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20.3 Resistance and resistivity  (Page 2/6)

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Calculating resistor diameter: a headlight filament

A car headlight filament is made of tungsten and has a cold resistance of 0 . 350 Ω size 12{0 "." "350" %OMEGA } {} . If the filament is a cylinder 4.00 cm long (it may be coiled to save space), what is its diameter?

Strategy

We can rearrange the equation R = ρL A size 12{R = { {ρL} over {A} } } {} to find the cross-sectional area A size 12{A} {} of the filament from the given information. Then its diameter can be found by assuming it has a circular cross-section.

Solution

The cross-sectional area, found by rearranging the expression for the resistance of a cylinder given in R = ρL A size 12{R = { {ρL} over {A} } } {} , is

A = ρL R . size 12{A = { {ρL} over {R} } "."} {}

Substituting the given values, and taking ρ size 12{ρ} {} from [link] , yields

A = ( 5.6 × 10 –8 Ω ⋅ m ) ( 4.00 × 10 –2 m ) 0.350 Ω = 6.40 × 10 –9 m 2 .

The area of a circle is related to its diameter D size 12{D} {} by

A = πD 2 4 . size 12{A = { {πD rSup { size 8{2} } } over {4} } "."} {}

Solving for the diameter D size 12{D} {} , and substituting the value found for A size 12{A} {} , gives

D = 2 A p 1 2 = 2 6.40 × 10 –9 m 2 3.14 1 2 = 9.0 × 10 –5 m . alignl { stack { size 12{D =" 2" left ( { {A} over {p} } right ) rSup { size 8{ { {1} over {2} } } } =" 2" left ( { {6 "." "40"´"10" rSup { size 8{ +- 9} } " m" rSup { size 8{2} } } over {3 "." "14"} } right ) rSup { size 8{ { {1} over {2} } } } } {} #=" 9" "." 0´"10" rSup { size 8{ +- 5} } " m" "." {} } } {}

Discussion

The diameter is just under a tenth of a millimeter. It is quoted to only two digits, because ρ size 12{ρ} {} is known to only two digits.

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Temperature variation of resistance

The resistivity of all materials depends on temperature. Some even become superconductors (zero resistivity) at very low temperatures. (See [link] .) Conversely, the resistivity of conductors increases with increasing temperature. Since the atoms vibrate more rapidly and over larger distances at higher temperatures, the electrons moving through a metal make more collisions, effectively making the resistivity higher. Over relatively small temperature changes (about 100º C size 12{"100"°C} {} or less), resistivity ρ size 12{ρ} {} varies with temperature change Δ T size 12{DT} {} as expressed in the following equation

ρ = ρ 0 ( 1 + α Δ T ) , size 12{ρ = ρ rSub { size 8{0} } \( "1 "+ αΔT \) ","} {}

where ρ 0 size 12{ρ rSub { size 8{0} } } {} is the original resistivity and α size 12{α} {} is the temperature coefficient of resistivity    . (See the values of α size 12{α} {} in [link] below.) For larger temperature changes, α size 12{α} {} may vary or a nonlinear equation may be needed to find ρ size 12{ρ} {} . Note that α size 12{α} {} is positive for metals, meaning their resistivity increases with temperature. Some alloys have been developed specifically to have a small temperature dependence. Manganin (which is made of copper, manganese and nickel), for example, has α size 12{α} {} close to zero (to three digits on the scale in [link] ), and so its resistivity varies only slightly with temperature. This is useful for making a temperature-independent resistance standard, for example.

A graph for variation of resistance R with temperature T for a mercury sample is shown. The temperature T is plotted along the x axis and is measured in Kelvin, and the resistance R is plotted along the y axis and is measured in ohms. The curve starts at x equals zero and y equals zero, and coincides with the X axis until the value of temperature is four point two Kelvin, known as the critical temperature T sub c. At temperature T sub c, the curve shows a vertical rise, represented by a dotted line, until the resistance is about zero point one one ohms. After this temperature the resistance shows a nearly linear increase with temperature T.
The resistance of a sample of mercury is zero at very low temperatures—it is a superconductor up to about 4.2 K. Above that critical temperature, its resistance makes a sudden jump and then increases nearly linearly with temperature.
Tempature coefficients of resistivity α size 12{α} {}
Material Coefficient α (1/°C) Values at 20°C.
Conductors
Silver 3 . 8 × 10 − 3 size 12{3 "." 8 times "10" rSup { size 8{ - 3} } } {}
Copper 3 . 9 × 10 − 3 size 12{3 "." 9 times "10" rSup { size 8{ - 3} } } {}
Gold 3 . 4 × 10 − 3 size 12{3 "." 4 times "10" rSup { size 8{ - 3} } } {}
Aluminum 3 . 9 × 10 − 3 size 12{3 "." 9 times "10" rSup { size 8{ - 3} } } {}
Tungsten 4 . 5 × 10 − 3 size 12{4 "." 5 times "10" rSup { size 8{ - 3} } } {}
Iron 5 . 0 × 10 − 3 size 12{5 "." 0 times "10" rSup { size 8{ - 3} } } {}
Platinum 3 . 93 × 10 − 3 size 12{3 "." "93" times "10" rSup { size 8{ - 3} } } {}
Lead 3 . 9 × 10 − 3 size 12{3 "." 9 times "10" rSup { size 8{ - 3} } } {}
Manganin (Cu, Mn, Ni alloy) 0 . 000 × 10 − 3 size 12{0 "." "000" times "10" rSup { size 8{ - 3} } } {}
Constantan (Cu, Ni alloy) 0 . 002 × 10 − 3 size 12{0 "." "002" times "10" rSup { size 8{ - 3} } } {}
Mercury 0 . 89 × 10 − 3 size 12{0 "." "89" times "10" rSup { size 8{ - 3} } } {}
Nichrome (Ni, Fe, Cr alloy) 0 . 4 × 10 − 3 size 12{0 "." 4 times "10" rSup { size 8{ - 3} } } {}
Semiconductors
Carbon (pure) − 0 . 5 × 10 − 3 size 12{ - 0 "." 5 times "10" rSup { size 8{ - 3} } } {}
Germanium (pure) − 50 × 10 − 3 size 12{ - "50" times "10" rSup { size 8{ - 3} } } {}
Silicon (pure) − 70 × 10 − 3 size 12{ - "70" times "10" rSup { size 8{ - 3} } } {}

Note also that α size 12{α} {} is negative for the semiconductors listed in [link] , meaning that their resistivity decreases with increasing temperature. They become better conductors at higher temperature, because increased thermal agitation increases the number of free charges available to carry current. This property of decreasing ρ size 12{ρ} {} with temperature is also related to the type and amount of impurities present in the semiconductors.

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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