Electromagnetic waves

Define — 4 marks

A mobile phone communicates with a nearby transmitter mast using radio waves. These waves travel through the air and carry signals that allow calls and data to be sent and received.

(a) Define what is meant by an electromagnetic wave. [2 marks]
(b) Define the wavelength of an electromagnetic wave. [1 mark]
(c) Radio waves are electromagnetic waves. State one property that all electromagnetic waves share, other than their ability to travel through a vacuum. [1 mark]

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(a) Award 1 mark for: A wave consisting of oscillating electric and magnetic fields (or: vibrating electric and magnetic fields / perpendicular electric and magnetic fields). Award 1 mark for: That propagates / travels through space (or: carries energy through space / transfers energy)
(b) Award 1 mark for: The distance between two adjacent points in phase on a wave (or: the distance between consecutive peaks/troughs / one complete oscillation)
(c) Award 1 mark for any one of: They all travel at the same speed (in a vacuum / 3 × 10⁸ m/s) / They are transverse waves / They can be reflected/refracted/diffracted / They carry energy / They obey c = fλ

Explain — 5 marks

A mobile phone transmits data using electromagnetic waves. These waves travel through the air and are received by a nearby mobile phone mast. The mast then sends the signal to other networks around the world.

(a) Explain what an electromagnetic wave is. [2 marks]
(b) Explain why electromagnetic waves do not need a medium to travel through. [2 marks]
(c) Explain how the mobile phone mast can receive the signal from the mobile phone even when they are separated by a large distance. [1 mark]

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(a) An electromagnetic wave is a transverse wave / has oscillating electric and magnetic fields
(a) The electric and magnetic fields oscillate perpendicular to the direction of travel / perpendicular to each other
(b) Electromagnetic waves are made of oscillating electric and magnetic fields / do not require particles to vibrate
(b) They can propagate through empty space / a vacuum because the fields can exist without a medium
(c) Electromagnetic waves travel at the speed of light and can travel long distances through air / space to reach the mast

Calculate — 2 marks

A local radio station broadcasts using radio waves with a frequency of 200 kHz. All electromagnetic waves travel at a speed of 3.0 × 10^8 m/s in air.

(a) Calculate the wavelength of the radio waves emitted by the station. [1 mark]
(b) The radio signal travels 60 km from the station to a listener's receiver. Calculate the time taken for the signal to travel this distance. [1 mark]

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(a) λ = v ÷ f OR 3.0×10^8 ÷ (200×10^3)
(a) 1500 m / 1.5 km
(b) t = d ÷ v OR 60000 ÷ 3.0×10^8
(b) 2.0×10^-4 s / 0.0002 s

Describe — 2 marks

Microwaves are used to transmit signals between satellites in space and ground stations on Earth. These signals can travel through the vacuum of space to reach orbiting communication satellites.

(a) Describe one property of electromagnetic waves. [1 mark]
(b) Describe how microwaves are used for satellite communication. [1 mark]

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(a) travel at the same speed (300 000 000 m/s) in a vacuum
(a) they are transverse waves
(a) they transfer energy
(a) they can travel through a vacuum
(b) microwaves carry signals to/from satellites
(b) microwaves transmit information between ground and space

Evaluate — 3 marks

A student is researching sun protection for a holiday. They read that ultraviolet waves from the Sun can damage skin, causing sunburn and increasing the risk of skin cancer. The student also learns that infrared waves from the Sun are responsible for the warming sensation on the skin.

(a) State one property that is the same for both ultraviolet waves and infrared waves. [1 mark]
(b) Evaluate why sunscreen is designed to absorb ultraviolet waves but does not need to absorb infrared waves. [2 marks]

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(a) They are both electromagnetic waves
(a) They both transfer energy
(a) They both travel at the speed of light (in a vacuum/air)
(a) They can both travel through a vacuum
(b) UV waves have higher frequency / higher energy (than infrared)
(b) UV (energy) can damage skin cells / cause sunburn / increase risk of skin cancer
(b) Infrared only causes heating / warming (which is not harmful in the same way)

Define — 4 marks

Explain — 5 marks

Calculate — 2 marks

Describe — 2 marks

Evaluate — 3 marks

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