What does a double arrow mean when it is used
between the reactants and the products in a
chemical equation?
a) twice as much product is present as reactant
b) the reaction is a forward reaction only
c) the reaction is reversible
d) the reaction requires a high activation energy
c
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Three coplanar parallel straight wires carry equal currents I to the right as shown below. Each pair of wires is a distance a apart. The direction of the magnetic force on the middle wire
A. is up out of the plane of the wires. B. is down into the plane of the wires. C. is in the plane of the wires, directed upwards. D. is in the plane of the wires, directed downwards E. cannot be defined, because there is no magnetic force on the middle wire.
A nanometer is
a. one thousandth of a meter. b. one millionth of a meter. c. one billionth of a meter. d. one trillionth of a meter. e. none of the above.
An electrical transmission line of 1.2 cm diameter carries a current of 200 Amps and has a resistance of 3 × 10–4 ohm per meter of length. If the air around this line is at 16°C, determine the surface temperature on a windy day, assuming a wind blows across the line at 33 km/h.
GIVEN
• An electrical transmission line on a windy day
• Line outside diameter (D) = 12 cm = 0.012 m
• Current (I) = 200b Amps
• Resistance per unit length (Re/L) = 3 × 10–4 ohm/m
• Air temperature (Tb) = 16°C
• Air velocity (V) = 33 km/h = 9.17 m/s
FIND
• The line surface temperature (Ts) ASSUMPTIONS
• Steady state conditions
• Air flow approaching line has low free-stream turbulence
SKETCH
PROPERTIES AND CONSTANTS
Thermal conductivity (k) = 0.0248 W/(m K) Kinematic viscosity (?) = 15.3 × 10–6 m2/s Prandtl number (Pr) = 0.71
Thermodynamic Devices: A heat pump absorbs heat from the atmosphere at a rate of 30 kW. If work is being done to run this heat pump at a rate of 7.7 kW, what is the coefficient of performance (COP) of the heat pump?
A. 3.9 B. 4.9 C. 2.9 D. 0.26 E. 22