Chapter 4 Flashcards
Physical Properties of Materials
Define density as a material property
weight per unit of volume
What is the difference in melting characteristics between pure metal elements and alloy metal
A pure metal element melts at one temperature (melting point) while an alloy begins melting at the solidus and finally becomes molten at the liquiduls. Between those two, the metal is a mix of solid and liquied
Describe the melting cahraceristics of a noncrystalline material
THe material begins to soften as temperature increases I gradual transition) it finally ocnverts to a liquid at the melting point
Define specific heat as a material property
Quantity of heat required to raise the temperature of a unit of mass of the material by one degree
What is thermal conductivity as a material property
It is the capacity of a material to transfer heat energy through itself by thermal movement only
Define thermal diffusivity
Thermal conductivity divided by the volumetric specific heat (heat energy required to raise the temperature of a unit of volume of a material by 1 degree)
What are the important variables that affect mass diffusion?
depends on the diffusion coefficient (increases with temperature, so it can also be considered an important variable) concentration gradient, contact area, and time
Define resistivity as a material property
A material’s capacity of resisnting electric flow of current
Why are metals better conductors of electricity than ceramics and polymers
Because of the metalic bond
Dielectric strength
Electrical potential requiered to break down the insulator per unit of thickness
What is an electrolyte
an ionized solution capable of conducting electric current by movement of ions
Which of the following metals has the lowest density (a) aluminum, (b) copper, (c) magnesium,
or (d) tin?
c magnesium
Thermal expansion properties of polymers are generally (a) greater than, (b) less than, or (c) the
same as those of metals?
a greater
In heating of most metal alloys, melting begins at a certain temperature and concludes at a higher temperature. In these cases which of the following marks the beginning of the melting (a)
liquidus or (b) solidus?
b solidus
Which one of the following has the highest specific heat (a) aluminum, (b) concrete, (c)
polyethylene, or (d) water?
d water
Copper is generally considered easy to weld because of its high thermal conductivity (a) true or (b)
false?
b false. Thigh high thermal conductivity of copper makes it difficult to weld because the heat flows away from the joint rather than being concentrated to permit melting of the metal
The mass diffusion rate dm/dt across a boundary between two different metals is a function of which of the following variables (four best answers): (a) concentration gradient dc/dx, (b) contact area, (c)
density, (d) melting point, (e) thermal expansion, (f) temperature, and (g) time?
a concentration gradient, b contact area, f temperature, g time:
Included in the equation dm = -D(dc/dt)A dt
Which of the following pure metals is the best conductor of electricity (a) aluminum, (b) copper, (c)
gold, or (d) silver?
d silver
A superconductor is characterized by which of the following (one best answer): (a) high conductivity,
(b) resistivity properties between those of conductors and semiconductors, (c) very low resistivity, or
(d) zero resistivity
d zero resitivity (at certain temperatures)
In an electrolytic cell, the anode is the electrode that is
positive
The starting diameter of a shaft is 25.00 mm. This shaft is to be inserted into a hole in an expansion
fit assembly operation. To be readily inserted, the shaft must be reduced in diameter by cooling.
Determine the temperature to which the shaft must be reduced from room temperature (20°C) in
order to reduce its diameter to 24.98 mm. Refer to Table 4.1. (steel)
T2 = -46.67°C
A bridge built with steel girders is 500 m in length and 12 m in width. Expansion joints are provided
to compensate for the change in length in the support girders as the temperature fluctuates. Each
expansion joint can compensate for a maximum of 40 mm of change in length. From historical
records it is estimated that the minimum and maximum temperatures in the region will be -35°C and
38°C, respectively. What is the minimum number of expansion joints required?
Solution: Assume L1 =500 m at -35 C, α=12x10-6
/C
L2 – L1 = αL1(T2 – T1)
L2 – L1 = 12x10-6
(500)(38 – (-35))
L2 – L1 = 0.42 m
Each expansion joint will control 40 mm = 0.04 m of expansion.
10 joints will provide 0.40 m of expansion. 11 joints will provide 0.45 m of expansion. Therefore, a
minimum of 11 joints are needed for coverage of the total length. Each bridge section will be 500/11
= 45.5 m long.
Aluminum has a density of 2.70 g/cm3 at room temperature (20°C). Determine its density at 650°C,
using data in Table 4.1 as a reference.
Solution: Assume a 1 cm3 cube, 1 cm on each side.
From Table 4.1, α = 24(10-6
) mm/mm/°C
L2 - L1 = αL1 (T2 - T2).
L2 = 1.0 + 24(10-6
)(1.0)(650 - 20) = 1.01512 cm
(L2 )
3 = (1.01512)3 = 1.04605 cm3
Assume weight remains the same; thus ρ at 650°C = 2.70/1.04605 = 2.581 g/cm3
With reference to Table 4.1, determine the increase in length of a steel bar whose length = 10.0 in, if
the bar is heated from room temperature of 70°F to 500°F.
Solution: Increase = (6.7 x 10-6 in/in/F)(10.0 in)(500°F - 70°F) = 0.0288 in
With reference to Table 4.2, determine the quantity of heat required to increase the temperature of an
aluminum block that is 10 cm x 10 cm x 10 cm from room temperature (21°C) to 300°C.
Solution. Heat = (0.21 cal/g-°C)(103 cm3
)(2.70 g/cm3
)(300°C - 21°C) = 158,193 cal.
Conversion: 1.0 cal = 4.184J, so heat = 662,196 J.
What is the resistance R of a length of copper wire whose length = 10 m and whose diameter = 0.10
mm? Use Table 4.3 as a reference.
Solution: R = rL/A, A = π(0.1)2
/4 = 0.007854 mm2 = 0.007854(10-6
) m2
From Table 4.3, r = 1.7 x 10-8 Ω-m2
/m
A 16 gage nickel wire (0.0508-in diameter) connects a solenoid to a control circuit that is 32.8 ft
away. (a) What is the resistance of the wire? Use Table 4.3 as a reference. (b) If a current was passed
through the wire, it would heat up. How does this affect the resistance?
Solution: (a) L = 32.8 ft = 393.6 in
Area A = π(0.0508)2
/4 = 0.00203 in2
R = r (L/A) = 6.8 x 10-8 (39.4)(393.6/0.00203) = 0.520 ohm
(b) If a current is passed through the wire causing the wire to heat up, the resistivity of the wire would
change. Since nickel is a metal, the resistivity would increase, causing the resistance to increase. This,
in turn, would cause slightly more heat to be generated.
Aluminum wiring was used in many homes in the 1960s due to the high cost of copper at the time.
Aluminum wire that was 12 gauge (a measure of cross-sectional area) was rated at 15 A of current.
If copper wire of the same gauge were used to replace the aluminum wire, what current should the
wire be capable of carrying if all factors except resistivity are considered equal? Assume that the
resistance of the wire is the primary factor that determines the current it can carry and the crosssectional area and length are the same for the aluminum and copper wires.
Solution: The area and length are constant between the types of wires. The overall change in
resistance is due to the change in resistivity of the materials. From Table 4.3:
For Aluminum r = 2.8 x 10-8
For Copper r = 1.7 x 10-8
The resistance will reduce by 1.7x10-8
/2.8x10-8 = 0.61
Since I = E/R and Rcu=0.61(Ral), then Icu = 1/0.61*Ial = 15/0.61= 25 A
Note that the code value is actually 20 A due to several factors including heat dissipation and
rounding down to the nearest 5 amp value.
