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Chapter 17

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Chapter 17

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  1. Chapter 17 Current and Resistance

  2. Example 17.3 – Page 578 • A) Calculate the resistance per unit length of a 22-gauge nichrome wire of radius 0.321mm. • B) If a potential difference of 10V is maintained across a 1m length of the nichrome wire, what is the current in the wire? • C) The wire is melted down and recast with twice its original length. Find the new resistance RN as a multiple of the old resistance R0.

  3. Example 17.3 – continue • A) • B) • C)

  4. Temperature Variation of Resistivity • For most metals, resistivity increases with increasing temperature • With a higher temperature, the metal’s constituent atoms vibrate with increasing amplitude • The electrons find it more difficult to pass through the atoms

  5. Temperature Variation of Resistivity, cont • For most metals, resistivity increases approximately linearlywith temperature over a limited temperature range • ρ is the resistivity at some temperature T • ρo is the resistivity at some reference temperature To • To is usually taken to be 20° C •  is the temperature coefficient of resistivity

  6. Temperature Variation of Resistance • Since the resistance of a conductor with uniform cross sectional area is proportional to the resistivity, you can find the effect of temperature on resistance

  7. Example 17.4 – Page 580 • A resistance thermometer, which measures temperature by measuring the change in resistance of a conductor, is made of platinum and has a resistance of 50Ω at 200C. • A) when the device is immersed in a vessel containing melting indium, its resistance increases to 76.8Ω. From this information, find the melting point of indium. • B) the indium is heated further until it reaches a temperature of 2350C. What is the ratio of the new current in the platinum to the current Imp at the melting point?

  8. Example 17.4 – continue • A) • B)

  9. Electrical Energy and Power • In a circuit, as a charge moves through the battery, the electrical potential energy of the system is increased by ΔQΔV • The chemical potential energy of the battery decreases by the same amount • As the charge moves through a resistor, it loses this potential energy during collisions with atoms in the resistor • The temperature of the resistor will increase

  10. Energy Transfer in the Circuit • Consider the circuit shown • Imagine a quantity of positive charge, DQ, moving around the circuit from point A back to point A

  11. Energy Transfer in the Circuit, cont • Point A is the reference point • It is grounded and its potential is taken to be zero • As the charge moves through the battery from A to B, the potential energy of the system increases by DQDV • The chemical energy of the battery decreases by the same amount

  12. Energy Transfer in the Circuit, final • As the charge moves through the resistor, from C to D, it loses energy in collisions with the atoms of the resistor • The energy is transferred to internal energy • When the charge returns to A, the net result is that some chemical energyof the battery has been delivered to the resistor and caused its temperature to rise

  13. Electrical Energy and Power, cont • The rate at which the energy is lost is the power • From Ohm’s Law, alternate forms of power are

  14. Electrical Energy and Power, final • The SI unit of power is Watt (W) • I must be in Amperes, R in ohms and DV in Volts • The unit of energy used by electric companies is the kilowatt-hour • This is defined in terms of the unit of power and the amount of time it is supplied • 1 kWh = 3.60 x 106 J