State two practical uses of photo cells.

Here are the solutions to the questions: (b) (i) State two practical uses of photo cells. 1. Automatic street lighting: Photo cells detect ambient light levels and switch streetlights on at dusk and off at dawn. 2. Automatic door openers: They can detect the presence of a person by sensing an interruption in a light beam, triggering the door to open. Other uses include burglar alarms, light meters in cameras, and solar panels for electricity generation. (b) (iii) A scientist uses a radioactive source with a half-life of 4 \, hrs as a clock. If the source initially emits 1220 particles per minute, what time interval has elapsed when he records 160 counts per minute? Step 1: Identify the given values. Initial activity (N_0) = 1220 counts/min Final activity (N) = 160 counts/min Half-life (T_1/2) = 4 \, hrs Step 2: Use the radioactive decay formula to find the number of half-lives (n). The formula for radioactive decay is: N = N_0 ((1)/(2))^n Substitute the given values: 160 = 1220 ((1)/(2))^n Divide both sides by 1220: (160)/(1220) = ((1)/(2))^n 0.1311475 ≈ ((1)/(2))^n Take the natural logarithm of both sides: (0.1311475) = n ((1)/(2)) -2.0316 ≈ n (-0.6931) Solve for n: n ≈ (-2.0316)/(-0.6931) n ≈ 2.931 Step 3: Calculate the total time elapsed (t). The total time elapsed is the number of half-lives multiplied by the half-life period: t = n × T_1/2 t ≈ 2.931 × 4 \, hrs t ≈ 11.724 \, hrs The time interval elapsed is 11.72 \, hrs. 3 done, 2 left today. You're making progress.