A device that converts mechanical energy into electrical energy is

Analyze the question: We need a device that changes mechanical energy (motion) into electrical energy.

Concept: Electromagnetic induction (Faraday's law) says that when a conductor moves relative to a magnetic field, or a coil rotates in a magnetic field, an e.m.f. is induced. A generator, also called a dynamo, uses this to turn mechanical energy into electrical energy.

Checking each option:

• A. Dynamo: A coil is rotated in a magnetic field by mechanical means (e.g. a turbine or bicycle wheel), producing electrical energy. This fits.
• B. Electric motor: It does the reverse, converting electrical energy to mechanical energy.
• C. Induction coil: It steps up a low d.c. voltage to a high voltage using a make-and-break circuit. It is an electrical-to-electrical device, not a mechanical one.
• D. Transformer: It changes the size of an a.c. voltage by mutual induction. It converts electrical energy to electrical energy, with no mechanical input.

Answer: A, a dynamo.

Exam tip: Remember the pair "Dynamo/Generator: mechanical → electrical" and "Motor: electrical → mechanical". The words dynamo and generator both mean the same thing in this context, and a motor is simply a generator in reverse.