Companion to the reading passage The Light and the Lightning: The Mind of Nikola Tesla. Answer each question, then click Check. Multiple-choice and number questions grade themselves; for written answers, compare yours with the model answer and checklist, then score yourself honestly.
Almost all of the electrical energy in the power grid comes from generators. In a generator, a coil of copper wire is turned inside a magnetic field. A turbine pushed by steam, falling water, or wind supplies the rotation. A group of students built a small hand-crank generator. They rotated its coil at different steady rates and used a meter to record the peak voltage across the coil’s ends. Their results are shown in Table 1.
Data are representative values for practice.
Table 1: Rotation Rate and Peak Voltage
| Rotation Rate (rotations/s) | Peak Voltage (V) |
|---|---|
| 2.0 | 1.5 |
| 4.0 | 3.0 |
| 6.0 | 4.5 |
| 8.0 | 6.0 |
1. Which graph best represents the relationship between the peak voltage and the rotation rate of the coil?
2. The students want their generator to produce a greater peak voltage. Place a check mark next to three changes that would increase the peak voltage.
In the 1880s and 1890s, Nikola Tesla and George Westinghouse promoted alternating current (AC) over Thomas Edison’s direct current (DC), largely because AC voltage can be changed easily with transformers. Engineers today must decide what voltage to use when sending energy from a power plant to a distant city. A power plant delivers 1.0 × 106 watts to a transmission line. The wires of the line have a total resistance of 5.0 Ω. Table 2 shows the current in the line for three possible transmission voltages.
Data are representative values for practice.
Table 2: Transmission Options for 1.0 × 106 W
| Transmission Voltage (V) | Current in Line (A) |
|---|---|
| 10 000 | 100 |
| 100 000 | 10 |
| 500 000 | 2.0 |
3. Substitute numerical values into a mathematical representation that can be used to determine the power lost as thermal energy in the 5.0-Ω line when the energy is transmitted at 10 000 volts. Then calculate the power lost.
4. An engineer makes the following claim.
Place a check mark in either the Support or Refute box. Justify your response using a mathematical relationship.
High-voltage lines are not free. As transmission voltage increases, the wires must be held farther from the ground and from each other, so towers must be taller. Longer insulators are also needed to keep charge from jumping from the wire to the tower. At each end of the line, transformers change the voltage. A transformer has two coils of wire wrapped around one iron core. A transformer changes voltage only while the current in its first coil is changing, which is why the grid uses alternating current that reverses direction 60 times each second.
5. Which row correctly describes the effects of increasing the transmission voltage while the power plant delivers the same power? (Columns: Current in the Line / Energy Lost as Heat in the Line / Tower Height and Insulation Needed)
6. Which statement best explains why a transformer works only when the current in its first coil is changing?