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PHYSICS 51 lecture 25.1

PHYSICS 51 lecture 25.1. Resistance and Resistivity. Resistance is the property of a material to “ slow down ” current Resistivity is a material property that measures how well a material conducts electricity  = E/J (definition) Unit (Ohms * m) Resistance

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PHYSICS 51 lecture 25.1

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  1. PHYSICS 51 lecture 25.1

  2. Resistance and Resistivity • Resistance is the property of a material to “slow down” current • Resistivity is a material property that measures how well a material conducts electricity •  = E/J (definition) • Unit (Ohms * m) • Resistance • R = L/A (L= length, A = area)

  3. Values of Resistivity • Conductors • Gold 2.4 x 10-8 • Copper 1.7 x 10-8 • Semiconductor • Silicon (undoped) 2.3 x 10+2 • Insulator • Glass 1.0 x 10+12

  4. Example 1: Cu wire • Resistance of Cu wire • 1.0m length of 1mm diameter wire has how much resistance

  5. Example 2: Mounted integrated circuits • Resistance of Au trace on a PC Board • How thick a gold deposition to get a 10.0mm trace that is 0.5mm wide below 1.0 Ohm • What is the approximate cost of the Au for the trace?

  6. High-performance BH chip • Buried Heterostructure (BH) chip design allows 30 mW fiber-coupled power • Other properties: • Low power dissipation • Low RIN • High SMSR • L-band version also in production • Fully qualifed according to Telcordia GR-468

  7. Current: CHARGES DRIFTING IN A CONDUCTOR Current Movement of charge I = dQ/dt Unit: Amp (C/s) Assumes (+) charge Not a vector! Current density J = Current/area

  8. ANALOGY OF ELECTRON MOTIONIN A CONDUCTOR 12 Volts 0 Volts

  9. ELECTRON MOTION IN A CONDUCTOR WITH AND WITHOUT AN ELECTRIC FIELD

  10. Resistors • Current – voltage relations • Linear • a resistor obeys Ohm’s Law • Constant slope = 1/R = I/ DV = or V = I R • Non- Linear • b) A vacuum tube diode • c) A semiconductor diode

  11. p-n Junction: diodes, light emitting diodes (LED) and laser diode Reverse Bias - + Forward Bias + -

  12. Ohm’s Law We can write Ohm’s Law two ways • Microscopic • E = J • Macroscopic • V = IR

  13. The simplest circuit I can think of V V = 9.0V R = 2.0W i R

  14. Source of EMF: A battery • EMF is the electro motive force (not a force) • It an energy per unit charge, a voltage • A battery is one source of EMF, it provides a a voltage difference across its poles • We will learn about other sources later • Any real battery has an internal resistance r which reduces the effective voltage • V = E – Ir • Has to be there, if not, shorted battery gives infinite current!

  15. Example1: battery and ohms law A 12.0V battery, with and internal resistance of 2.0 ohms is connected to an external resistor of 4.0 ohms. • What is the current • What is the voltage drop across each resistor • What is the terminal voltage of the battery

  16. Electric potential (V) rises and drops in a circuit

  17. Example2: battery and ohms law A 12.0V battery, with and internal resistance of 2.0 ohms is connected to an external resistor of 1000.0 ohms. • What is the current • What is the voltage drop across each resistor • What is the terminal voltage of the battery

  18. A few Conclusions • When the external load is very large compared to the internal resistance of the battery, you can ignore the battery resistance • When the external load is large compatred to the resistance of the wires that from the circuit you can ignore the resistance of the wires • When the external load is small compared to the internal resistance of the battery, you loose all your power inside the Battery!

  19. Plan for the day • Feedback Results • HW # 6 • Simple Electric Ideas • Resistance and Resistivity • Current • Ohm’s Law • EMF and batteries • Power in circuits and resistors • All about light bulbs • Quiz

  20. Homework #6 • Read Ch. 25: Sections 1-5 (skip sec 6) • Do Exercises: 25.3, 25.18, 25.28, 25.39, 25.51 • Due: Tuesday, March 11

  21. Approach to simple circuits • The voltage around a complete loop of a circuit is 0 • Battery is a voltage rise in direction it wants to push current • V = IR • Resistors are a voltage drop in the direction of current flow

  22. Equivalent Resistance Series connection Same current in each element Same Voltage in each element Parallel connection

  23. Source of EMF: A battery • EMF is the electro motive force (not a force) • It an energy per unit charge • A battery is one source of EMF, it provides a a voltage difference across its poles • Any real battery has an internal resistance r which reduces the effective voltage • V = E – Ir • Has to be there, if not shorted battery gives infinite current!

  24. Electrical Power Phy 50: Power = (energy or work)/time Unit = Watt (Nm/s or J/s) • P = IV (C/s x J/C) = (J/s) = Watt • Because V = IR , we can rewrite power • P = I2R or P = V2/R

  25. Electrical Power (2) Battery delivery power to a circuit • P = IV Resistors dissipate power from a circuit • P = I2R

  26. Electric potential (V) rises and drops in a circuit

  27. Clicker #1 Consider two identical resistors wired in series (one behind the other). If there is an electric current through the combination, the current in the second resistor is 1. equal to 2. half 3. smaller than, but not necessarily half the current through the first resistor.

  28. Clicker #2 As more identical resistors Rare added to the parallel circuit shown here, the total resistance between points Pand Q 1. increases. 2. remains the same. 3. decreases.

  29. Clicker #3 Charge flows through a light bulb. Suppose a wire is connected across the bulb as shown. When the wire is connected, 1. all the charge continues to flow through the bulb. 2. half the charge flows through the wire, the other half continues through the bulb. 3. all the charge flows through the wire. 4. none of the above

  30. Resistors and Light bulbs • We will often represent a resistor as a light bulb • When we ask does the bulb get brighter or dimmer we mean is there more or less current going through it • P = I2R

  31. Clicker #4 The circuit below consists of two identical light bulbs burning with equal brightness and a single 12 V battery. When the switch is closed, the brightness of bulb A 1. increases. 2. remains unchanged. 3. decreases.

  32. Clicker #5 If the four light bulbs in the figure are identical, which circuit puts out more light? 1. I. 2. The two emit the same amount of light. 3. II.

  33. Digression into light bulbs 20% of all electricity generated worldwide is used in lighting

  34. Incandescent Light bulb Filament It is just a resistor

  35. Incandescent Bulb: History • 1802 -1860 Scientific demonstrations • Poor electric voltage/current sources • Short bulb life • 1860 – 1890 Commercial introduction • Lighting was the initial application that drove electrical power commercialization • Eliminate gas flames! • 2009 – 2020 Elimination • 2009 Outlawed in Australia • 2014 Current US law bans sale at 100W t

  36. Light Bulb efficiency • Light efficiency = output power/input power • Input power easy: P = i2R or iV • Output power more complicated, depends on what the eye can see • For a normal incandescent bulb most of the energy is outside the visible spectrum Standard incandescent light bulbs give about 5% of the input energy into light that is visible. The rest goes to heat

  37. Efficiency and Efficacy • Two new words • Lumen (lm): output power multiplied by eye sensitivity • Efficacy (lm/W): Measure of the efficiency of a light source that includes eye sensitivity

  38. Photonic eye response curve

  39. Lumens from Various sources Source Luminous flux (lumens) 37 mW "Superbright" white LED 0.20 15 mW green laser (532 nm wavelength) 8.4 1 W high-output white LED 25–120 Kerosene lantern 100 40 W incandescent lamp 325 7 W high-output white LED 450 100 W incandescent lamp 1700 40 W fluorescent lamp 2800 35 W xenon bulb 2200–3200 100 W fluorescent lamp 8000 127 W low pressure sodium vapor lamp 25000 400 W metal halide lamp 40000

  40. How to measure light-bulbs Old Way Measure power into bulb 40 Watt bulb means 40W into bulb, says nothing about light out of bulb New Way Measure the actual useful light 450 Lumens (direct measure of useful light) “”40W” equivalent, uses 9W

  41. Two more ideas • Color Temperature • Measure of how red (low color temp) or blue (high color temp) is in the light • Sun has a temperature of about 5800K • Color Rendering Index (CRI) • Measure of how much of the complete spectrum is in the light • Sunlight has a CRI of 100 • Sodium lights have CRI of about 20

  42. Incandescent Bulbs Attributes • Nice “warm” light • Instant on and easily dimmable • Very cheap to make Concerns • Poor efficiency • Modest lifetime

  43. CFL Bulbs Attributes • 3X more efficient than incandescent • Cost dropping fast Concerns • Warm up time • Color often poor • Big source

  44. LED bulbs Attributes • More efficacy than CFL (2X) • Different color temperatures easy • Instant on and dimmable Concerns • Early versions were not very bright • Way too expensive

  45. What you need to know • “Light bulb” is resistor with a fixed resistance (like incandescent) • Light bulb is Brighter when it has • More current • More voltage • Light bulb is off (no light) when • Shorted • Open circuit

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