If a different current source caused RL to dissipate power into the water at a rate of 1.0 W. how long would it take to increase the temperature of the water by 1.0°C? [ Note:Assume that the heat used to heat the heating element and insulation is negligible.]
A. 70s
B. 420 s
C. 700 s
D. 4200 s
Read the passage and then answer the following questions:
Electric power is generally transmitted to consumers by overhead wires. To reduce power loss due to heat, utility companies strive to reduce the magnitudes of both the current ( I) through the wires and the resistance ( R) of the wires.
A reduction in R requires the use of highly conductive materials and large wires. The size of wires is limited by the cost of materials and weight. The table lists the resistances and masses of 1000-m sections of copper wires of different diameters at two different temperatures.
Diameter | Resistance | Resistance | Mass |
(m) | per 103 m at 25°C (Ω) | per 103 m at 65°C (Ω) | per 103 m (kg) |
6.6 × 10-2 | 7.2 × 10-3 | 8.2 × 10-3 | 2.4 × 104 |
2.9 × 10-2 | 3.5 × 10-2 | 4.1 × 10-2 | 4.6 × 103 |
2.1 × 10-2 | 7.1 × 10-2 | 8.2 × 10-2 | 2.3 × 103 |
9.5 ×10-3 | 3.4 × 10-1 | 3.8 × 10-1 | 4.9 × 102 |
Safety and technical equipment considerations limit voltage. Because electricity is transmitted at high-voltage levels for long-distance transmission, transformers are needed to lower the voltage to safer levels before entering residences.
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