Current & Resistance. Electric Fish. Electric eel Torpedo ray. Dipole Potential

Transcription

1 Current & Resistance Current is the flow of charge. Such a flow is produced by a potential difference. The current depends on the potential difference and the resistance. The resistance is a property of the material and the dimensions. A current dissipates energy in a resistor. This is the basis of many common devices. Circuit rules let us simplify complex circuits. We ll look at series and parallel combinations. Electric Fish. Electric eel Torpedo ray The electric eel and the torpedo ray both use bioelectricity to stun and capture prey. The eel can produce a larger voltage, but the ray a larger current. Note the difference in body shape. Which one lives in salt water and which in fresh? Dipole Potential

2 Depolarization Field of the Heart The Electrocardiogram

3 Positive or negative? Big or small? A B C Positive or negative? Big or small? A B C Positive or negative? Big or small? A B C

4 Direction of dipole moment? 3.0 mv 4.0 mv 2.0 mv Solving Problems: Think Concepts. Sources of Potential V=.5 V Work is done to raise charges to a higher potential. 2 V=70 mv Na - K - - Exchange pump - In the cell (another source of potential) this work is done by ion pumps.

5 Potential and the motion of charges DV bat Increasing U Ion flow 2 I Decreasing U Definition of current!! E r!!!!! I I = ΔQ Δt C s = amp = A The discharge of an electric eel can transfer a charge of 2.0 mc in a time of 2.0 ms. What current, in A, does this correspond to?

6 How much is too much? Physiological AC current DC current effect (rms) (ma) (ma) Threshold of sensation 3 Paralysis of respiratory muscles 5 60 Heart fibrillation, likely fatal >00 >500 What determines the current? Our basic circuit: I R bulb DV bat E DV bulb 2 I I = ΔV R

7 Human Circuits Hand-to-hand resistance of a human with dry skin is quite variable, but we can assume a typical value of about 50 kω. If the energy ball produces a potential difference of about.5 V, what is the current through your body? Is this enough to feel? Safety is a matter of voltage, but it s also a matter of resistance. Determining resistance R = ρl A Carbon filament lamp Lewis Latimer

8 How do they fit it in there? The filament of a 00 W bulb carries a current of 0.83 A at the normal operating voltage of 20 V. a. What is the resistance of the filament? b. If the filament is made of tungsten wire of diameter mm, how long is the filament? Tungsten resistivity: at bulb's operating temperature, ρ = Ω m Resistivity as a measure of body composition Energy and Power

9 Energy and Power in Resistors Resistive Heating An electric blanket has a wire that runs through the interior. A current causes energy to be dissipated in the wire, warming the blanket. A new, low-voltage electric blanket is rated to be used at 8 V. It dissipates a power of 82 W. What is the resistance of the wire that runs through the blanket? Warming up. Suppose that a molecular ion of charge -0e is embedded within the 5.0- nm-thick cell membrane of a cell with membrane potential -70 mv. What is the electric force on the molecule?

10 Standard electric outlets in the United States run at 20 V and can carry up to 5 A. In England, the outlets are 230 V and run at up to 3 A. An electric kettle has a coiled wire inside that dissipates power when it carries a current, warming the water in the kettle. A kettle designed for use in England runs at the maximum current of 3 A; a kettle designed for use in the United States will typically run at a maximum of 3 A as well. If a kettle that is designed to work in England is redesigned to work in the United States. How must the resistance of the coil of wire that heats the water be changed? Will the US version of the kettle heat water more or less rapidly than the UK version? Warming Up: Photoresistor A battery is connected to a photoresistor. When light shines on this resistor, it increases the number of charge carriers that are free to move in the resistor, decreasing the resistivity. Now, the room lights are turned off, so less light falls on the photoresistor. How does this affect the current in the circuit? A.! The current increases. B.! The current decreases. C.! The current is not affected.

11 Warming Up: Blood Resistivity Variations in the resistivity of blood can give valuable clues to changes in the blood s viscosity and other properties. The resistivity is measured by applying a small potential difference and measuring the current. Suppose a medical device attaches electrodes into a.5-mm-diameter vein at two points 5.0 cm apart. What is the blood resistivity if a 9.0 V potential difference causes a 230 µa current through the blood in the vein? The hematocrit, the % of blood volume taken up by red blood cells, has a very large effect on blood resistivity. Cooking with Electricity Rwiener = 200 ohms Many websites describe how to add wires to clothing to wear while riding your motorcycle to keep you warm. A current from the 2 V battery of the motorcycle passes through the wires, warming them. One recipe for a vest calls for 0 m of 0.25-mm-diameter copper wire. How much power will this vest provide to warm the wearer?

12 Increasing Potential, Part I A resistor is connected to a 3.0 V battery; the power dissipated in the resistor is.0 W. The battery is now traded for a 6.0 V battery. The power dissipated by the resistor is now!.0 W!! 2.0 W! 3.0 W! 4.0 W Increasing Potential, Part II You ve brought your 000 W (at 20 V) hair dryer on vacation to Europe, where the standard outlet voltage is 230 V. Assuming that the hair dryer can operate safely at this higher voltage, and that the resistance of the hot wire doesn t change, can you actually use it if the outlet is rated at 5 A? Series circuits In Trial, a battery is connected to a single lightbulb, and the brightness noted. Now, in Trial 2, a second, identical, lightbulb is added. How does the brightness of these two bulbs compare to the brightness of the single bulb in Trial? A.! The brightness is greater. B.! The brightness is the same. C.! The brightness is less.

13 Compound Circuits Rank the bulbs in the following circuit according to their brightness. Conservation of Current Details... The wires below carry currents as noted. Which is the biggest: IA, IB or IC?

14 A power and resistance puzzle A 60 W and a00 W bulb are connected in series. Think about the current in the circuit, and through each bulb. a. Which bulb has higher resistance? b. When connected in series,which is brighter?? Warming Up: What s the voltage output of your stereo? Suppose a stereo speaker (with a resistance of 8 Ω) uses a power of 20 watts. What is the voltage across the speaker? Can the output run a lightbulb? Adding Voltages in Series An electric eel may have 6,000 electrocytes connected in series. Each can produce a voltage of 50 mv. What total voltage could the eel produce? (There are losses; observed voltages are less than this.)

15 Strongly Electric Fish 600 V head-to-tail 50 V Orientation of electric organs Torpedo ray Electric eel top-to-bottom 6000 cells in the stack 400 cells in the stack Which one lives in fresh water and which in salt? Electric Catfish The electric catfish, found in rivers in Africa, produces a voltage pulse with stacks of electrocytes. As the fish grows in length, the magnitude of the voltage pulse the fish produces will grow as well. The best explanation is that, as the fish grows: A. The voltage produced by each electrocyte increases. B. More electrocytes are added to each stack. C. More stacks of electrocytes are added in parallel to the existing stacks. D. The thickness of the electrocytes increases. Kirchhoff s Laws

16 Which Is Brighter? a. Which bulb is brightest? b. Which bulb is dimmest? Another Complex Circuit What is the current in the resistor? Reducing Complex Circuits, Part I

17 Reducing Complex Circuits, Part II A Complex Circuit Puzzle When we blow on the warm filament, it cools. This decreases the resistance. How does this affect the brightness of the bulb?! A.!It gets brighter! B.!It gets dimmer Which is Brighter, Part II a. Which bulb is brightest? b. Which bulb is dimmest? Suppose a wire is connected between points and 2. Does the brightness of each bulb:! A.!Increase! B.!Decrease

18 Which is Brighter, Part II a. Which bulb is brightest? b. Which bulb is dimmest? Suppose a wire is connected between points and 2. Does the brightness of each bulb:! A.!Increase! B.!Decrease A power and resistance puzzle A 25 W and a 60 W bulb are connected in series. Think about the current in the circuit, and through each bulb. a. Which bulb has higher resistance? b. When connected in series,which is brighter?? What s the Resistance? There is a current of.0 A in the circuit below. What is the resistance of the unknown circuit element?

19 What s the Current? What is the current supplied by the battery in the following circuit?