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Lecture #25 What’s in the wall?? Last Time We discussed how charge can be studied. There are two kinds of charge – NEGATIVE electrons, light, mobile and easy to push around – POSITIVE protons, located in the nucleus of atome. “heavy”, tough and not to be messed with. Charges interact with each other. Results of our Study Charges can be created by mechanical rubbing or separation of bound surfaces. Charge cannot be created or destroyed – It is there before we rub – All we mortals can do is move it around Like charges repel each other Un-Like charges attract each other. So… insulator what about this balloon trick? insulator A Van de Graaff generator What about our Volunteer?? This thing also has the capacity to store charge + + + + + + + - This is called a capacitor Charging a Capacitor takes WORK The work per unit charge is called the Potential Difference + + + + + + + - + + + + + + + - Capacitor separated charge POTENTIAL DIFFERENCE Units are VOLTS It takes one Joule of energy (work) to move one coulomb through a potential difference of one volt. We usually refer to potential differences as “the voltage” Another Definition If a capacitor has a potential difference V from one terminal (plate) to the other And if the capacitor has a charge Q on the plate, Then Q C V where C is measured in Farads. Example A 0.001 F capacitor has 5 volts across it. How much charge is on the capacitor? Q C V so Q CV 0.001F 5volts 0.005Coul. Big Cap Demo What’s Happening? + + + + + + + - Huh? The electrons were “pushed” by the difference in potential (voltage). They came around to the (+) plate and neutralized the positive plate. The two plates and the wires are said to form an electrical circuit. The movement of charge is called electrical current. Symbols & Definitions “Stand” at one point in a circuit where electrical current is flowing. Count the amount of charge in coulombs that pass where you are standing in one second. This amount of charge, divided by the time of passage is defined as the CURRENT. The current is measured in AMPERES and we use the symbol I. One ampere is a current of one coulomb per second. The Battery The battery behaves in a way like a capacitor. As charge is withdrawn from one side of the battery the battery replaces it via a chemical reaction in the battery cell. The battery therefore can maintain a constant potential difference or voltage between its terminals. Electricity made a giant leap in 1800 when Volta invented the electric cell and the battery (of cells). For the first time investigators had a source of steady electric current. Alessandro Volta, (1745-1827) Pretty Complicated Lots of sources on the internet to find out how these things really work. The Commercial Battery Something that we ALL Know! The Circuit – A Thunker I1 A I2 Current must be the same! V A I Define Resistance R V R (Unit Ohm ) I V IR Ohm Sweet Ohm The more resistance, the less the current. Like stepping on a garden hose. If you stepped in two spots on a garden hose, the flow would even be less. The “resistance” would double. So let’s look at a few bulbs. Example 6 volts 10 ohm V IR V 6 I 0.6 A R 10 Add a Bulb B 6V C 10 10 Total = 20 I=V/R=6/20=0.3 amperes … ½ the previous current Parallel Circuit