2.) Is the total concentration of current carriers (free electrons and holes) in a semicon- ductor equal to the concentration of valence electrons? 3.) There are two types of current carriers in semiconductors (free electrons and holes) and only one type in metals. Yet, metals are much better conductors of electric current. Is this because the currents of electrons and holes oppose each other?

If there is one free electron per atom of copper, what is the electron number density of this metal?

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?

A tightly wound solenoid at 4.0 K is 50 cm long and is constructed from Nb wire of radius 1.5 mm. What maximum current can the solenoid carry if the wire is to remain superconducting?

A valence electron in a crystal absorbs a photon of wavelength, =0.300nm . This is just enough energy to allow the electron to jump from the valence band to the conduction band. What is the size of the energy gap?

If you pick up and shake a piece of metal that has electrons in it free to move as a current, no electrons fall out. Yet if you heat the metal, electrons can be boiled off. Explain both of these facts as they relate to the amount and distribution of energy involved with shaking the object as compared with heating it.

The measured density of a NaF crystal is 2.558 g/cm3 . What is the equilibrium separate distance of Na+ and Flions?

The collector current of a transistor is 3.4 A for a base current of 4.2 mA. What is the current gain?

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.

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?