EmSAT Physics 4

Question 1

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

Accepted Answer

1080 kJ

Answer

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?

Accepted Answer

1/8

Answer

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

Question 3

Which type of electromagnetic radiation has the highest frequency?

Accepted Answer

Gamma rays

Answer

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?

Accepted Answer

22.5 cm in front

Answer

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?

Accepted Answer

Refraction

Answer

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

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