2. Write 10 questions of all kinds to the text

3. Define the key sentences and give a short summary of the text

2 Section 2

2.1 Part 1

2.1.1 Mind the pronunciation of these words, remember them

constant, increase n, decrease n, observe, value, evidently, limitation, potential, surrounding, velocity, overcome, extremely, vary, increase v, decrease v, cathode, thereby, presence, toward, urge, eventually, constitute, sufficient, range, precisely, characteristic, rare, resistance, nevertheless, concept, quantity, ratio, diode, neighborhood, pulse , approximately, succession, unidirectional.

2.1.2 Find 2 conditional sentences in paragraph 2 and define their types

State the function of the infinitive to obtain in paragraph 6

SPACE CHARGE

1. If the plate potential is held constant at E_b1 and the heater current I_f is varied, the lower curve of Figure 1 is obtained. On increasing the plate voltage to a higher value Eb₂ it is observed that the plate current is increased, but only at the higher cathode temperatures (larger values of If). Evidently temperature saturation is responsible for the agreement of the two curves in the lower region. Both curves are known to be nearly horizontal in the upper part, and this limitation of current is caused by the presence of a space charge near the cathode.

2. As electrons are given off by the cathode, they form a cloud of negative charges in the surrounding space. These negative charges lower the potential in that region, and thereby produce a field which ugres the electrons back toward the cathode. If there were no plate, all the electrons emitted would eventually return to the cathode. Even with the plate at a positive potential, only the electrons emitted with the sufficient velocity will have enough energy to penetrate the cloud, and their motion will constitute the plate current. If the plate voltage is increased, it will further overcome the effect of space charge, and thereby produce an increase of plate current.

3. This effect of space-charge limitation is extremely important , as the useful working range of nearly all vacuum tubes is precisely the region in which a mission is limited by temperature saturation are very rare.

4. Although the relationship between plate current and plate voltage is not one of simple proportion and cannot be expressed in terms of Ohm's Law, one may nevertheless speak of the resistance of the internal plate circuit of the tube. The ratio of plate potential to plate current is known as the direct current plate resistance, and is show in Figure 2 as OA/AP. concept of D.C. plate resistance is useful in many cases where the current through the tube is steady or constant, but a much more important quantity is the alternating current plate resistance, or the ratio of change in plate voltage to change in plate current. In yhe neighbourhood of point P of Figure 4 the A.C. plate resistance is given by QB/BR. For many diodes, the D.C. plate resistance is approximately twice as great as the A.C. plate resistance.

5. It is clear from the figure that neither of these resistances is constant for all voltage, and that both tend toward lower values as plate voltage and plate current are increased .

6. If the plate of the diode is made negative with respect to the cathode, the electron will be driven back to the cathode and no plate current will flow. Hence, if an alternating voltage is applied to the plate, current will flow only in the positive half cycles, and it will consist of a succession of pulses, always in the same direction. The tube therefore function as a rectifier, in which an alternating voltage applied to the tube produces a unidirectional current. Wide use is made of this characteristic to obtain direct voltages and currents an A.C. source.

Notes to the text