Deck 18: The Flow of Charges

Question 1

Which of the following is NOT a material property (that is, independent of the geometry)?

A)resistivity
B)conductivity
C)current density
D)density of mobile charges

Answer 1

current density

Question 2

Given two wires (with circular cross-section), one of copper having a length of 1.0 m and a diameter of 1.0 mm, the other of gold with a diameter of 2.0 mm, what length must the gold wire be in order to have the same resistance as the copper wire?

A)1.4 m
B)2.8 m
C)4.2 m
D)5.6 m

Answer 2

2.8 m

Question 3

By what fraction would the voltage across a nerve cell membrane drop 2.5 ms after the cell fired?

A)16%
B)37%
C)63%
D)84%

Answer 3

84%

Question 4

Which of the following metal wires of circular cross-section, length l, and diameter d has the least resistance?

A)l = 20 cm, d = 3 mm
B)l = 17 cm, d = 4 mm
C)l = 13 cm, d = 2 mm
D)l = 10 cm, d = 1 mm

Question 5

Iron has two conductance electrons in its outer orbital shell. Its mass density is 7.87 g/cm³, and the molar mass is 55.85 g/mol. What is the drift velocity when a voltage of 12 V is applied across a 2.0 m length of iron wire?

A)0.1 mm/s
B)0.7 mm/s
C)1.4 mm/s
D)2.2 mm/s

Question 6

Which of these statements is an accurate description of current?

A)The current is proportional to the current density.
B)The current through an area is proportional to the density of ions.
C)The number of free electrons in a current is proportional to the gap.
D)The current passing through an area is directly proportional to the time interval.

Question 7

Which of the following describes how electrical current in a metal is formed?

A)by thermal motion of electrons loosely bound to the ions
B)by thermal motion of electrons and ions
C)by slow drift motion of loosely bound electrons
D)by slow drift motion of ions

Question 8

What is the resistance of a wire made of a material with a resistivity of 3.20 × 10^-8 Ù m if its length is 3.50 m and its diameter is 0.50 mm?

A)0.14 Ù
B)0.40 Ù
C)0.57 Ù
D)1.60 Ù

Question 9

A copper wire of length 1.5 km and diameter 2 mm has a potential of 110.0 V applied across its ends. What current would an ammeter measure when placed in the circuit?

A)3.4 A
B)6.8 A
C)10.2 A
D)13.6 A

Question 10

Suppose the membrane potential for a nerve cell changed from -70.0 mV to -50.0 mV. By what factor would the relative concentration of Cl^- between the axoplasm and interstitium change, given a body temperature of 37°C?

A)0.48
B)0.96
C)2.1
D)3.5

Question 11

For any double-logarithmic plot, which one of these types of equations relating x to y gives a straight line?

A)y = ax²
B)y = ax^b
C)y = ax + b
D)y = alnx + b

Question 12

What would happen to the decay time constant of a resting nerve cell if the membrane thickness increased?

A)The time would increase.
B)The time would depend on the initial voltage of the preceding nerve pulse.
C)The time would remain the same.
D)The time would decrease.

Question 13

A mass flow rate of 0.6 kg/s of cooling water is needed to remove heat from an X-ray tube operating at 3.0 MV, in which the temperature rise of the water is 30°K. What is the beam current in the X-ray tube?

A)12.8 mA
B)24.2 mA
C)25.1 mA
D)41.2 mA

Question 14

A wire of length 3.0 m and diameter 1.5 mm has a resistance of 1.25 Ù. What is the resistivity of the material used to make the wire?

A)3.0 × 10^-6 Ù m
B)7.4 × 10^-7 Ù m
C)1.8 × 10^-7 Ù m
D)9.4 × 10^-8 Ù m

Question 15

At what phase of the heart cycle does repolarization of the atrial chambers occur?

A)soon after the P-peak
B)at the start of the Q-peak
C)immediately following the R-peak
D)between the S- and T-peaks

Question 16

What is the resistance of a tube of mercury of length 1.0 m and diameter 1.0 mm?

A)1.3 Ù
B)1.6 Ù
C)3.2 Ù
D)5.2 Ù

Question 17

A voltage of 25.0 V is applied across a hollow iron pipe with an inner diameter of 2.0 cm and a thickness of 1.5 mm. A current of 1.5 A is measured. What is the length of the pipe?

A)1.1 km
B)2.5 km
C)6.8 km
D)8.1 km

Question 18

Which of the following properties is NOT directly accessible to experimental measurement?

A)current
B)voltage
C)cross-sectional area
D)conductivity

Question 19

What units does the constant of proportionality between speed of impulse transport and unmyelinated axon radius have?

A)m s^-1
B)m²s^-1
C)m^1/2s^-1
D)m^1/2s^-1/2

Question 20

During the firing of a nerve cell, the membrane potential rises above -70 mV. The equilibrium ratio of the concentration of Cl^- ions in axoplasm to interstitium before the pulse arrives is 0.072. In which direction will the Cl^- ions flow during the passage of the pulse?

A)They will flow from interstitium to axoplasm.
B)They will flow from axoplasm to interstitium.
C)No migration of Cl^- ions takes place.
D)As the system is no longer in equilibrium, we cannot use Nernst's equation to determine the direction of flow

Question 21

In the absence of the Na/K pump, membrane potentials would tend toward zero.

Question 22

Why is the electrotonus spread inadequate for the propagation of signals through unmyelinated nerves?

Question 23

Think of resistivity in a metal conductor as arising from collisions between fast electrons and the ions in the lattice. It is known experimentally that resistivity increases with temperature. Can you offer an explanation, in terms of the collision model, of why this should be so?

Question 24

When the potential difference across a conductor is doubled, the current is found to increase by a factor of 1.5. What can you conclude about the conductor?

Question 25

The conductivity of the nerve cell membrane remains constant when the cell fires.

Question 26

The equilibrium interstitium cation concentration increases, and the interstitium anion concentration decreases relative to their values in the axoplasm when temperature is lowered.

Question 27

Given a fixed mass of conducting metal, what factors need to be considered in maximizing the resistance of a length of wire made from this metal if the diameter of the wire must NOT be less than a prescribed value? How sensitive is this maximum to the minimum value of the diameter?

Question 28

Erroneous statements such as "10000 volts of electricity passed through the person's body" are often reported. How would you correct such a statement?

Question 29

If it takes several hours for electrons to travel from a light switch to the light, why does it NOT take that long for the light to come on?

Question 30

For an X-ray tube, it is important to be able to transport the energy generated by the beam striking the electrode without raising the temperature of the cooling fluid (usually water) above a certain amount. What limitations does this put on the design of an X-ray tube?

Question 31

The nerve membrane functions only as a capacitor.

Question 32

The diameter of a metal conductor decreases from one end to the other, while maintaining a steady flow of current. How does this affect the drift speed of the electrons?

Question 33

The duration of passage of a nerve impulse, as registered on an ECG, is given by the time interval between the first rise in measured skin potential and the first fall in this potential, relative to its resting value.

Question 34

What distinguishes the mechanism for the beating of the heart from that which operates on nerve cells in the brain?

Question 35

What factors made the Hodgkin-Huxley experiment difficult to perform?

Question 36

The signal decay time of a resting nerve does NOT depend on the thickness or area of the membrane.

Question 37

For ECG studies, the direction of the polarization vector can be determined by only two electrodes.

Question 38

The random thermal motion of electrons in a metal conductor does NOT contribute to the electric current when an external electric field is applied.

Question 39

Minimum Gibbs free energy is NOT attained by a semipermeable membrane.

Question 40

Current density is a material property of a conductor.