For each question: list what you know, decide which equation to use, then use FIFA. The solution starts by showing how to pick the equation. Speed of electromagnetic waves = 3.0 × 108 m/s.
Q1 (F) A wave has a frequency of 50 Hz and a wavelength of 0.40 m. Calculate its speed.
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Which equation? Know frequency and wavelength → v = fλ
I: v = 50 × 0.40
A: 20 m/s
Q2 (F) A wave has a frequency of 25 Hz. Calculate its period.
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Which equation? Frequency and period → T = 1 / f
I: T = 1 ÷ 25
A: 0.040 s
Q3 (F) Sound travels at 340 m/s. A note has a frequency of 170 Hz. Calculate its wavelength.
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Which equation? Know speed and frequency → v = fλ
I: 340 = 170 × λ
F: λ = 340 ÷ 170
A: 2.0 m
Q4 (F) A wave has a period of 0.020 s. Calculate its frequency.
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Which equation? → T = 1 / f
I: 0.020 = 1 ÷ f
F: f = 1 ÷ 0.020
A: 50 Hz
Q5 (F/H, S only) A lens forms an image 12 mm tall of an object 4.0 mm tall. Calculate the magnification.
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Which equation? Image and object heights → magnification = image height ÷ object height
I: magnification = 12 ÷ 4.0
A: 3 (no units)
Q6 (F/H) A radio station broadcasts at 1.0 × 108 Hz. Calculate the wavelength.
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Which equation? Electromagnetic wave, know speed and frequency → v = fλ
I: 3.0 × 108 = 1.0 × 108 × λ
A: 3.0 m
Q7 (F/H) Ultrasound travels through tissue at 1500 m/s with a wavelength of 0.75 mm. Calculate its frequency.
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Which equation? → v = fλ
Convert: 0.75 mm = 0.00075 m
I: 1500 = f × 0.00075
F: f = 1500 ÷ 0.00075
A: 2.0 × 106 Hz
Q8 (H) A 0.20 m length of wire carrying 3.0 A is at right angles to a magnetic field of 0.50 T. Calculate the force on the wire.
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Which equation? Current in a magnetic field → F = BIl
I: F = 0.50 × 3.0 × 0.20
A: 0.30 N
Q9 (H) A 15 cm wire carrying 2.0 A feels a force of 0.12 N in a magnetic field. Calculate the magnetic flux density.
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Which equation? → F = BIl
Convert: 15 cm = 0.15 m
I: 0.12 = B × 2.0 × 0.15
F: B = 0.12 ÷ 0.30
A: 0.40 T
Q10 (H, S only) A transformer has 1000 turns on the primary coil and 50 turns on the secondary. The primary p.d. is 230 V. Calculate the secondary p.d.
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Which equation? Turns and p.d. → Vₚ / Vₛ = nₚ / nₛ
I: 230 ÷ Vₛ = 1000 ÷ 50
F: Vₛ = 230 × 50 ÷ 1000
A: 11.5 V
Q11 (H, S only) A 100% efficient transformer has a primary p.d. of 230 V and a primary current of 0.50 A. The secondary p.d. is 12 V. Calculate the secondary current, to 2 s.f.
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Which equation? Currents, no turns given, 100% efficient → VₚIₚ = VₛIₛ
I: 230 × 0.50 = 12 × Iₛ
F: Iₛ = 115 ÷ 12 = 9.58…
A: 9.6 A
Q12 (H) Microwaves in an oven have a wavelength of 12 cm. Calculate their frequency, in standard form.
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Which equation? Electromagnetic wave → v = fλ
Convert: 12 cm = 0.12 m
I: 3.0 × 108 = f × 0.12
F: f = 3.0 × 108 ÷ 0.12
A: 2.5 × 109 Hz
