Chapter Eight : Low Temeprature physics (Cryogenics)

Basic Concepts: - Low temperature physics deals with the study of materials at temperature near absolute zero. - Van de Waal's effect expresses the mutual interaction between molecules and is different from chemical interaction between atoms which leads to formation of molecules. - The mechanism for achieving very low temperature depnds on drawing energy from the material, this may be done by putting the material to be cooled in contract with a cooler material such as liquified gas. Super fluidity: - Somliquefied gases can flow without resistance or without friction at temperatures close to absolute zero. Helium liquid is a superfluid its viscosity vanishes. It can also flow up along the walls of the container againest gravity and friction and has low specific heat. - Superonductivity: some metals have excessive electrical conductivity (zero resistance) at very low temperatures. Meissner effect: If a permenant magnet is placed above a superconductor the current induced in the superconductor generates a magnetic field which repels the permanent magnet so the permanent magnet remains hanging in air.

Chapter Nine : The Electric Current and Ohm's Law

Basic concepts 1- The current intensity: " I " * The quantity of chargflowing through a given section in circuit per second. ( I = Q / t ) * It is measured in " Amperes " or C/s . 2- The potential difference : " V " * Is the work done to transfer unit charge between two points. ( V = W / Q ) * It is measured in " Volts " or J/C . 3- Ohm's law : * The current produced in a given conductor - kept at constant temperature - is directly proportional to the potential difference across its terminals. i.e 4- Ohm's law for a closed circuit : mp Where V_B is the e.m.f of the source r is its internal resistance and R is the external resistane. 5- Parameters affecting the resistance: 8- Resistors in series: 9- Resistors in parallel:

Chapter Ten : Magnetic effect of electrical current and Electrical Measuring Instruments

1- A magnetic field is produced around a current - carrying wire. 2- The intensity of the magnetic field produced around a current - carrying wire, increased by: a) getting closer to the wire         b) increasing the current. 3- The direction of the magnetic field produced around a current - carrying straight wire is determined by Ampere's right - hand rule. 4- The lines of force around a current - carrying wire forming a circular loop resemble to a great extent those of a short bar magnet. 5- The magnetic flux density at the center of a current - carrying circular loop depends on: a) The number of loop turns         b) The current intensity in the loop b) The radius of the loop. 6- The magnetic field produced by a current flowing through a solenoid (coil of several closely spaced loops) resembles to a great extert that of a bar magnet. 7- The magnetic flux density at any point on the axis of a current - carrying solenoid depends on: a) The current intensity         b) The number of turns per unit length. 8- Right hand screwule is used determine the polarity of a solenoid carrying current. 9- The unit of magentic flux density is Web/m² , (Tesla or N/Am) 10- The force exerted by a magnetic field on a current - carrying wire placed in the field depends on: a) The length of the wire       b) The current intensity c) The magnetic flux density.       d) The angle between the wire and the direction of the magnetic field. 11- The direction of the force due to magnetic field acting on a straight wire carrying current is determined by Fleming left hand rule. 12- Tesla: the unit of magnetic flux density: It is the magnetic flux density which will exert a force of one Newton on a current carrying wire of one meter meter length perpendicular to the field when the current is one ampere. 13- A moving coil glavanometer is an instrument used to detect, measure and determine the polarity of very weak electric currents. 14- The oparation of a moving coil galvanometer is based on the torque acting on a current - carrying loop in the presence of a magnetic field. 15- The senstivity of a galvanometer is defined as the scale deflection per unit current intensity flowing through its coil. 16- The ammeter is a device which is used through a calibrated scale to measure directly the electric current. 17- To extend the range of the glavnometer, a low resistor known as a shunt is connected in parallel with the coil of the galvanometer. 18- The total resistance of the ammeter (with the shunt) is very small, therefore, it does not appreciably change the current to be measured in a closed circuit. 19- The voltmeter is a device used to measure the potential difference across two points of an electric circuit. It is basically a moving coil glavanometer having a very high resistance called a multiplier resistance connected in series with its coil. 20- Since the total resistance of the voltmeter is very great, it does not affect much the flow of current through the element across which it is connected to measure its potential difference. 21- The ohmmeter is an instrument which is used to measure an unknown resistance. 22- An ohmmeter is basically a microammeter connected in series with a constant cell, resistance, a variable resistance and a 1.5 volt battery. If its terminals are in contact (sc) the pointer gives full - scale deflection (FSD). If a resistore is inserted between its terminals, the current flowing decreases. Hence the pointer deflection decreases, and indicates directly the value of the inserted resistor through a calibrated scale. Basic Laws: 1- The magnetic flux density "B" at a point which is d "m" away from a wire carrying current I(A) is detemined by the relation weber/m² (or Tesla) 2- The magnetic flux density B at the center of a circular loop of radius r carrying current I and of N turns is: (tesla) 3- The magnetic flux density at any point on the axis of a solenoid of length number of turns N and carrying a current I is (tesla) 4- The magnetic force acting on a wire of length carrying current I and placed in a magentic field of "B" flux density is where  is the angle between the wire and the direction of the magnetic field. 5- The torque acting on a rectangular loop of face area A , number of turns N, carrying current I and placed prallel to magnetic field of flux density B is: 11- The value of the shunt of an ammeter is given from the relation: where R_s is the shunt resitance and I_g is the maximum current in the glavnometer coil, R_g is the resitance of the galvanometer coil and I is the full scale defection (FSD) current. 12- The multiplier resistance R_m in a voltmeter is given by the realtion where V is the FSD voltage, V_g, the voltage drop across the galvonometer coil and I_g is the current flowing in the galvnometer which corresponds to full scale deflection (FSD).