Which of the following materials best conducts electric current? A Gold b. Silver c. Copper d. Glass E Aluminum F. Platinum

Integrated Concepts How much time is needed for a surgical cauterizer to raise the temperature of 1.00 g of tissue from 37.0°C to 100°C and then boil away 0.500 g of water, if it puts out 2.00 mA at 15.0 kV? Ignore heat transfer to the surroundings.

A close analogy exists between the flow of energy by heat because of a temperature difference (see Section 19.6) and the flow of electric charge because of a potential difference. In a metal, energy dQ and electrical charge dq are both transported by free electrons. Consequently, a good electrical conductor is usually a good thermal conductor as well. Consider a thin conducting slab of thickness dx, area A, and electrical conductivity , with a potential difference dV between opposite faces. (a) Show that the current I = dq/dt is given by the equation on the left: ChargeconductionThermalconductiondqdt=A|dVdx|dQdt=kA|dTdx| In the analogous thermal conduction equation on the right (Eq. 19.17), the rate dQ/dt of energy flow by heat (in SI units of joules per second) is due to a temperature gradient dT/dx in a material of thermal conductivity k. (b) State analogous rules relating the direction of the electric current to the change in potential and relating the direction of energy flow to the change in temperature.

Integrated Concepts (a) Redo Exercise 20.25 taking into account the thermal expansion of the tungsten filament. You may assume a thermal expansion coefficient of 12 10-6/°C. (b) By what percentage does your answer differ from that in the example?

A Pb wire wound in a tight solenoid of diameter of 4.0 mm is cooled to a temperature of 5.0 K. The wire is connected in series with a 50resistor and a variable source of emf. As the emf is increased, what value does it have when the superconductivity of the wire is destroyed?

The current I is measured through a sample of an ohmic material as a voltage V is applied, (a) What is the current when the voltage is doubled to 2V (assume the change in temperature of the material is negligible)? (b) What is the voltage applied is the current measured is 0.27 (assume the change in temperature of the material is negligible)? What will happen to the current if the material if the voltage remains constant, but the temperature of the material increases significantly?

Explain the properties of myelinated nerves in terms of the insulating properties of myelin.

Do impurities in semiconducting materials listed in Table 9.1 supply free charges? (Hint Examine the range of resistivity for each and determine whether die pure semiconductor has the higher or lower conductivity.)

Check Your Understanding What conditions are necessary for superconductivity?

What does the term fibrillation mean in connection with heart function? Give two important causes of heart fibrillation in human beings. How do pacemakers and defibrillators help to prolong life for patients suffering from this condition?