Electricity & Magnetism Lecture Notes PDF

Electricity & Magnetism FUNDAMENTALS OF RADIATION AND RADIATION SAFETY MODULE TIMELINE 2024-25 Mock...

Electricity & Magnetism FUNDAMENTALS OF RADIATION AND RADIATION SAFETY MODULE TIMELINE 2024-25 Mock Assessment Seminar Seminar Seminar 3 1 2 MCQ Exam Lecture Lecture Lecture Lecture 2 4 6 7 Lecture Lecture Lecture Lecture Lecture Lecture 1 3 5 8 9 10 Holidays SEPTEMBER OCTOBER NOVEMBER DECEMBER JANUARY Virtual 1 3 Simulations 2 1 3 On-Campus Simulations 2 TOPICS KEY: Skills for module Fundamentals of completion Radiation Radiation Safety Radiation Protection Assessment Skills & in Context Assessment Module Learning Objectives This Photo by Unknown Author is licensed under CC BY-SA This Photo by Unknown Author is licensed under CC BY-SA This Photo by Unknown Author is licensed under CC BY- NC-ND Today’s learning outcomes Understand the principles of Understand the nature of charge magnetism Understand the relationship Understand how electric current between electric current and flows magnetic fields Have an awareness of how electrical Have an awareness of how components can be used to control electromagnetic induction is used an electrical supply in the production of X-rays  An understanding of electricity and magnetism is essential in understanding the function and operation of equipment used in diagnostic radiography X-ray tubes, Image intensifiers TV systems IT systems generators and the X- ray circuit Radiation detection Magnetic Resonance devices Imaging Song of the week! Rate the song! ⓘ Start presenting to display the poll results on this slide. Understand the nature of charge Electricity There are two types of The SI unit of charge is the electric charge Coulomb (C) The smallest unit of 1 coulomb is equivalent to the negative charge is the charge on an electron (e) charge on 6 x 1018 electrons The smallest unit of That’s positive charge is that of 600,000,000,000,000,0000 a proton (p) electrons! Charge Electrical charges exert forces on each other even when they are separated by a vacuum The forces are mutual, equal and opposite Like charges repel each other while unlike charges are attracted to each other. Two small balls of metal foil suspended by thread behave in the following manner when charges (q) are applied to them Unlike No charge charges applied applied + - Like charges applied + + - - What will affect the magnitude of the attraction and repulsion? ⓘ Start presenting to display the poll results on this slide. The magnitude of these mutual forces of attraction and repulsion are influenced by The magnitude of the The inverse square of the individual charges (q) distance between the charged bodies (F ∝ 1/d2) Greater charge -> greater force (F ∝ q1xq2) The medium in which they are embedded At a maximum in a vacuum Understand how electric current flows  What is an electric field? In an electric field, charge Electric field strength is defined as: experiences an electric force intensity of an electric field at a particular location September 2024 Electricity & magnetism 16 A Simple Circuit… Voltage Voltage Switch Switch open closed A A Circuit broken - Circuit complete – No current flows Current flows A practical example: The electric field between two electrodes looks like this………… It would move (accelerate) under the influence of the electrostatic force along the Cathode Anode lines of the electric field, to the e- positive electrode, the anode and gain kinetic energy whilst it was moving What would happen if an electron (-) was placed at the negatively charged electrode? Movement of Current Electrical energy is that which enables an electron to move within a given system Electrical potential is when positive & negative charges are separated In an electric circuit there is a positive terminal (anode) and a negative terminal (cathode) Electrons move through the circuit from the cathode to the anode Current The SI unit of current is the ampere (A) Charge (C) = Current (A) x Time (s) ampere second (As) or for small charge milliamp second (mAs) The coulomb is therefore the electric charge delivered by 1 ampere constant current in 1 second  Some examples: Describe as fully as you can what happens in each situation 1 volt What would happen if the voltage was increased to: 1kilovolt (kV)? 50 kilovolts (kV)? What name is given to the phenomenon of (a) flowing electron(s)? 100 volts Potential Difference The potential Potential difference is In an electric energy of the the difference in the circuit charge electron’s charge amount of energy that moves under the is converted to charge carriers have influence of the between two points in another form of electric field a circuit. energy Potential Difference **Measured in Volts: ** Potential difference (p.d.) is measured in volts (V) and is also called voltage. The energy is transferred to the electrical components in a circuit when the charge carriers pass through them. A potential difference of 1 volt exists between two points if 1 joule of energy is used moving 1 coulomb of charge between the two points Electron volt In an X–ray tube electrons are emitted (through thermionic emission) at the cathode and move towards the anode. We’ll revisit The charge on a single electron is 1.6 x 10-19 this in a later C which is a very small charge. lecture… The electronvolt is a measure of the work done moving an electron through a potential difference of 1 volt Electron volt When an electron travels from the cathode to the anode in an X-ray tube its potential energy is converted to kinetic energy If a potential difference of 70 kV is applied across an X–ray tube each electron will gain 70 keV of kinetic energy Resistance Electrical resistance of a material is caused by impedance to the flow of electrons in the material Ohms law Resistors can be added to a circuit to control the current supplied to a component. If a potential difference of 1V drives a current of 1A through a conductor the resistance of the conductor is one ohm (1Ω) Measurement of resistance Ohm’s law: V = IR Capacitance A capacitor is a device that temporarily stores electric charge Capacitance is the amount of charge it can hold per unit potential Dielectric Capacitance is measured in farads (F) Coulomb Farad = Volt Electrodes What determines capacitance of a capacitor? The area of electrodes The distance between the electrodes The dielectric constant of the dielectric material Dielectric constant is a measure of its ability to store energy Electrical energy and power To get an electric current to flow, the electrons must be driven by a potential difference As the electrons move they are involved in collisions with the atoms of the conductor and so energy must be expended to keep the electrons moving in one direction Electrical energy and power Colliding electrons dissipate energy in the form of heat We use this phenomenon to heat up the filament (cathode) of the x-ray tube to produce electrons - thermionic emission Electrical power – the Watt The rate at which this heat is dissipated is an indication of Watt = Volts x Amperes the electrical power (in Watts) So, why do we care? kV - This is the voltage (in kilovolts) across the x-ray tube – the accelerating potential that drives electrons across the X-ray tube mA - This is the values of the electric current (in milliamperes) formed by the flow of electrons across the tube s - This is the exposure duration (in seconds) – the total time for which the electron current flows So, why do we care? kV, mA and time all have a significant effect on the number (quantity) and energy (quality) of X-ray photons generated within the X-ray tube  kiloVoltage kiloVoltage (kV) (kV) Exposure Exposure switch open switch closed Xray tube Xray tube mA mA Simplified (!!) X-ray circuit… Magnetism, as you recall from physics class, is a powerful force that causes certain items to be attracted to refrigerators. Magnetism Magnetism is caused by moving electrical charges In a magnetic material the circulation and spin of charged electrons in the orbits of the atoms produces atomic dipoles Some materials form permanent magnets Other materials can exhibit temporary magnetic behavior (induced magnetism) Magnetic dipoles Individual molecular magnets randomly arranged A magnetic dipole is a magnetic north pole and a magnetic south pole separated by a small distance Magnetism is induced in the material as all the molecular dipoles become aligned Arrow head represents north pole of molecular magnet Magnetic domains Some material have A magnetic domain is magnetic domains region in which the magnetic fields of atoms are grouped together and aligned In a magnetised material the magnetic field of the domains all point in the same direction Magnetic Fields The magnetic field is the area around a magnet in which the effect of magnetism is felt. We use the magnetic field as a tool to describe how the magnetic force is distributed in the space around and within the magnet Measuring magnetism The total number of magnetic lines of force is called the magnetic flux and the SI unit is the weber (Wb) The flux density is a measure of the magnetic flux per unit area The SI unit of flux density is the tesla (T) 1 T = 1 Wb.m-2 Earth’s magnetic field strength is ~25 - 65µT Types of magnetic material Diamagnetic materials are those materials that are Diamagnets freely magnetized when placed in a magnetic field Temporarily magnetic Paramagnets are generally known as materials with Paramagnets atoms carrying weakly interacting permanent magnetic moments Ferromagnetic materials are those materials which Ferromagnets exhibit a net magnetisation at the atomic level, even in the absence of an external magnetic field The Atomic Nucleus Of interest for MRI in A magnetic field exists whenever a Semester 2 charge moves N Protons are found in the nucleus of atoms and have a +ve charge S Protons also rotate on their axis and thereby create a magnetic field around them  Electromagnetism When electrons move through a conductor a magnetic field is produced around the conductor This is electromagnetism Direction of magnetic field Direction of electron flow The direction of the current will determine the direction of the magnetic field If the wire is coiled several times it is called a solenoid When a current is passed through the wire the magnetic lines of force become that of a bar magnet Electromagnetic Induction Electromagnetic Induction Electricity can be induced in a wire by placing it in a magnetic field. This is known as electromagnetic induction. It works because the magnetic field around the coil is changing when the coil ‘cuts through’ the magnetic field line. It can be done by moving a bar magnet inside a coil of wire or by moving a coil of wire in a magnetic field. Because the magnet is being moved back and forth, this changes the direction of the current, creating alternating (a.c.) current. Everyday uses of electromagnetic induction Generating electricity (converting mechanical to electrical energy) using an AC generator Motors – converting electrical to mechanical energy Using a “pencil” to write on a tablet Rotation of the anode in a rotating anode X-ray tube  Electricity is supplied as either direct current (DC) or alternating current (AC) Direct current waveform Alternating current waveform +V time time -V The magnetic field associated with the direct current does not change As the potential difference changes with the AC supply the associated magnetic field grows and collapses and then reverses with the changing current direction Mutual Induction When two coils are brought in proximity to each other, the magnetic field in one of the coils tends to link with the other. This further leads to the generation of voltage in the second coil. This property of a coil which affects or changes the current and voltage in a secondary coil is called mutual inductance. Transformers A transformer uses mutual Volt meter induction to alter the voltage supply When current flows through the primary coil a magnetic field is Secondary coil induced within the iron ring Iron The alternating current produces ring a changing magnetic field in the iron ring Primary coil This changing magnetic field induces a current in the secondary circuit Transformers The voltage running through the wires can be changed by altering the number of turns between the primary and secondary coils The voltage in the two circuits is proportional to the number of turns in the two coils Np = number of turns in primary coil Np Vp Ns = number of turns in secondary coil = Vp = voltage in primary circuit Ns Vs Vs = voltage in secondary circuit Step up transformers A transformer has 10 coils in its primary windings and 15 in its secondary, if supplied with 230 volt alternating current what voltage will be induced in the secondary circuit? Np Vp 10 230 = = 10 x Vs = 15 x 230 Ns Vs 15 Vs 15 x 230 Vs = = 345 V 10  Most X – ray sets are supplied with 415 V. If the transformer has 100 coils in the primary windings and 20,000 in the secondary what will the induced voltage be in the secondary circuit? 100 415 20000 ∗ 415 = 𝑉𝑠 = 20000 𝑉𝑠 100 𝑉𝑠 = 83000𝑉 = 83𝑘𝑉 Summary Tell me one thing that you have learned from today's session ⓘ Start presenting to display the poll results on this slide. Thank You [email protected]