ACT Science Practice Test 26

Every time a lightbulb is switched on, an electrical circuit is formed. When plugged into outlets that provide a certain voltage, current begins to flow through the bulbs. Current depends on the resistance of the bulb and the voltage of the power supply. The power output of a bulb is a measure of the amount of energy the bulb requires each second. Power is calculated by multiplying current and voltage. Table 9.3 contains data that relate these variables for a basic circuit consisting of one lightbulb and a power supply.

TABLE 9.3

Figure 9.2 shows three configurations of bulbs. When the bulbs are connected in series, they form one path to the power supply. If any bulb in the pathway breaks, all the lights go out because the circuit is no longer complete. In contrast, bulbs connected in parallel are all independently connected to the power supply-in essence, forming their own circuits. Bulbs wired in parallel across a power supply continue to work even when one bulb goes out because each branch forms an independent circuit.

Figure 9.2

Table 9.4 shows how the number of bulbs in series and parallel affect the current and power values. It gives data for 240-ohm bulbs connected to a 120-V power supply.

TABLE 9.4

Since ancient times, scientists, philosophers, and other thinkers considered the smallest pieces of matter to be tiny spherical structures called atoms that were stacked in various arrangements. The ancient Greek philosopher Democritus also suggested that these atoms were indivisible and indestructible. The turn of the twentieth century brought with it a renewed interest in exploring atoms. Two ground-breaking experiments challenged the idea that atoms are the smallest constituent of matter. These experiments were very different, but both pointed to the fact that atoms contained even smaller parts.

Experiment 1: The 1897 Cathode Ray Tube Experiments

Scientists in the late 1890s investigated a curious new device known as a cathode ray tube (CRT). The CRT consisted of a glass tube with a metal wire coming out of each end. When all of the air was removed from the tube and a voltage was applied across the wires, mysterious green rays appeared at one end of the tube. These rays were called cathode rays. It was not immediately known if these rays were a type of wave or a type of particle, but several experiments were conducted on them. First it was discovered that the rays would always be attracted to an area of excess positive charge and away from an area of excess negative charge. Precise measurements could not determine the exact mass of the cathode rays, but it was determined that they did have mass. After that discovery, the rays were considered a particle instead of a wave.

The mass-to-charge ratio of the cathode rays was determined to be more than 1,000 times smaller than the same ratio for any known atom or ion. After subsequent experiments, researchers determined that cathode rays were small particles that broke off of an atom when a voltage was applied.

Experiment 2: The 1911 Gold Foil Experiment

At the time, the accepted model of the atom was a relatively solid sphere, similar to a ball of chocolate chip cookie dough. An experiment in 1911 brought that model into question. A beam of alpha particles (small, high-energy, positively charged particles) was shot into a piece of gold foil approximately 8.6 × 10^-8 m in thickness. The alpha particles were thought to have enough energy to pass straight through the foil and hit a detector on the other side, and most of the particles did just that. However, a small fraction of alpha particles were deflected a few degrees as they passed through the foil. Upon closer examination of the data, a more startling fact was found-some alpha particles never hit the detector.

More detectors were added around the gold foil, and it was discovered that a tiny portion of the alpha particles, 1 out of every 20,000 particles, was deflected 90 degrees or more from the beam. Some particles even bounced straight back toward the alpha particle source. The scientist was so surprised by the results that he stated, "It was as if you fired a 15-inch shell at a sheet of tissue paper and it came back to you." It was concluded that there must be some particle inside an atom causing these major deflections of alpha particles.