INTRODUCTION TO BIOPHYSICS. II semester, week /2015

Transcription

1 INTRODUCTION TO BIOPHYSICS II semester, week /2015

2 INTRODUCTION TO ELECTRICITY Electricity phenomena associated with interaction between electrically charged objects

3 PARTICLES AND ELECTRIC CHARGE Electron - electrically negative particle Proton electrically positive particle Neutron electrically neutral partice The Law of Electric charges: Like charges repel each other, and unlike charges attract each other.

4 SI UNIT OF CHARGE IS COULOMB (C) Electron, e = x C m e = x kg Proton, p = x C m p = x kg Each electric charge (q) can be expressed as q = n x e, where e - charge of one electron, n - number of excessive or deficient electrons

5 THE LAW OF CONSERVATION OF CHARGE The Law of conservation of charge states that the net charge of an isolated system remains constant.

6 EXERCISE : If you shuffle across a carpeted floor on a dry day and acquire a net charge of -2.0mC, will you have a deficiency or excess of electrons? How many missing or extra electrons will you have? Answer: you have acquired 1.3x10 13 electrons. In this example, (a) what type of charge does the carpet acquire? (b) How much charge does the carpet acquire? (c) Does it have an excess or deficiency of electrons? (d) How many electrons? What about shuffling across a wooden floor?

7 DISTINGIUSHING BETWEEN ELECTRICAL CONDUCTORS AND INSULATORS Conductors valence electrons of atoms are loosely bound CONDUCTORS Iron Mercury Silver Carbon Copper Insulators valence electrons are more tightly bound and are not readily moved INSULATORS Wood Glass Rubber

8 SEMI-CONDUCTORS Semi-conductors are a very useful intermediate class, not as conductive as metals but considerably more conductive than insulators. By adding certain impurities to semi-conductors in the appropriate concentrations the conductivity can be wellcontrolled. SEMICONDUCTORS Germanium (Transistors) Silicon (computer chips)

9 Electrostatic charging - a process by which an object receives a net charge Charging by friction Charging by contact or by conduction Charging by induction

10 Electrostatic charging Charging by friction - this is useful for charging insulators. If you rub one material with another, electrons have a tendency to be transferred from one material to the other. Charging by conduction - useful for charging conductors. If a charged object touches a conductor, some charge will be transferred between the object and the conductor, charging the conductor with the same sign as the charge on the object. Charging by induction - also useful for charging conductors. Again, a charged object is used, but this time it is only brought close to the conductor, and does not touch it.

11 Electrostatic charging Examples: Clothes or papers sticking together Electrostatic air cleaners Electrostatic copiers Electrostatic spark discharge can cause an explosion in the presence of flammable gases

12 COULOMB S LAW F 1 F 2 +q 1 +q 2 r F 1 F 2 +q 1 -q 2 F 1 F 2 k q 1 k = 8.99x10 9 Nm 2 /C 2 r q 2 2

13 EXERCISE: In a certain organic molecule, the nuclei of two carbon atoms are separated by a distance of 0.25 nm. What is the magnitude of the electric repulsion between them? The problem concerns electric interaction (force) between two charged objects (two nuclei of carbon) Force between two charges is described by Coulomb s law

14 F k q 1 2 r q 2 k = 8.99x10 9 Nm 2 /C 2 F F +q 1 +q 2 r In a carbon nuclei there are six protons, so q 1 = q 2 = 6 x 1.6 x C r = 0.25 nm = 0.25 x 10-9 m F = x 10-6 N = mn

15 ELECTRIC FIELD The space surrounding an electric charge has a property called an electric field. This electric field exerts a force on other electrically charged objects.

16 ELECTRIC FIELD The electric field (E) is a vector field with SI units of newton per coulomb (N C 1 ). The strength of the field at a given point is defined as the force that would be exerted on a positive test charge of +1 coulomb placed at that point; the direction of the field is given by the direction of that force. E F q

17 ELECTRIC FIELD - ELECTRIC FIELD LINES

18 ELECTRIC FIELD - ELECTRIC FIELD LINES

19 ELECTRIC FIELD - ELECTRIC FIELD LINES

20 Charge separation by polarization Nonpolar molecule _ Induced molecular dipole _ Electric dipole gives us a model for permanently polarized molecules, like the water molecule.

21 CONDUCTORS AND ELECRTIC FIELD The electric field is zero inside a charged conductor Any excess charge on an isolated conductor resides entirely on the surface of the conductor Excess charge tends to accumulate at sharp points, or locations of highest curvature. Electric field lines are always perpendicular to the surface

22 UNIFORM ELECTRIC FIELD BETWEEN TWO PARALLEL PLATES E 4 kq A A surface area of the plate Q charge on the plate

23 ENERGY OF A CHARGE IN AN ELECTRIC FIELD

24 ELECTRIC FIELD A charge (q) moving in an electric field (E) along the lines of electric field over a distance (d) aquires energy (ΔU). To move a charge (q) against the electric field (E) over a distance (d) a work (DU) has to be done on it. U q E d One can say also that a charge (q) in an electric field (E) has potential energy equal to that work (ΔU). That potential of electric field to move electric charges and to change the energy of electric charges is expressed using a term: voltage

25 VOLTAGE Any difference in charge between two objects will result in the development of a DIFFERENCE OF POTENTIAL (V) between them. We call this difference of potential VOLTAGE. SI unit of electric potential difference is volt 1V = 1J/1C Potential difference in a uniform electric field is V = Ed

26 THANK YOU!