Mean drift velocity

A student experiments with microwaves emitted from a transmitter. The frequency f of the microwaves from the transmitter can be adjusted.

The microwaves are produced by an alternating current in the transmitter.

In one experiment, f is 11 GHz. In a wire in the transmitter, the magnitude of the maximum alternating current is 20 mA. The wire has cross-sectional area 1.4 × 10^–8 m² and is made of a metal with free electron number density 8.0 × 10²⁸ m^–3.

The student models the average motion of the free electrons in the wire as simple harmonic motion.

Use your answer to (i) to calculate the amplitude A of this motion.

Without further calculation, explain how the maximum acceleration of a free electron varies as the frequency f is adjusted, provided that the maximum alternating current remains constant.

The student connects two microwave transmitters X and Y, and places them in front of a microwave detector D, as shown in the diagram below.

The transmitters X and Y produce coherent vertically polarised microwaves with the same frequency f.

The detector D is sensitive to vertically polarised microwaves only.

When the detector D is moved along the line PQ, a pattern of maximum and minimum intensity is observed. Adjacent maxima are separated by a distance x.

* Explain:

• why this pattern of intensity occurs

• the expected relationship between the frequency f and the distance x

• how to verify this relationship experimentally.

The circuit diagram of an electrical circuit is shown below.

The positive terminals of the batteries are connected together. One battery has electromotive force (e.m.f.) 4.5 V and internal resistance 0.80 . The other battery has e.m.f. 2.4 V and internal resistance 0.50 . R is a coil of insulated wire of resistance 1.2 at room temperature.

The switch S is closed.

On the diagram, draw an arrow to show the direction of the conventional current.

Calculate the current I shown by the ammeter.

The current measured by the ammeter is smaller than that calculated in (ii). This is because the temperature of R increased due to heating by the current.

Without any changes to the circuit itself, state and explain what practically can be done to make the measured current the same as the calculated current..

Which sequence shows the materials arranged in the order of increasing number density of charge carriers?

A conductor, insulator, semiconductor

B conductor, semiconductor, insulator

C insulator, semiconductor, conductor

D semiconductor, insulator, conductor

Your answer

Derive the S.I. base units for resistance.

Fig. 16.1 shows the I-V characteristics of two electrical components L and R.

The component L is a filament lamp and the component R is a resistor.

Show that the resistance of R is 40 .