Students should be able to: describe, with examples, processes which…

The idea

This is the skill the section builds to, and it is Higher tier physics only. You need a worked example in both directions. Sound to vibration: pressure variations in air push on the ear drum, the drum vibrates, the ossicles pass the vibration to the cochlea, and nerve impulses travel to the brain. Vibration to sound: in a loudspeaker a varying current in a coil sitting in a magnetic field feels a varying force, so the coil and cone vibrate, and the moving cone pushes on the air to create the compressions and rarefactions of a sound wave. Each converter works only over a limited frequency range because the vibrating solid has mass and stiffness and cannot follow oscillations that are too fast. For the ear this sets the audible range at 20 Hz to 20 kHz.

What earns the marks

• Saying the current in the loudspeaker circuit is varying and passes through a coil in a magnetic field.
• Saying the coil experiences a force whose size and direction follow the current, so the cone vibrates.
• Linking the vibrating cone to pressure variations, compressions and rarefactions, in the air.
• Explaining that the conversion works only over a limited frequency range and relating that to 20 Hz to 20 kHz for the ear.

Common mistakes and how to put them right

• Describing the loudspeaker cone as making sound energy rather than pressure variations in the air.

• Giving only one direction of conversion when the question asks for processes in both.

• Omitting the magnetic field, so the force on the coil has no cause.

• Stating the frequency limit without saying what physically causes it.

Exam tips

• These are often six mark levels questions; sequence your points so each one follows from the last.
• Name the medium at each step, air or solid, so the conversion between them is explicit.
• Finish with the consequence for hearing, since the specification pairs the process with the frequency range.