OCR GCSE Combined Science: Physics: Waves and Radioactivity

The electromagnetic (EM) spectrum is a continuous family of transverse waves that all travel at the same speed in a vacuum but have different wavelengths and frequencies.

Describe how the wavelength, frequency and energy of EM waves change across the spectrum from radio waves to gamma rays, and, for any two named parts of the spectrum, give a use and an associated danger where relevant. (6 marks)

A sound wave travelling through air has a frequency of 170 Hz. The speed of sound in the air is 340 m/s.

(a) Calculate the wavelength of the sound wave. Use $v = f\lambda$v=fλ. Show your working and give the unit. (3 marks)

(b) State whether a sound wave is transverse or longitudinal. (1 mark)

A radioactive isotope of caesium has a half-life of 30 years. A sealed source initially contains this isotope with an activity of 800 Bq.

(a) Calculate the activity of the source after 90 years. Show your working. (2 marks)

(b) State what is meant by the half-life of a radioactive isotope. (1 mark)

Uranium-238 is a radioactive isotope that decays by emitting an alpha particle to form an isotope of thorium (Th). The decay is represented by the equation below, in which $a$a and $b$b are missing numbers:

$^{238}_{92}\text{U} \;\rightarrow\; ^{a}_{b}\text{Th} + ^{4}_{2}\text{He}$92238​U→ba​Th+24​He

(a) State the value of $a$a (the mass number of the thorium nucleus) and the value of $b$b (its atomic number). (2 marks)

(b) State what an alpha particle is made of. (1 mark)

Carbon-14 is a radioactive isotope that decays by emitting a beta particle to form nitrogen (N). The decay is shown below:

$^{14}_{6}\text{C} \;\rightarrow\; ^{14}_{7}\text{N} + ^{0}_{-1}\text{e}$614​C→714​N+−10​e

Explain, using the equation, why the atomic number increases from 6 to 7 during beta decay, even though the mass number stays the same. (2 marks)

Two atoms are described as isotopes of the same element.

State what is meant by isotopes. (1 mark)