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Sinai University Faculty of Engineering Science Department of Basic Science 5/25/2017 w3 1 Chapter 1-2 Electrodynamics 5/25/2017 w3 2 Electric potential The electrical potential is defined as the work performed when moving an electric charge q between infinity and a potential level, divided by that charge. Whereas the electrical potential difference is defined as the work performed when moving an electric charge q between two potential levels, divided by that charge. But since the electric field is a force per unit charge, the electric potential must be energy per unit charge. E=Force/charge 5/25/2017 N/C V=Energy/Charge w3 J/C (Volts) 3 Electric potential energy, U V c=U t c /q t. J/C(=Volt) U t c = q t V c. CV(=J) Electric potential energy = q tE L in uniform electric field, E A new energy unit in atomic and nuclear scale 1eV= 1.6x10-19 CX1 V=1.6x10-19 J 5/25/2017 w3 4 Example A fresh flashlight battery has a potential difference of about 1.5 Volts between its terminals regardless of what their absolute potentials really are. If an electron were allowed to move from the negative terminal of such a battery to the positive one it would gain a kinetic energy of 1.5 electron Volts but its potential would drop by 1.5 Volts. 5/25/2017 w3 5 Example The potential energy has been converted to kinetic. If we wanted to move the electron back to the negative electrode we would have to perform work of 1.5 electron Volts because we would be moving the electron against the repelling force of excess electrons residing in the negative terminal. 5/25/2017 w3 6 Electric current An electric current is a flow of charged particles. Current is measured using an ammeter. Iav=nAvdq, The unit of current is the Ampere, A. 5/25/2017 7 Electric potential DV=1.5 V DW=q DV DW= KE +ve charges 5/25/2017 w3 8 Electric Charges, DQ F E + + + 5/25/2017 w3 9 Electric current No electric Field 5/25/2017 There is an electric Field w3 10 Ideal Current Source An ideal current source is defined as having the ability to force its nominal current into any load. Rin is to be infinite An approximation to an ideal current source is a battery of very high voltage V in series with a very large resistance R. Such approximation would supply a current V/R into any load that has a resistance much smaller than R. 5/25/2017 w3 11 Ideal Voltage Source The ideal voltage source is defined to have the nominal potential difference between its terminals and a zero internal resistance. A voltage source must be able to maintain an excess electron density on one electrode and an equal but opposite rarefaction on the other, regardless of how many electrons may be leaving any one terminal. In terms of the microscopic events we can define the voltage source as a device which can withdraw electrons from one of its terminals and deposit them onto the other. This can be done by mechanical means as in a generator or by chemical means as in a battery. 5/25/2017 12 w3 R L . A Electric resistance Measures the relative ease with which a current flows in a medium. L R . A 5/25/2017 w3 13 Electrical Resistivity, 5/25/2017 w3 14 1.0 104 . Sample Problem 1 A rectangular block of iron has dimensions 1.2 cm x 1.2 cm x 15 cm. (a) What is the resistance of the block measured between the two square ends? (b) What is the resistance between two opposing rectangular faces? The resistivity of iron at room temperature is 9.68 x 10-8 Solution (a) The area of a square end is (1.2 x 10-2 m)2 or 1.44 x 10-4 m2. From Eq. 1-6, . L (9.68 108 .m)(0.15m) R A 1.44 10 4 m2 1.0 10 . 4 0.15 m (b) The area of a rectangular face is (1.2 x 10-2 m)x(0.15 m) or 1.80 x l0-3 m2. From Eq. 1-6 L (9.68 108 .m)(1.2 102 m) R A 1.44 10 3 m2 5/25/2017 6.5 107 0.65 w3 15 Ohmic Resistance A conducting device obeys Ohm's law if the resistance between any pair of points is independent of the magnitude and polarity of the applied potential difference. 5/25/2017 w3 16 Electric circuit components 5/25/2017 w3 17 The rules for solving simple circuits 1-Draw a loop inside the simple circuit, and specify a certain direction in such loops. 2- Follow the following sign convention, a- The current is positive if it is in the same direction of the loop direction. b- The emf, e, is positive if its direction, from negative to its positive pole, is in the same direction of the loop direction. j Vi 3- Make an equation for each simple loop. j i 6- Solve these equations simultaneously to obtain circuits unknown 7- If you get any current with negative sign, this means that the direction of this current is opposite to that considered in step 3, but its value is correct. 5/25/2017 w3 18 Vab Va Vb iR Example + ,r - + R ir iR i{r R} i 5/25/2017 {r R} w3 19 Example i a 1,r1 2,r2 R c 5/25/2017 b w3 20 Power in DC Circuits P= P= =IV P= Energy conversion 5/25/2017 w3 95% into light 21 Types of Electric Currents The current we have considered so far is called a D.C. (Direct Current) since its direction does not change with time. An A.C. (Alternating Current) is one in which the current changes direction with time, and hence its sign, with some fixed frequency. V volt OR I ampere Time 5/25/2017 w3 22 Assignment Artificial Electrical pacemaker Structure, theory of operation, uses 5/25/2017 w3 23 Capacitor Storing Electrical Energy A device that stores electrical energy based on opposite charges is called a capacitor. C= o A/d A d E 5/25/2017 w3 24 Charge and Discharge of a Capacitor,Pacemakers 5/25/2017 w3 25 Circuit Topology PARALLEL SERIES 1 1 1 . Req R1 R2 Req= R1 + R2 1 1 Req n Rn1 5/25/2017 NOT SERIES Neither PARALLEL w3 Req Rn n 26 Home work Solve the following problems 2,4,6,11,15,16 20, 24, 25,36,39 Hand it next week [email protected] 5/25/2017 w3 27 5/25/2017 w3 28