What is the wavelength in the Balmer series corresponding to a value of n = 15?
A) 277.1 nm
B) 371.1 nm
C) 188.6 nm
D) 656 nm
E) 754.2 nm
B
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An electric furnace is to be used for batch heating a certain material with specific heat of 670 J/(kg K) from 20 to 760°C. The material is placed on the furnace floor which is 2m x 4m in area as shown in the accompanying sketch. The side walls of the furnace are made of a refractory material. Parallel to the plane of the roof, but several inches below it, a grid of round resistor rods is installed. The resistors are 13 mm in diameter and are spaced 5 cm center to center. The resistor temperature is to be maintained at 1100°C, under these conditions the emissivity of the resistor surface is 0.6. If the top surface of the stock is assumed to have an emissivity of 0.9, estimate the time required for heating a 6 metric ton batch. External heat losses from the furnace may be neglected, the
temperature gradient through the stock can be considered negligibly small, and steady-state conditions can be assumed.
GIVEN
- Batch heating of material in the furnace shown above
- Specific heat of material (c) = 670 Jkg K
- Material temperatures
- Initial 
- Final 
- Furnace dimensions: 2 m x 4 m x 1.3 m high
- Side walls are refractory material
- Resistor rod diameter (Dr) = 13 mm = 0.013 m
- Resistor center to center distance (s) = 5 cm = 0.05 m
- Resistor temperature (T2) = 1100°C = 1373 K
- Emissivity of the resistor surface (?2) = 0.6
- Emissivity of the material surface (?1) = 0.9
- Mass of material (m) = 6 metric tons = 6000 kg
FIND
- The time required (t) for heating the 6 metric ton batch
ASSUMPTIONS
- Quasi-steady state conditions
- External heat losses are negligible
- Temperature gradient through the material is negligible (negligible internal thermal resistance)
- Material is gray
- Convective heat transfer is negligible
PROPERTIES AND CONSTANTS
the Stephan-Boltzmann constant (?) = 5.67 x 10–8 W/(m2 K4
A uniaxial composite material is made from 75 vol %t continuous kevlar fibers and 25 vol %t epoxy. The kevlar fibers have an elastic modulus of 131×109 Pa and a tensile strength of 4.0×109 Pa. The epoxy resin has an elastic modulus of 5×109 Pa and a tensile strength of 50×106 Pa.
(a) In a uniaxial test to failure, what component will fail first, and at what strain? (b) What is the composite stress at fracture of the first component? Assume that both the kevlar fibers and the epoxy matrix are elastic up to their failures.
Ten seconds after starting from rest, a car is moving at 40 m/s. What is the car's acceleration?
A) 0.25 m/s2 B) 2.8 m/s2 C) 4.0 m/s2 D) 10 m/s2 E) 40 m/s2
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