Physics Flashcards

A 60 W light bulb operates for 5 hours. How much electrical energy does it consume?

Answer: 1080 kJ

E = P × t = 60 W × 18000 s = 1,080,000 J = 1080 kJ.

The half-life of a radioactive isotope is 8 days. What fraction of the original sample remains after 24 days?

Answer: 1/8

24 days = 3 half-lives; remaining fraction = (½)³ = 1/8.

Which type of electromagnetic radiation has the highest frequency?

Answer: Gamma rays

Gamma rays have the highest frequency (and shortest wavelength) in the electromagnetic spectrum.

A concave mirror has a focal length of 15 cm. An object is placed 45 cm from the mirror. Where does the image form?

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.

Which phenomenon explains why a straw appears bent when placed in a glass of water?

Answer: Refraction

Refraction — the bending of light as it passes from water to air — causes the apparent displacement of the straw.

In a series RLC circuit at resonance, which statement is correct?

Answer: Inductive reactance equals capacitive reactance

At resonance, X_L = X_C, so they cancel and the impedance equals the resistance (minimum impedance).

A proton enters a uniform magnetic field perpendicular to the field lines. The resulting path of the proton is:

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.