A 24-kW electric furnace is connected to a 240-V line. What is the resistance of the furnace?
A)
1000 ?
B)
10 ?
C)
2.4 ?
D)
0.42 ?
E)
100 ?
C
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Overhead electric transmission lines get heated (due to the electric resistivity of the cable material) and lose heat to the ambient air; this transmission energy loss due to heat transfer is referred to as the Joule effect in the conduction material. Consider a 3- cm diameter horizontal cable that dissipates heat at the rate of 50 W per unit length, and that the heat transfer due to surrounding air is only by natural convection. Determine the surface temperature of the cable when (a) in the summer time the air temperature is 35°C and (b) in the winter time the air temperature is 25°C.
GIVEN
• Overhead heated electric transmission lines loosing heat due to convection
• Heat dissipation rate= 50 W/m
• Diameter of transmission lines (D) = 3 cm= 0.03 m
• Ambient air temperature during summer (T?)
At summer = 35°C At winter= 250C
FIND
(Surface temperature of cable in (a) Summer (b) Winter
SKETCH
ASSUMPTIONS
Air is still
Surface temperature is uniform and constant
Air properties are constant along the range of temperature.
PROPERTIES AND CONSTANTS
for dry air at the assumed mean temperature of 50°C
Thermal expansion coefficient (?) = 0.003095 1/K
Thermal conductivity (k) = 0.0272 W/(m K)
Kinematic viscosity (?) = 18.5 × 10–6 m2/s
Prandtl number (Pr) = 0.71
The horizontal speed of a projectile
a. is zero. b. stays constant. c. continuously increases. d. continuously decreases. e. sometimes decreases and sometimes increases.
What is meant by ‘like charges?'
a. charges that are both positive b. equal amounts of charge c. charges that are both negative d. A and C
A crystal diffracts a beam of electrons, like a grating, as they hit it perpendicular to its surface
The crystal spacing is 0.18 nm, and the first maximum scattering occurs at 80° relative to the normal to the surface. (e = 1.60 × 10-19 C, melectron = 9.11 × 10-13 kg, h = 6.626 × 10-34 J ? s) (a) What is the wavelength of the electrons? (b) What potential difference accelerated the electrons if they started from rest?