EmSAT Physics 4 — Questions and Answers
Question 1: A 60 W light bulb operates for 5 hours. How much electrical energy does it consume?
- 300 J
- 1080 J
- 1080 kJ (Correct answer)
- 300 kJ
Correct answer: 1080 kJ
E = P × t = 60 W × 18000 s = 1,080,000 J = 1080 kJ.
Question 2: The half-life of a radioactive isotope is 8 days. What fraction of the original sample remains after 24 days?
- 1/2
- 1/4
- 1/8 (Correct answer)
- 1/16
Correct answer: 1/8
24 days = 3 half-lives; remaining fraction = (½)³ = 1/8.
Question 3: Which type of electromagnetic radiation has the highest frequency?
- Infrared
- Ultraviolet
- X-rays
- Gamma rays (Correct answer)
Correct answer: Gamma rays
Gamma rays have the highest frequency (and shortest wavelength) in the electromagnetic spectrum.
Question 4: A concave mirror has a focal length of 15 cm. An object is placed 45 cm from the mirror. Where does the image form?
- 22.5 cm in front (Correct answer)
- 30 cm in front
- 45 cm behind
- 22.5 cm behind
Correct answer: 22.5 cm in front
Using 1/f = 1/v + 1/u: 1/15 = 1/v + 1/45 → 1/v = 1/15 – 1/45 = 2/45, so v = 22.5 cm in front.
Question 5: Which phenomenon explains why a straw appears bent when placed in a glass of water?
- Reflection
- Diffraction
- Refraction (Correct answer)
- Dispersion
Correct answer: Refraction
Refraction — the bending of light as it passes from water to air — causes the apparent displacement of the straw.
Question 6: In a series RLC circuit at resonance, which statement is correct?
- Inductive reactance equals resistance
- Capacitive reactance equals resistance
- Inductive reactance equals capacitive reactance (Correct answer)
- Total impedance is maximum
Correct answer: Inductive reactance equals capacitive reactance
At resonance, X_L = X_C, so they cancel and the impedance equals the resistance (minimum impedance).
Question 7: A proton enters a uniform magnetic field perpendicular to the field lines. The resulting path of the proton is:
- A straight line
- A parabola
- A circle (Correct answer)
- A helix
Correct answer: A circle
A charged particle moving perpendicular to a uniform magnetic field follows a circular path because the magnetic force is always perpendicular to its velocity.
EmSAT Physics 4
A 60 W light bulb operates for 5 hours.
How much electrical energy does it consume?