ELECTRICITY AND MAGNETISM

1. ELECTRICITY AND MAGNETISM All matter is made up of atoms and molecules These atoms or molecules contain protons, electrons and neutrons Usually the atom is neutral but if the is an excess of protons (positively charged) or electrons (negatively charged) it is an ion… Cation – positively charged atom Anion – negatively charged atom

2. Charge Carriers – is a particle or a group of particles which is associated with a certain quantity of excess positive or negative electric charge • Conductors are materials or devices which contain a significantly large number of mobile charge carriers which can move along a conductor in a specific direction when a voltage is applied across a conductor • Insulators are materials or devices that contain a small number of mobile charge carriers or immobile charge carriers (electrons bound firmly). Eg. Non-metals, plastics, glass, felt, rubber, paper, organic material, solid ionic compounds such as salts • Static electricity – the transfer of electrons to and from a material ( acquires deficient electron surfaces and deficient proton surfaces

3. Properties of charge: • Positive or negative • Like charges repel, opposite charges attract • Excess charge lies on the external surface of a charged conductor • Excess charge usually concentrate and accumulate where charged surface is sharply curved • Negative charges move from high region of concentration to low region of concentration however positive charge ‘move” from high electric potential to low electric potential • The Earth is at zero electric potential, is an infinite source for electrons and is a sink for electrons • Conventional flow of current in a circuit is in the direction of positive charge carriers • The principle of conservation of charge: whenever a quantity of charge of one sign is produced then an opposite sign is produced

4. Rate of directed flow of charge is called Current, I, it magnitude is I = Q/t where I – current, Q – given quantity of charge, t – time taken for movement past point Therefore Q = I x t and Q = n x q Where n – number of charge carriers passing a given point Q – charge on each one The unit of charge (Q and q) is called the Coulomb (C ) The unit of current is called the Ampere (A ) The unit of time is called the second (C ) One coulomb is 1 ampere per second

5. Methods of charging • By friction – mainly for insulators. The insulator is rubbed with a dry cloth which essentially transfers electrons from one surface to the other to produce a net negative charge on one surface and a net positive charge on the other. (some danger in igniting/explosion of flammable material by “jumping charges”) • By electrostatic induction – mainly for conductors. • A charged insulator is brought close to a metallic conductor on an insulating stand. • The free electrons are attracted to side closest to the positively charged insulator (excess + charge on opposite side of conductor) • The excess + charge side of the conductor is earthed to allow a flow of electrons to neutralize + charge ( a brief current flow I occurs) • Earth connection is broken and charged insulator removed Effect of charged object on an uncharged object -the uncharged object receives induced charge

7. Ionization – Action at a point • A point on a charged conductor that is sharply curved will have a high concentration of charge at that point… strong electric field created…electrons of surrounding air stripped …ionization occurs. • Ionization also occurs due to collision between atoms and molecules • Same sign charges repel (electric wind occurs), opposite attract (neutralizes some of opposite charge

8. Lightning conductor • Clouds become negatively charged as air passes over it constantly (charging by friction) • A positive charge becomes induced on the roofs of buildings, lightning rods, etc. by the cloud • Ions flow to the rods and from rod to the cloud to neutralize some charges…reduction of chance of lightning striking • But if it does strike, charges flow to earth

9. Practical applications of charging • Spray painting – positively charged droplets of painting move towards earthed object following the lines of the electric field • Use of weedicides, pesticides, insecticides for crop sraying – charged droplets by use of sprayer so that they repel one another to create a wide spray, following electric field lines to move towards earthed plants • Dust extraction mechanisms – eg removal of dust in chimneys, etc. • Ink jet printers – control of ink droplets towards paper

10. ELECTRIC FIELD • Electric field is the region surrounding an electric charge • An electric force is exerted by this field on any other object in the field • It consists of electric field lines – an imaginary line drawn in an electric field such that its direction at any point in the field gives the direction • The direction of an electric field line at any point in the electric field is that along which a small unit of positive charge placed at that point would move…lies along the tangent to the field line at that point in the electric field

12. Potential difference • P.D., symbol V, between two points in an electric field is the work done, W or Energy used, E, per unit charge in transferring a quantity of charge, Q from one point to another V = W/Q or V= E/Q so E=W=VxQ • Unit of P.D. is the volt (V)…one joule per coulomb

13. Electric field strength, E • E is defined as the force per unit charge at a given point in an electric field • It is a vector quantity with unit NC-1 or Vm-1 • For a uniform electric field, E= p.d. between the conducting plates, V linear distance between the plates,d

14. Current electricity • The rate of flow of charge carriers essentially constitutes a current • In a circuit, the conventional flow of current is given by the direction of positive charge carriers (these don’t actually move)

15. Unit for electric current is the Ampere (A) defined as the force exerted between two straight, parallel, current-carrying conductors • Unit of electric charge is the coulomb defined as one ampere per second Q = I x t Q – charge, I – current, t –time Current may be d.c. or a.c. d.c. – direct current is the flow of current in one direction (variations on one side of the time axis) a.c. – flow of current in opposite directions over time…flow and reversed flow continuously (variations on both sides of the time axis)

16. Steady dc current Square wave dc current ac current

17. The Period, T is the time to complete one cycle or variationThe frequency, f, is the number of cycles per second…f = 1/T and T = 1/fThe peak value or amplitude is the maximum value of V or I in either direction/sense.Typical effects of an electric current include1. heating2. magnetic3. chemical T 2T 1 cycle

19. CIRCUITS Fuse V Voltmeter Ammeter A /Rheostat Connecting wires Fuse used to prevent overflow of current Rheostat can be arranged to work as a Potentiometer (or potential divider… An arrangement for tapping off a variable fraction of an applied voltage.

21. Voltage or potential difference (or emf) across a circuit allows a current flow because the electrical energy is being converted to other forms of energy per unit charge flowing through it. • E. M.F. is the maximum voltage obtained between the terminals of an electrical power supply obtained such as from a cell. (chemical or kinetic energy converted to electrical energy

22. Resistance • In a circuit all components provide some resistance to flow of current dependent on the amount of load it is. • A resistance wire for example is dependent on the length of wire and is inversely proportional to its cross sectional area. It is also dependent on the nature of the material the wire is made from (silver, gold, copper and aluminum – low resistances)

23. Resistors in series • I = I1 +I2+I3…the same current flows through components in series • V = V1 +V2+V3…the total voltage across components in series is the sum of all the individual voltages • R = R1 +R2+R3…the total resistance of resistors in series is the sum of the individual resistances I1 I1 I2 V1 V2 V3 I2 I3 I3

24. Resistances in parallel • I = I1 +I2+I3…the same current flows through components in series • V = V1 +V2+V3…the total voltage across components in series is the sum of all the individual voltages • 1/R=1/R1 +1/R2+1/R3 …the total resistance of resistors in series is the sum of the individual resistances

25. Note • Cells in series: E= E1+E2-E3+E4-E5+E6 V =V1 +V2+V3+V4 +V5+V6 • Identical cells in parallel: E= E1=E2=E3=E4 1/V=1/V1+1/V2+1/V3+1/V4

26. Ammeters • Have a very low resistance compared with other devices in the circuit Voltmeters • Resistance of voltmeter is very large / infinite resistance to draw negligible current

27. OHM’s LAW • States that a voltage applied across a metallic conductor is directly proportional to the current through the conductor, provided that physical conditions remain constant. • V= I x R • For metallic conductors at constant temperature there is a linear relationship between V and I and a graph of V vs I is a straight line through the origin…Ohmic conductor. • Non Ohmic conductors do not have I-V graphs that go through the origin

28. House circuits • There are usually 3 wires used in household electrical wiring: • There are 3 wires from the pole to the house: 2 live 110V and 1 neutral • The Earth wire, a safety device provides an alternative route for current in case a live wire is touching the housing of an electrical device • A fuse or circuit breaker is also a safety device to minimize overload (eg a metal strip that breaks when too much heat is applied

29. Houses are usually wired with parallel wiring • So that each appliance can work independently • If any malfunction it does not affect the operation of other appliances • Each appliance can operate with the same voltages However, • Current surges can cause overload and lead to appliance damage and electrical fires • Current or voltage overload leading to a device operating below its rated power or not at all

30. Devices • A Diode is a device with little or no resistance in one direction (forward bias) and almost infinite resistance in the opposite direction (reverse bias) • In forward bias the diode will allow current to flow through it (will conduct), but in reverse bias the diode will not conduct • The diode can be used to rectify an a.c. current • Four diodes connected to form a bridge rectifier can fully rectifiy an ac current

31. Cell-device that converts chemical energy to electrical energy by producing electrons during a chemical reaction 2 types of cells: • Primary cell – cannot be recharged (eg. Dry cell – zinc carbon cell)

32. Seal usually with a vent to allow gases to escape • Carbon rod acts as the positive terminal or anode • Zinc container acts as negative terminal or cathode also • Porous casing/bag to enable electrical contact between materials inside and outside of the bag • Electrolyte (ammonium chloride in a moist paste – to allow electrical contact between materials within the cell, liquids will spill) • Mixture of deplorizar (MnO2) and powdered graphite to improve conduction

33. Limitations: • Local action – impurities in zinc cathode causes cell to constantly produce current so the active material is used up even when cell is not in use…limited shelf life. Zinc is amalgamated (covered with a protective layer such as mercury) • Polarization – formation of hydrogen bubbles on anode causes production of back emf and an increase in internal resistance of the cell. Depolarizer employed to reduce polarization NB. Do not leave zinc carbon cells in equipment

34. 2. Secondary cell or accumulator can be recharged since chemical reactions that are reversible, eg. Lead acid or nickel cadium (Nicad) cell and nickel iron (Nife) cell To charge a secondary cell an ac current is used and the voltage of The charging agent is more than the emf of the cell/battery itself

35. Advantages Disadvantages

36. Comparison

37. Electromagnetism • In a long, straight conductor