Chapter 25 Electric Currents and Resistance - PowerPoint PPT Presentation

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Chapter 25 Electric Currents and Resistance

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  1. Chapter 25Electric Currents and Resistance

  2. 24-6 Molecular Description of Dielectrics - + - + - + - + - + - + - + - + + - + - - + + - + - + - + - - + + + - - + + - - E 1. Dielectric is inserted +Q -Q Eo -Q +Q 2. Dielectric is polarized 3. Dielectric has internal Electric field 4. Electric Field is reduced by the dielectric constant,  E weaker, V is smaller, but C is bigger

  3. 24-6 Molecular Description of Dielectrics This means that the electric field within the dielectric is less than it would be in air, allowing more charge to be stored for the same potential. This reorientation of the molecules results in an induced charge – there is no net charge on the dielectric, but the charge is asymmetrically distributed. The magnitude of the induced charge depends on the dielectric constant:

  4. 25-1 The Electric Battery Volta discovered that electricity could be created if dissimilar metals were connected by a conductive solution called an electrolyte. This is a simple electric cell.

  5. 25-1 The Electric Battery A battery transforms chemical energy into electrical energy. Chemical reactions within the cell create a potential difference between the terminals by slowly dissolving them. This potential difference can be maintained even if a current is kept flowing, until one or the other terminal is completely dissolved.

  6. 25-1 The Electric Battery Several cells connected together make a battery, although now we refer to a single cell as a battery as well.

  7. 25-2 Electric Current Electric current is the rate of flow of charge through a conductor: The instantaneous current is given by: Unit of electric current: the ampere, A: 1 A = 1 C/s.

  8. 25-2 Electric Current A complete circuit is one where current can flow all the way around. Note that the schematic drawing doesn’t look much like the physical circuit!

  9. 25-2 Electric Current By convention, current is defined as flowing from + to -. Electrons actually flow in the opposite direction, but not all currents consist of electrons.

  10. 25-2 Electric Current Example 25-1: Current is flow of charge. A steady current of 2.5 A exists in a wire for 4.0 min. (a) How much total charge passed by a given point in the circuit during those 4.0 min? (b) How many electrons would this be?

  11. 25-3 Ohm’s Law: Resistance and Resistors Experimentally, it is found that the current in a wire is proportional to the potential difference between its ends:

  12. 25-3 Ohm’s Law: Resistance and Resistors The ratio of voltage to current is called the resistance: This is Ohm’s Law

  13. Problem 5 5. (II) An electric clothes dryer has a heating element with a resistance of 8.6Ω (a) What is the current in the element when it is connected to 240 V? (b) How much charge passes through the element in 50 min? (Assume direct current.)

  14. 25-2 Electric Current Conceptual Example 25-2: How to connect a battery. What is wrong with each of the schemes shown for lighting a flashlight bulb with a flashlight battery and a single wire?

  15. 25-3 Ohm’s Law: Resistance and Resistors In many conductors, the resistance is independent of the voltage; this relationship is called Ohm’s law (Ohmic Materials). Materials that do not follow Ohm’s law are called nonohmic. Unit of resistance: the ohm, Ω: 1 Ω = 1 V/A.

  16. 25-3 Ohm’s Law: Resistance and Resistors Example 25-4: Flashlight bulb resistance. A small flashlight bulb draws 300 mA from its 1.5-V battery. (a) What is the resistance of the bulb? (b) If the battery becomes weak and the voltage drops to 1.2 V, how would the current change?

  17. Problem 9 9.(II) A 12-V battery causes a current of 0.60 A through a resistor. (a) What is its resistance, and (b) how many joules of energy does the battery lose in a minute?

  18. 25-3 Ohm’s Law: Resistance and Resistors Standard resistors are manufactured for use in electric circuits; they are color-coded to indicate their value and precision.

  19. 25-3 Ohm’s Law: Resistance and Resistors This is the standard resistor color code. Note that the colors from red to violet are in the order they appear in a rainbow.

  20. 25-3 Ohm’s Law: Resistance and Resistors Some clarifications: • Batteries maintain a (nearly) constant potential difference; the current varies. • Resistance is a property of a material or device. • Current is not a vector but it does have a direction. • Current and charge do not get used up. Whatever charge goes in one end of a circuit comes out the other end.

  21. 25-4 Resistivity The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area: The constant ρ, the resistivity, is characteristic of the material.ρ is in Ω.m, A is the area

  22. 25-4 Resistivity

  23. 25-4 Resistivity For any given material, the resistivity increases with temperature: Semiconductors are complex materials, and may have resistivities that decrease with temperature.0 and T0 are the resistivity and temperature at 0ºC or 20ºC.

  24. 25-4 Resistivity Example 25-7: Resistance thermometer. The variation in electrical resistance with temperature can be used to make precise temperature measurements. Platinum is commonly used since it is relatively free from corrosive effects and has a high melting point. Suppose at 20.0°C the resistance of a platinum resistance thermometer is 164.2 Ω. When placed in a particular solution, the resistance is 187.4Ω. What is the temperature of this solution? ( α=3.927 10-3 °C-1)

  25. Problem 23 23. (II) A length of aluminum wire is connected to a precision 10.00-V power supply, and a current of 0.4212 A is precisely measured at 20.0°C. The wire is placed in a new environment of unknown temperature where the measured current is 0.3818 A. What is the unknown temperature? α=0.00429°C-1

  26. 25-5 Electric Power Power, as in kinematics, is the energy transformed by a device per unit time: or

  27. 25-5 Electric Power The unit of power is the watt, W. For ohmic devices, we can make the substitutions:

  28. 25-5 Electric Power Example 25-8: Headlights. Calculate the resistance of a 40-W automobile headlight designed for 12 V.

  29. 25-5 Electric Power What you pay for on your electric bill is not power, but energy – the power consumption multiplied by the time. We have been measuring energy in joules, but the electric company measures it in kilowatt-hours, kWh: 1 kWh = (1000 W)(3600 s) = 3.60 x 106 J.