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

Sub-atomic particle Relative mass Relative charge Location in the atom

Proton 1 a.m.u. +1 In the nucleus

Neuton 1 a.m.u. 0 In the nucleus

Electron 1/1840 a.m.u. -1Elementary charge of 1.60 x 10-19 C

Outside the nucleus

• 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

Type of conductor example Type of charge carrier

Metallic Pure metals, alloys, graphite

Mobile electrons (free electrons)

Electrolyte Ionic/electrovalent compounds in aqueous solution or molten (eg. Acids, bases, salts)

Mobile ions (cations, anions

Semi conductor Silicon and germanium (eg in diodes)

Holes and free electrons

• 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

• 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 pointQ – charge on each oneThe 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

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. 1) A charged insulator is brought close to a metallic conductor on an insulating stand. 2) The free electrons are attracted to side closest to the positively charged insulator (excess + charge on opposite side of conductor)3) The excess + charge side of the conductor is earthed to allow a flow of electrons to neutralize + charge ( a brief current flow I occurs)4) Earth connection is broken and charged insulator removed

Effect of charged object on an uncharged object-the uncharged object receives induced charge

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

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

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

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

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

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

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)

• 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 –timeCurrent 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)

Steady dc current

ac current

Square wave dc current

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

1 cycle

T 2T

QUANTITY TYPICAL SYMBOL UNIT & TYPICAL SYMBOL INSTRUMENT OF MEASUREMENT

CALCULATING EQUATION

Unit charge q(=e=1.6x10-19C)

Coulomb – C Q = nq

Number of charge carriers

n

Total quantity of charge

Q Coulomb –C

Time t Second - s Watch or clock

Current I Ampere - A Ammeter I=V/R

Voltage (Potential difference, p.d. Electromotive force e.m.f.)

V Volt - V Voltmeter V=IR

Resistance R Ohm - Ω Ohmmeter R=V/I

Electrical potential Energy

E Joule-J (& kilowatt hour / KWh = 1000Wx1h= 1000x3600=3,6000,000J

Joulemeter W=E=QV ;E = Pt; E=IVt,; E=I2Rt; E = (V2/R)t

Electrical Work W Joule - J(& kilowatt hour / KWh

Joulemeter W= E= QV

Power P Watt P=IV = I2R = V2/R

CIRCUITS

A

/Rheostat

V

Connecting wires

AmmeterVoltmeter

Fuse

Fuse used to prevent overflow of currentRheostat can be arranged to work as a Potentiometer (or potential divider…An arrangement for tapping off a variable fraction of an applied voltage.

• 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

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)

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

I2 I3 I3

V1

V2

V3

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

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

Ammeters• Have a very low resistance compared with other

devices in the circuitVoltmeters• Resistance of voltmeter is very large / infinite

resistance to draw negligible current

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

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

Wire International colour code

OLD NEW

Live, L Red Brown

Neutral Black Blue

Earth, E Green Green and Yellow

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

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

Cell-device that converts chemical energy to electrical energy by producing electrons during a chemical

reaction2 types of cells:• Primary cell – cannot be recharged (eg. Dry cell – zinc carbon

cell)

• 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

• 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

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 ofThe charging agent is more than the emf of the cell/battery itself

Advantages

Disadvantages

Primary cell Secondary cell

Light weight Very small internal resistance

Portable since it contains no liquids that can spill

Can supply large maximum current

Cheap Supplies a slightly larger emf

No maintenance required Can be recharged

Available in dry

Available with robust containers

Available maintenance free

Primary cell Secondary cell

Supplies smaller current Heavy

Cannot be recharged Liquid within it can spill out…not portable

Cannot be used for very long continuous periods of time

Expensive

Limited shelf life Cumbersome

Some types require regular maintenance

Comparison Characteristic Lead acid battery Zinc carbon cell

Terminal voltage/emf 12 V 1.5 V

Maximum current 80 A 2.0 A

Internal resistance 0.15 Ω 0.75Ω

Portability Not as portable Portable

Rechargeability rechargeable Not rechargeable

Electromagnetism

• In a long, straight conductor