2. HIGH FREQUENCY CURRENT ارتفاع وتيرة الحالية Definition of high frequency current Oscillating system & Oscillating current

3. Definition of H.F.C A high frequency current is a current which alternates البديل so rapidly بسرعة that it does not stimulate حفز motor or sensory nerves الأعصاب الحسية. The frequency more than approximately 500000 cycles per second & it is often termed an oscillating current. High frequency (HF) / radio frequencies are between 3 and 30 MHz's

4. Oscillating Systems - Introduction Each day we encounter many kinds of oscillatory motion, such as swinging pendulum يتأرجح البندول of a clock, a person bouncing on a trampoline, a vibrating guitar string, and a mass on a spring. Oscillation ذبذبة is the repetitive variation الاختلاف المتكررة. Examples include a swinging pendulum and AC power.

5. Examples of oscillating systems A spring A pendulum

6. Pushing a swing is a simple example of using a “driving” force قيادة " قوة to produce oscillations. (Producing Oscillations) The number of cycles per unit time is called the “frequency” تردد(f). 1 Hz = 1 cycle/sec.

7. Oscillating system تتأرجح نظام The magnitude of the maximum displacement from equilibrium توازن is called the amplitude سعة of the motion.

8. Oscillating system تتأرجح نظام In an oscillating system such as the oscillation of a simple pendulum, the oscillation does not continue with the same amplitudes indefinitely.

9. Explaining simple Oscillating system نظام بسيط تأرجح 1.The simplest mechanical oscillating system is a mass attached to a linear spring subject to no other forces. 2.The system is in an equilibrium state توازن الدولة when the spring is static. 3.If the system is displaced from the equilibrium, there is a net restoring force on the mass, tending to bring it back to equilibrium. 4.However, in moving the mass back to the equilibrium position, it has acquired momentum which keeps it moving beyond that position, establishing a new restoring force in the opposite sense.

10. The flow of energy تدفق الطاقة As the pendulum oscillates there energy flows: Kinetic → Potential → Kinetic → Potential … Here the bob stops (soon to turn back) Energy is in the form of Stored one & is called as =potential energy الطاقة الكامنة Here the bob reaches its highest speed & is called as =kinetic energy الطاقة الحركية amplitude

11. When we add a small damping force, the amplitude gradually تدريجيا decreases النقصان to zero but the frequency changes by a negligible amount. For real oscillator, there may be friction, air resistance act on the system, the amplitude will decrease.

12. Driving f x fofo This loss in amplitude الخسارة في السعة is called “damping” التخميد and the motion is called “damped harmonic motion”. Amplitude Small damping Large damping

13. compare the motion of un-damped غير مخمد and damped ثبط oscillators. x t t x

14. An Oscillating current It is produced أنتج by discharging تفريغ a condenser مكثف through a low ohmic resistance أومية مقاومة منخفضة containing an inductance. ملف محاثة

15. An Electromagnetic LC Oscillator An Electromagnetic الكهرومغناطيسية LC Oscillator مذبذب Capacitor initially charged. متهم Initially, current is zero, energy is all stored in the capacitor. A current الحالية gets going, energy gets split between the capacitor مكثف and the inductor. Capacitor discharges completely, yet current keeps going. Energy is all in the inductor. The magnetic field المجال المغنطيسي on the coil starts to collapse, which will start to recharge إعادة شحن the capacitor. Finally, we reach the same state we started with (with opposite polarity) and the cycle restarts.

16. An Electromagnetic LC Oscillator The electrons flow تدفق الالكترونات from one plate to the other & back again, passing forwards & backwards several times with diminishing force until the current dies away. The oscillations of the current is due to the self- induced EMF الذاتي الناجم عن المجالات الكهرومغناطيسية which is set up in the inductance coil as the current varies in intensity. The amplitude of each flow of current is less than that of the preceding one as there is some loss of energy in the circuit.

17. The frequency of the oscillations can be calculated by the formula;- F=1/2∏√LC F = Frequency ( Number of cycles per second) C = Capacity of condenser, measured in farads L = Inductance of the circuit, measured in Henries. The frequency of the oscillating current is Inversely proportional تناسبا عكسيا to the product of the capacity of the condenser & the inductance of the circuit.

18. Physical properties الخواص الفيزيائية of H.F.Current 1.Damping of oscillations التخميد من التذبذبات : - As system oscillates Energy lost to the surrounding medium Each excursion is less than the preceding.

19. Physical properties of H.F.Current 2. Heat Production إنتاج الحرارة : – In accordance with Joules Law. 3. Electromagnetic waves موجات كهرومغناطيسية :- As the H.F. current oscillates, electromagnetic waves are setup. These has the same frequency as the current & are wireless or Hertzian waves. Note:- Electromagnetic waves were first postulated by James Clerk Maxwell and subsequently confirmed by Heinrich Hertz. 1884, James Clerk Maxwell

20. As an oscillating electric field generates an oscillating magnetic field, the magnetic field in turn generates an oscillating electric field, and so on. These oscillating fields together form an electromagnetic wave in accordance with - Maxwell's equations. 1888, Heinrich Hertz was the first to generate and detect electromagnetic waves in a laboratory setting