Electric Current- Hewitt Lecture

Transcription

1 Energy/Charge= Voltage Joules/Coulomb Electrical Pressure Current ~ Voltage Difference Electric Current- Hewitt Lecture Analogy: Water in a pipe with a piston at each end. 5 lbs of pressure on one end. 5 lbs of pressure on the other end. Will the water flow? No But wait there is pressure so why wonʼt it flow? The water needs a pressure difference. 5 lbs of pressure on one end. 3 lbs of pressure on the other end. Will the water flow? Yes Why? Because it has a pressure difference. Basic idea: What causes water to flow? Not just a pressure, it needs a pressure difference! Demo: Show 12V battery Connect both ends of a light to just one end. It doesnʼt light up. Connect both ends of a light to the other end. It doesnʼt light up. Connect ends of a light across the two terminals. It lights up. Why?

2 As long as you have a conductor it will light up. It is not the pressure, it is the difference of pressure. Stories Guy who worked on his car but put one hand behind his back. To prevent current from going through his heart. If he touched parts of the car that had a voltage difference that would push current through his heart. Electricians never grab wires with their hands. Why? Remember the biology experiment with the frog? The give it electrical shock and the frog muscle contracts, even on a dead frog. Like us closing our hands. Everyone do that- open and close your hand, do it again and again. What is going on? An electrical signal is coming down your arm causing this to happen. Electricians always touch wires with the back of their hands to prevent from grabbing electrical wires. Bird puts one leg down, and doesnʼt get fried Then it puts another leg down, it doesnʼt get fried, Itʼs at 10,000 V!! Why doesnʼt it get fried? There is no voltage difference so no current is flowing. Third rail and the boston MTA. Cherry Picker and Tahoe Storm Guy had to pee and died because that made the connection between. If you step between the main voltage rail and the ground rail and are wearing rubber shoes you are fine. Why? Resistance. Great electrical insulator can increase the resistance between you and the tracks!

3 So what was a proportionality can become an exact equation when I add the idea of resistance. Current ~ Voltage Difference So we can modify the equation now... Current = Voltage Difference/Resistance So, the amount of current flow through a conductor depends on two things 1.The voltage difference 2. The resistance This usually has to do with the geometry and the material that the conductor is made from. Current flows more easily through a big piece of copper than a narrow piece of copper. This is like water in a pipe. More water flows through a big pipe than a skinning pipe. Demo: Hold up the light you used earlier. The copper wire on either end is relatively thick and allows electrons to flow easily. Inside the light is a very narrow filament. Guess where most of the resistance is in this system? The filament You can think of resistance like friction. What happens when you rub your hands together? They get hot. What happens to the light? It gets hot! In fact, so hot that it begins to glow and give off light. Genius of Tommy Edison was to find something that offered a lot of resistance. This caused it to get super hot. What was his challenge? To prevent it from burning up! Found a material that had high resistance, but didnʼt burn up, so it glowed bright.

4 Tungsten without oxygen. Demo this using the battery. We measure in the following units: Current (Amperes) Voltage Difference (Volts) Resistance (Ohms) I = V/R The old song and dance with this equation. If you know any two of these you can find a third. If I tell you this light has a resistance of 1 ohm and you put it across a battery. What is the current? 12 Amps This is called Ohmʼs Law DC- direct current AC- alternating current Use of a washing machine analogy with water sloshing back and forth in two pipes that are connected to the wall. Pistons are inside the wall doing this. Compare this to what the power plant does. How many electrons come from the power plant? None They simply cause the electrons, that you supply, to slosh back and forth through the wire and transfer energy to your appliances. The power plant supplies energy from one place to another by simply shaking the electrons. The electric field causes this.

5 The electrons do this at 60 Hz. Electrons are sloshing back and forth at 60 times per second. Demo: Use the battery Have light at the bottom Move the light up and ask again. Move the light up and ask again. I could take the voltage across campus and take the light and ask And the light would light up, over there. What is a wire? Two leads coming from a power source that is located somewhere else! You have a switch to turn it on and turn it off. Show by disconnecting and reconnecting the alligator clip. What if I connect lamps in series? What will happen? Now connect the lamps in series. They are less bright. What is the voltage across these two lights? 12V What is the voltage across one of these two lights? 6V Figure out the current across one bulb. 6V and 1A give us how much current? 6A

6 Follow the amount of current everywhere. 6 A, 6 A, 6 A, etc even through the battery. How about up at the top of the wire? zero- no current. Since we have the twice the resistance, whatʼs the current? Half as much as before- 6 A instead of 12 A Ohmʼs Law says it all. Add a third bulb in series Brighter or dimmer? Dimmer Voltage across whole system Resistance is 3 x so current is 1/3 4 A Voltage is 4 V across Now add them parallel. What will happen when I add the same bulb in parallel? 12 A Another bulb 12 A Now I have more current flowing- the battery will wear down quickly. Think of cars going down a street.

7 12 A 24 A 36 A Who is getting tired? The battery! Question As you add more resistors does the battery see an overall resistance that is more or less, and why? Think about that.