Companion to the reading passage The Stone That Points North: A History of Magnetism in Human Hands. 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.
In 1820, Hans Christian Oersted noticed that a compass needle swung when an electric current flowed in a nearby wire. A group of students repeated his investigation with modern equipment. They kept a steady current of 10 amperes in a long, straight wire. They used the magnetometer sensor in a smartphone to measure the strength of the magnetic field produced by the wire at different distances from it. The students subtracted the effect of Earth’s magnetic field from each reading. Their results are shown in Table 1.
Data are representative values for practice. 1 μT = 1 × 10−6 tesla.
Table 1: Magnetic Field Near a Wire Carrying 10 A
| Distance from Wire (m) | Magnetic Field Strength (μT) |
|---|---|
| 0.010 | 200 |
| 0.020 | 100 |
| 0.040 | 50 |
| 0.080 | 25 |
1. Which graph best represents the relationship between magnetic field strength and distance from the wire?
2. A student claims that the magnetic field around the wire behaves like gravity: doubling the distance from the wire reduces the magnetic field strength to one-fourth of its original value.
Place a check mark in either the Support or Refute box. Justify your response with evidence from Table 1.
A compass needle is a small, freely turning magnet. Far from any current-carrying wire, it lines up with Earth’s magnetic field, which has a strength of about 50 μT at Earth’s surface. Around 1600, William Gilbert proposed that Earth itself behaves like a giant magnet. Science textbooks often show Earth’s magnetic field using Model 1.
Model 1: A bar magnet is drawn inside a circle that represents Earth. The magnet’s south pole points toward Earth’s geographic North Pole. Curved magnetic field lines leave one end of the magnet, loop around the outside of Earth, and enter the other end. Compass needles drawn on Earth’s surface line up along these field lines.
3. Sketch (on paper) the magnetic field around a single bar magnet labeled N and S. Draw at least four field lines, and include arrows on the field lines to show the direction of the magnetic field. Then describe your sketch below: where the lines start and end, which way the arrows point, and where the lines are closest together.
4. Which statement describes a limitation of Model 1?
A magnetic resonance imaging (MRI) scanner surrounds a patient with a strong, steady magnetic field of 1.5 tesla, about 30 000 times stronger than Earth’s field. This field is produced by an electromagnet: a large coil of wire carrying a current of several hundred amperes. The scanner then sends pulses of radio waves with a frequency of about 6.4 × 107 hertz into the body. Hydrogen nuclei in the body’s water absorb and re-emit these radio waves, and a computer uses the signals to build an image of soft tissue. Because the field is so strong, iron and steel objects are never allowed in the scanner room.
5. Which row describes three changes that would each increase the strength of the magnetic field produced by an electromagnet? (Columns: Current in the Coil / Number of Turns of Wire / Material Inside the Coil)
6. Substitute numerical values into a mathematical representation that can be used to determine the wavelength in air of the 6.4 × 107-hertz radio waves used by the MRI scanner. Then calculate the wavelength. (Radio waves travel at about 3.00 × 108 m/s in air.)