CHAPTER 6 ALTERNATING CURRENT

Transcription

1 HDR102 PHYSICS FOR RADIOGRAPHERS 1 CHAPTER 6 ALTERNATING CURRENT PREPARED BY: MR KAMARUL AMIN BIN ABDULLAH SCHOOL OF MEDICAL IMAGING FACULTY OF HEALTH SCIENCES

2 LEARNING OUTCOMES At the end of the lesson, the student should be able to:- Explain what is electrical current including the concept of electron and current. Differentiate between alternating and direct current including the source and usage. Explain the generation of alternating current. Describe the principles and build up of generators. Explain the single and three phases generators. Explain the sine wave including the frequency, distance and amplitude with factors affecting it. Slide 2 of 52

3 OUTLINES INTRODUCTION 6.1 Electrical Current 6.2 Alternating and Direct Current 6.3 The Principles and Build Up of Generators 6.4 Generation of Alternating Current 6.5 One and Three Phases Generators 6.6 Sine Waves 6.3 References Slide 3 of 52

4 INTRODUCTION The concept of electron and current What is an electric current? What are charge carriers? Slide 4 of 52

5 6.1 Electrical Current For current to flow you must have a complete circuit. A power source is needed to create a POTENTIAL DIFERENCE. When these conditions are met free electrons move about inside the metal. Direction of current flows in the circuit. Direction of electrons flow in the circuit. Slide 5 of 52

6 6.1 Electrical Current The unit of current is the Ampere (A) The symbol for current is I The unit of charge is the coulomb (C) The coulomb is defined as equal to the charge flow in one second when the current is one Ampere. The symbol of charge is Q The equation is Q = It Slide 6 of 52

7 6.2 Alternating and Direct Current Direct Current (DC) The current from a battery is always in the same direction. One end of the battery is positive and the other end is negative. The direction of current flows from positive to negative. This is called direct current, or DC. Figure 1: DC circuit and waveform. Slide 7 of 52

8 6.2 Alternating and Direct Current Alternating Current If voltage alternates, so does current. When the voltage is positive, the current in the circuit is clockwise. When the voltage is negative the current is the opposite direction. This type of current is called alternating current, or AC. Figure 2: AC circuit and waveform. Slide 8 of 52

9 6.2 Alternating and Direct Current Advantages of Alternating Current (AC) AC current is used for almost all high-power applications because it is easier to generate. It is cheaper than Direct Current (DC). It can be transmitted over long distances. The value of voltage AC can be changed easily, using transformer. Slide 9 of 52

10 6.3 The Principles and Build Up Generators The basic principle behind generator is Faraday s law itself when ever:- a) a conductor cut a magnetic flux and an emf is induced in it and b) if the circuit is closed a current will through it. Figure 3: The generator. Slide 10 of 52

11 6.3 The Principles and Build Up Generators Therefore the essential parts of a generator include magnetic field and conductors to cut the magnetic flux produced by the field. Figure 4: The AC generator. Slide 11 of 52

12 6.4 Generation of AC A = is moving along (parallel to) the lines of force - cutting NO lines of force, NO emf is induced. B = it cuts more and more lines of force per second/ more directly across the field (lines of force), the emf induced in the conductor is maximum. Slide 12 of 52

13 6.4 Generation of AC C = the loop is once again moving in a plane parallel to the magnetic field, and no emf is induced in the conductor D = it cuts fewer and fewer lines of force. The induced emf (voltage) decreases from its peak value. Slide 13 of 52

14 6.5 Single and Three Phase Generators A generator is a device that converts mechanical energy (motion) into electrical energy (current voltage). Slide 14 of 52

15 6.5 Single and Three Phase Generators Basic Generator The basic generator consists of a loop of wire wound on an armature drum residing within a magnetic field (B) produced by a permanent magnet. Each end of the loop is connected to a slip ring which conducts electricity. Attached to each slip ring are electrical contactors called brushes. Figure 5: The basic generator. Slide 15 of 52

16 6.5 Single and Three Phase Generators Single Phase Generator A generators with a single source or AC voltage is called a Single-Phase generator. d l Flux density B [T] Figure 6: Generator for single phase Figure 7: Current produced at terminal Slide 16 of 52

17 6.5 Single and Three Phase Generators Three Phases Generator Three-Phase Generator is a generator built with three loops at 120 degrees. Figure 8: Instead of using one coil only, three coils are used Slide 17 of 52

18 6.6 Sine Waves Waves transfer energy from one point to another without transferring matter. They consist of disturbances which transfer the energy in the direction the wave travels without transferring matter. The basic shape of the wave generated is that of a sine wave. Slide 18 of 52

19 6.6 Sine Waves Cycle Is composed of several repeating parts called "cycles". One complete cycle having a maximum value above, and a maximum value below, the reference line. One cycle has one crest (peak +ve) and one trough (peak ve). Figure 9: A sine wave. Slide 19 of 52

20 6.6 Sine Waves Wavelength Is the distance between peak to another peak/ crest to crest/ trough to trough. Wavelengths vary from a low to high frequencies. express wavelengths in meters. Figure 9: A sine wave. Slide 20 of 52

21 6.6 Sine Waves Amplitude The height of a wave crest above the reference line is called the amplitude of the wave. The amplitude of a wave gives a relative indication of the amount of energy the wave transmits - in other words the signal's strength. Figure 9: A sine wave. Slide 21 of 52

22 6.6 Sine Waves Frequency The number of cycles of a continuous wave per unit of time is called the frequency of the wave Is measured in Hertz. One Hertz (abbreviated Hz) is one cycle per second. Therefore, if 5 waves pass a point in one second, the frequency of the wave is 5 cycles per second or 5 Hz. Figure 9: A sine wave. Slide 22 of 52

23 6.6 Sine Waves Factors Affecting the Waves Factors that reduce the range include:- Signal attenuation. This is a fancy way of saying, the farther a signal goes, the weaker it gets. Attenuation can be by absorption, reflection, or refraction of the radio wave energy. Interference. The atmosphere is FULL of EM waves of all kind - the more there are in the area the radio is being used, the more interference there is and the poorer the range and quality of transmission and reception. Slide 23 of 52

24 6.6 Sine Waves Factors Affecting the Waves Factors that increase range include: The ability of radio waves to penetrate through or bend around certain obstructions. The power output of the radio - the greater the power the less the attenuation of the waves. Antenna height - obviously the taller the transmission and reception antennae, the greater the range. Slide 24 of 52

25 SAMPLE OF ACTIVITY Quiz Answer the questions. Which of the following statements BEST describe wavelength? It is the time taken for the electromagnetic field to undergo one complete cycle of oscillation It is the number of waves that passes a particular point in a given time frame. It is the distance between two successive points in a wave. It is the intensity of the wave defined by its maximal height Slide 25 of 52

26 SUMMARY The unit of current is the Ampere (A) The unit of charge is the coulomb (C) The coulomb is defined as equal to the charge flow in one second when the current is one Ampere. The Direct Current is always in the same direction. In Alternating Current, if voltage alternates, so does current. A generator is a device that converts mechanical energy (motion) into electrical energy (current voltage). Slide 30 of 52

27 NEXT SESSION PREVIEW CHAPTER 7: TRANSFORMERS In chapter 7, students will be introduced to the principles and concepts of transformer. Slide 31 of 52

28 6.7 References No. REFERENCES 1 Ball, J., Moore, A. D., & Turner, S. (2008). Essential physics for radiographers. Blackwell. 2 Bushong, S. C. (2008). Radiologic science for technologists. Canada: Elsevier. Slide 32 of 52

29 APPENDIX FIGURE Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 SOURCE B7-2b.JPG pg Slide 33 of 52