ACT Science Practice Test 51

PASSAGE II

The Earth's magnetic field is one of the planet's most significant natural phenomena. For centuries, the field has been used to aid navigation and exploration, and has been vital to many major discoveries. The magnetic field of the Earth extends several thousands of miles into space. It has the effect of shielding the Earth from solar wind, protecting the planet from dangerous high-energy particles and radiation. The exact source of the Earth's magnetic field is not certain. The following two Scientists attempt to explain the phenomenon.

Scientist 1

The Earth's magnetic field is similar to that of a bar magnet tilted 11 degrees from the spin axis of the Earth. The magnetic field of a bar magnet, or any other type of permanent magnet, is created by the coordinated motions of electrons within iron atoms. It is widely accepted that the Earth's core consists of metals. The inner core is 70% as wide as the moon and consists of a solid iron ball, which would exhibit properties of ferromagnetism (the natural magnetic tendency of iron). The core has its own rotation and is surrounded by a "sea" of molten rock. The magnetic field grows and wanes, and the Earth's poles drift and occasionally flip as the rotation of the core changes. The poles of the magnetic field have "flipped" many times due to the fluctuations in the rotation of the solid inner core. Other fluctuations in the magnetic field that can occur on a daily basis are largely the result of interference by solar wind.

Scientist 2

The Earth's magnetic field is attributed to a dynamo effect of circulating electric current in the molten outer core. Electric currents cause magnetic fields; therefore, the circulating electric currents in the Earth's molten metallic core are the origin of the magnetic field. When conducting fluid flows across an existing magnetic field, electric currents are induced, creating another magnetic field. When this magnetic field reinforces the original magnetic field, a dynamo is created which sustains itself.

Sitting atop the hot, iron inner core, the Earth's molten outer core churns and moves. The outer core also has cyclones or whirlpools powered by the Coriolis effects of Earth's rotation. These complex and unpredictable motions generate the fluctuating magnetic field. The outer core is seething, swirling, and turbulent, which has been detected by the constant changes and reversals in polarity throughout the planet's history. Further, iron has a special characteristic. When it is hotter than 1043 K, its Curie temperature, iron loses its magnetic properties. Therefore the Earth's magnetic field is caused not by magnetised iron deposits, but mostly by electric currents in the liquid outer core.

PASSAGE III

Radioactive decay is a natural process by which an atom of a radioactive isotope (chemical element) spontaneously decays into another element. The unstable nucleus disintegrates by emitting alpha or beta particles, or gamma rays. This process changes the composition of the nucleus and continues to take place until a stable nucleus is reached. Half-life refers to the amount of time it takes for half (50%) of the atoms in a sample to decay.

Figure 1 shows the decay from Radon 222 to Polonium 218 and other decay products.

Figure 1

Figure 2 shows the decay from Mercury 206 to Thallium 206 to Lead 206.

Figure 2

Table 1 shows decay products and associated energy in MeV, million electron volts, and the type of particle emitted from the decay.