Consider the following statements about the microphone and the speaker of a mobile phone : 1. The microphone converts sound to a mechanical signal. 2. The microphone converts sound to an electrical signal. 3. The speaker converts a mechanical signal to sound. 4. The speaker converts an electrical signal to sound. Which of the statements given above are correct?
- (a)1 and 3
- (b)1 and 4
- (c)2 and 3
- (d)2 and 4
Correct — D, 2 and 4. A microphone is an input device: it takes the pressure variations of a sound wave and delivers a varying voltage that the phone's circuitry can handle, so it converts sound into an electrical signal, which is statement 2. A speaker is the output device and runs the same chain backwards, taking the electrical signal from the circuit and pushing air with it to make sound, which is statement 4. Statements 1 and 3 stop halfway. A diaphragm does move inside the microphone and a cone does move inside the speaker, but movement is an internal stage of each device, not what the device delivers to the rest of the phone.
- (a)1 and 3 — Describes only the mechanical middle of each device and leaves the phone with nothing to work with. A circuit cannot process a moving diaphragm; it needs a voltage, which is what statement 2 supplies.
- (b)1 and 4 — Gets the speaker right and the microphone wrong. If the microphone stopped at a mechanical signal, no electrical output would reach the transmitter and the person at the other end would hear nothing.
- (c)2 and 3 — Gets the microphone right and then breaks the return path. A speaker fed a mechanical signal is a contradiction — what reaches it down the wire is a current, and turning that current into air movement is the whole job.
A transducer is any device that turns energy of one kind into energy of another, and a phone carries a matched pair of them. In a common moving-coil microphone the arriving sound presses on a thin diaphragm attached to a coil sitting in a magnetic field; as the coil moves, a varying voltage is induced in it, so sound energy has become electrical energy. A loudspeaker is the same construction driven in reverse: the varying current in the coil makes it move in the magnetic field, the cone attached to it pushes the air, and electrical energy has become sound.
The item is written so that the two halves of each pair look interchangeable, and the way through is to ask what each device hands to whatever comes next. The microphone hands a signal to the electronics; the speaker hands a sound to the listener. The mechanical stage in statements 1 and 3 is real — sound really does start by moving a diaphragm — but it is machinery inside the device rather than its output, so those statements describe the wrong end of each conversion. The same logic identifies transducers elsewhere: a solar cell turns light into electricity, an electric motor turns electricity into motion, a thermocouple turns a temperature difference into a voltage.
- A microphone converts sound energy into an electrical signal; a loudspeaker converts an electrical signal into sound.
- A moving-coil microphone and a moving-coil loudspeaker are essentially the same device operated in opposite directions.
- The microphone works by electromagnetic induction — a coil moving in a magnetic field generates a voltage.
- The loudspeaker works by the motor effect — a current-carrying coil in a magnetic field experiences a force and moves.
- Any device that turns one form of energy into another is called a transducer.
Ask what each device hands to the next part of the phone, and the pair 2 and 4 falls out at once.
- Choosing the mechanical statement because a diaphragm genuinely does move inside the device.
- Reversing the pair and letting the speaker take sound in.
- Assuming both devices need separate designs, when one is close to the other run backwards.
Usually as a one-line item on what a microphone or a speaker converts, or as a statements item like this that adds a plausible mechanical stage to each.
A microphone converts
- (a) electrical signals to sound waves
- (b) sound waves to electrical signals
- (c) microwaves to sound waves
- (d) sound waves to microwaves
Answer(b) sound waves to electrical signals
Exactly half of this item, asked on its own. Once the microphone is fixed as sound going in and a voltage coming out, the speaker is the same conversion run the other way, and the pair 2 and 4 follows.
- practice — not a real PYQ
A loudspeaker in a mobile phone works mainly on which one of the following principles?
- (a)A current-carrying coil in a magnetic field experiences a force
- (b)A moving charge produces a magnetic field of its own
- (c)Light falling on a semiconductor releases electrons
- (d)A conductor heats up when a current passes through it
Answer(a) A current-carrying coil in a magnetic field experiences a force — the coil and the cone attached to it move, and the moving cone pushes air to make sound.
- practice — not a real PYQ
A solar cell is a transducer that converts
- (a)electrical energy into light energy
- (b)light energy into electrical energy
- (c)heat energy into sound energy
- (d)chemical energy into light energy
Answer(b) light energy into electrical energy — light falling on the semiconductor junction frees charge carriers and drives a current.