4. A well-insulated electric water heater, consisting of a coil made of Nichrome, can warm 5.0L of water from 20.0°C to 49°C in 20 minutes. Density of water = 1 g/cm3 Specific heat of water = 4.186 J/g.°C Resistivity of Nichrome = 1.1x10-6 Ω.m Temperature Coefficient of Nichrome = 0.00041/°C Density of Nichrome = 8.4 g/cm3 (a) How much power is the water heater drawing if the heater has an efficiency of 90%? (b) The water heater is connected to a 240V source. Calculate the resistance of the coil. (c) The total mass of the nichrome coil is 800g. Find the length and diameter of the wire.

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?

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.