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AP Physics Electric Current and Resistance

AP Physics Electric Current and Resistance. Homework for Chapter 18. Read Chapter 17 HW 17.A: p.562: 6-9, 12-17. HW 17.B: p. 562-563: 23-25; 28-34. HW 17.C: p. 564: 53-56; 60-64. Batteries and Direct Current.

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AP Physics Electric Current and Resistance

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  1. AP PhysicsElectric Current and Resistance

  2. Homework for Chapter 18 • Read Chapter 17 • HW 17.A: p.562: 6-9, 12-17. • HW 17.B: p. 562-563: 23-25; 28-34. • HW 17.C: p. 564: 53-56; 60-64.

  3. Batteries and Direct Current

  4. battery - a device that converts chemical potential energy into electrical energy. • Allesandro Volta constructed one of the first practical batteries. • He used zinc and copper electrodes in a weak sulfuric acid solution. circuit - any complete loop consisting of wires and electrical devices ex: batteries and light bulbs. anode (+) - the positive terminal of a battery. cathode (-) - the negative terminal of a battery. electric current - the net rate at which charge flows past a given point. direct current (dc) - in a battery circuit, the electrons can only flow in one direction, from negative terminal to positive terminal.

  5. 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.

  6. 18-1 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.

  7. 18-1 The Electric Battery Several cells connected together make a battery, although now we refer to a single cell as a battery as well.

  8. electromotive force (emf) - the potential difference across the two terminals of a battery (or any dc power supply) when not connected to an external circuit. • emf is NOT a force; it is a voltage, measured in volts.

  9. terminal voltage - is the voltage across a battery or power supply when it is connected to an external circuit. • Also called operating voltage • Terminal voltage is always less than the emf because of internal resistance of the battery. • A battery’s internal resistance depends on its age, type of electrolyte, and electrode material. • The terminal voltage is what a battery actual delivers; it can be considerably less than the emf.

  10. V =  - Ir terminal voltage = emf – (current) x (internal resistance of the battery) Example: A battery has an emf of 8.40 V and an internal resistance of 0.5 . It can supply a current of 0.084 A. What is its terminal voltage? V =  - Ir = 8.40 V – (0.084 A) (0.5) = 8.36 V

  11. Batteries in Series • Notice the symbol for battery and resistance. A resistance is anything in the circuit that opposes the charge flow. • When batteries are connected in series, their voltages add and the voltage across the resistance R is the sum of the voltages. • Example: Car batteries. These generally consist of six 2-volt cells connected in series.

  12. Batteries in Parallel • When batteries of the same voltage are connected in parallel, the voltage across the resistance is the same, as if only a single battery were present. • In this arrangement, each battery supplies a fraction of the total current. • Example: jumping your car. The strong battery (low resistance) delivers most of the current to help the weak battery (high resistance).

  13. What is happening in a series? • A series connection adds the voltage of the two batteries, but it keeps the same amperage rating (also known as Amp Hours). For example, these two 6-volt batteries joined in series now produce 12 volts, but they still have a total capacity of 10 amps. • To connect batteries in a series, use jumper wire to connect the negative terminal of the first battery to the positive terminal of the second battery. Use another set of cables to connect the open positive and negative terminals to your application. • Note: Never cross the remaining open positive and open negative terminals with each other, as this will short circuit the batteries and cause damage or injury. • Be sure the batteries you're connecting have the same voltage and capacity rating. Otherwise, you may end up with charging problems, and shortened battery life.

  14. Parallel? • Parallel connections will increase your current rating, but the voltage will stay the same. • In the “Parallel” diagram, we're back to 6 volts, but the amps increase to 20 AH. It's important to note that because the amperage of the batteries increased, you may need a heavier-duty cable to keep the cables from burning out.

  15. Current and Drift Velocity

  16. Electric Current Electric current is the rate of flow of charge through a conductor: 1 A(amperes) = 1 C(coulombs)/s(seconds)

  17. Electric Current A complete circuit is one where current can flow all the way around.

  18. Electric Current In order for current to flow, there must be a path from one battery terminal, through the circuit, and back to the other battery terminal. Only one of these circuits will work:

  19. Electric Current By convention, current is defined as flowing from + to –. Electrons actually flow in the opposite direction.

  20. drift velocity - the average velocity of the electron flow in a metal wire. • much smaller than the random velocities of the electrons themselves. • drift velocity is approximately 1 mm/s • drift velocity is opposite the direction of the electric field, towards the positive terminal of the battery. • The electric field, which is what pushes the charges in the wire, travels down the wire at close to the speed of light (on the order of 108 m/s). • This is why the current starts “instantly” in all parts of the circuit.

  21. Example 17.1: If 3.0 x 1015 electrons flow through a section of a wire of diameter 2.0 mm in 4.0 s, what is the electric current in the wire?

  22. Check for Understanding • When a battery is placed into a complete circuit, the voltage across its terminal is its • a) emf • b) terminal voltage • c) power • d) all of these • Answer: b • 2. As a battery gets old, its • a) emf increases • b) emf decreases • c) terminal voltage increases • d) terminal voltage decreases • Answer: d

  23. Check for Understanding 3. When four 1.5 volt batteries are connected in parallel, the output voltage of the combination is a) 1.5 V b) 3.0 V c) 6.0 V d) none of these Answer: a 4. The unit of electrical current is a) C b) C/s c) A d) both b and c Answer: d

  24. Homework for Sections 17.1 & 17.2 HW 17.A: p.562: 6-9, 12-17.

  25. 17.3 Resistance and Ohm’s Law

  26. resistance (R) - the ratio of the voltage to the resulting current R = V or V = IR Ohm’s Law I • The SI unit of resistance is the volt per ampere (V/A) or ohm (). ohmic - a resistor is said to be ohmic if it has constant resistance. * example – wire, electric stove • Not all materials are ohmic: example: lightbulbs, semiconductors

  27. Example 17.2: A resistor with a resistance of 20  is connected to a 12-volt battery. What is the current through the resistor?

  28. Factors That Influence Resistance • The major factors that influence resistance of a conductor of uniform cross-section are: 1) the type of material or the intrinsic resistive properties 2) its length (L) 3) its cross-sectional area (A) 4) its temperature (T) Resistivity () • determined by the resistive properties of a material (partly due to intrinsic atomic properties) R =  L or  = RA A L where R is resistance • the SI unit of resistivity is the ohm-meter ( · m)

  29. conductivity() – the inverse of resistivity  = 1  • the SI unit of conductivity is 1/ohm-meter [( ·m)-1]

  30. Example 17.3: Calculate the current in a piece of 10.0 m long 22-gauge (the radius is 0.321 mm) nichrome wire if it is connected to a source of 12.0 V. Assume the temperature is 20°C.

  31. Check for Understanding • The unit of resistance is the • a. V / A • b. A / V • c. W • d. V • Answer: a • 2. For an ohmic resistor, current and resistance • a) vary with temperature • b) are directly proportional • c) are independent of voltage • d) none of these • Answer: d (ohmic resistors have constant resistance by definition)

  32. Warmup: Power Up! Daily Physics Warmup #77 Power is the rate at which energy is used. When an appliance is labeled with a certain power, such as 1,200 watt hair dryer, it means that during each second of operation the dryer transforms 1,200 joules of energy from one type of energy into other types of energy. ************************************************************************ Identify the type of energy that operates the appliance and the type or types of energy it produces. Device Energy In Energy Out toaster portable generator electric dryer flashlight heat (light) electrical chemical electrical heat electrical Electrical or chemical light (heat)

  33. Electric Power

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

  35. Electric Power Power, as in kinematics, is the energy transformed by a device per unit time:

  36. 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. One kWh = (1000 W)(3600 s) = 3.60 x 106 J

  37. Power in Household Circuits The wires used in homes to carry electricity have very low resistance. However, if the current is high enough, the power will increase and the wires can become hot enough to start a fire. To avoid this, we use fuses or circuit breakers, which disconnect when the current goes above a predetermined value.

  38. Power in Household Circuits Fuses are one-use items—if they blow, the fuse is destroyed and must be replaced.

  39. Power in Household Circuits Circuit breakers, which are now much more common in homes than they once were, are switches that will open if the current is too high; they can then be reset.

  40. Alternating Current Current from a battery flows steadily in one direction (direct current, DC). Current from a power plant varies sinusoidally (alternating current, AC).

  41. Summary of Chapter 18 • A battery is a source of constant potential difference. • Electric current is the rate of flow of electric charge. • Conventional current is in the direction that positive charge would flow. • Resistance is the ratio of voltage to current:

  42. Summary of Chapter 18 • Ohmic materials have constant resistance, independent of voltage. • Resistance is determined by shape and material: • ρ is the resistivity. • Power in an electric circuit:

  43. joule heat – the thermal energy expended in a current-carrying resistor • also known as I2R losses (“I squared R” losses) • can be undesirable (ex: electrical transmission lines) • can be the intended purposes (ex: hair dryers, toasters) • heat = power  time (J/s  s = J) kilowatt-hour (kWh) – a unit of work (energy) 1 kWh = (1000 W)(3600 s) = ( 1000 J/s)(3600 s) = 3.6 x 106 J

  44. Example : What amount of heat is generated in a 10  resistor that carries 0.3 A of current for 3 minutes?

  45. Example 17.5: What is the operating resistance of a 100 W household light bulb? The operating line voltage of household electricity is 120 V.

  46. Check for Understanding • Electric power has units of • A2· • J/s • V2/  • all of these • Answer: d • 2. If the voltage across an ohmic resistor is doubled, the power expended in the resistor • a) increases by a factor of 2 • b) increases by a factor of 4 • c) decreases by half • d) none of these • Answer: b, since P=V2/R

  47. Check for Understanding 3. If the current through an ohmic resistor is halved, the power expended in the resistor a) increases by a factor of 2 b) increases by a factor of 4 c) decreases by half d) decreases by a factor of 4 Answer: d, because P = I2R 4. Assuming your hair dryer obeys Ohm’s law, what would happen if you plugged it directly into a 240-volt outlet in Europe if it is designed to be used in the 120-volt outlets of the US? Answer: Since P = V2/R, its power output would quadruple, and it would overheat at least.

  48. Homework for Section 17.4 • HW 17.C: p. 564: 53-56; 60-64.

  49. Chapter 17 Formulas V =  - Ir Defines terminal voltage in terms of emf, current, and internal resistance of a battery. I = q Define electric current in terms of charge flow. t V = IR Ohm’s Law. R =  L Defines the resistivity of a material. A • = 1 Conductivity is the reciprocal of resistivity.  P = IV = I2R = V2 Computes the electric power delivery to a resistor. R

  50. 18-3 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: I V

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