiple of the basic unit of charge – we call this e = 1.6 x10-19 C charges cannot be divided into smaller units than this.

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

Pure water is non-conducting clean water will not conduct electricity if salt or acid is added, however, it will conduct electricity H2O carbon electrodes

A salt water solution is a conductor When salt NaCl (sodium chloride) is added to water H2O, the NaCl molecule dissociates into a positive ion Na+, and a negative ion Cl- . Thus the solutions contains both positive and negative ions, both of which can conduct electricity. Electric current can pass through dirty bath water and through you also! we are conductors – water + Na+ + Cl–

Gas discharges PLASMA High Voltage Source When a high voltage is applied to a gas-filled tube, the gas can become ionized, one or more electrons are separated from each atom. Since positive and negative charges are present the ionized gas conducts electricity. The gas atoms are excited and emit light of a color characteristic of the gas. PLASMA Gas in tube not blood! High Voltage Source

examples of electrical discharges fluorescent lamp the Aurora neon lights

Charging by friction If you rub plastic with cat’s fur, electrons are rubbed onto the plastic making it negative if you rub glass or plastic with silk, electrons are rubbed off the glass making it positive the charge can be transferred to other objects. only the electrons can be transferred

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.

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 (not protons) can be transferred from one object to another by rubbing (friction)

Non-conductors can be charged too! Even though non-conductors do not have free electrons meandering about, they can be charged by friction 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 as a result your comb ends up with a net negative charge and attracts your hair which is now positive.

Example Object A has a charge of -5 C and Object B has a charge of +5 C If -10 Coulombs of negative charge are transferred from object A to object B. What is the final charge on each object? ANSWER: object A has a net charge of +5 C object B has a net charge of -5 C. -10 C -5 C +5 C A B

One Coulomb is a HUGE charge To get a charge of one Coulomb on an object we would have to remove 6.250 x 1018 electrons from it! In the capacitor discharge demo, only 0.01 C of charge were involved.

Attracting uncharged objects + 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 uncharged metal sphere

You can bend water with charge! charged rod The water molecule has a positive end and a negative end. When a negative rod is brought near the stream of water, all the positive ends of the water mole- cules turn to the right and are attracted to the negative rod. stream of water

We can cause the 2 x 4 to move The Magic Wand 2 x 4 We can cause the 2 x 4 to move with electric forces

Seeing the effects of charge: the electroscope 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.

Danger High Voltage ! The van de Graff can charge the sphere to more than 50,000 volts! This is enough to cause discharges to the surrounding air  ionization or breakdown The sparks excite air molecules which give off light

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 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 9 Volt battery

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 Charging units spray electrons on the soot particles soot

Electrostatic shielding

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 also interfere

Lightning- outdoor spark causes 80 million dollars in damage each year in the US On average, kills 85 people a year in the US is all 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 leader & streamer leader meets streamer lightning bolt

Frog's leg Batteries in 18th 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. acid zinc electrode copper electrode

Inside a Duracell 1.5 Volt battery Metal Cap plastic case DURACELL + Zinc outer electrode Carbon center electrode - Bottom electrode Electrolyte paste

Current– flow of electric charge 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 and around.  this is called current – flow of charge copper An electric circuit! Duracell +

Electric current (symbol I) Electric current is the flow of electric charge q (Coulombs) It is the amount of charge q that passes a given point in a wire in a time t, I = q ÷ t Current is measured in amperes 1 ampere (A) = 1 C / 1 s q

Potential difference or Voltage (symbol V) Voltage is what causes charge to move in a conductor It plays a role similar to pressure in a pipe; to get water to flow there must be a pressure difference between the ends, this pressure difference is produced by a pump A battery is like a pump for charge, it provides the energy for pushing the charges around a circuit

Voltage and current are not the same thing You can have voltage, but without a path (connection) there is no current. An electrical outlet voltage