Basic Electric Circuits & Components IntroductionSI Units and Common PrefixesElectrical CircuitsDirect Currents and Alternating CurrentsResistors, Capacitors and InductorsOhms and Kirchhoffs LawsPower Dissipation in ResistorsResistors in Series and ParallelResistive Potential DividersSinusoidal QuantitiesCircuit Symbols

IntroductionThis lecture outlines the basics of Electrical CircuitsFor most students much of this will be familiarthis lecture can be seen as a revision session for this materialIf there are any topics that you are unsure of (or that are new to you) you should get to grips with this material before the next lecturethe following lectures will assume a basic understanding of these topicsWe will return to look at several of these topics in more detail in later lectures

SI Units

QuantityQuantity symbolUnitUnit symbolCapacitanceCFaradFChargeQCoulombCCurrentIAmpereAElectromotive forceEVoltVFrequencyfHertzHzInductance (self)LHenryHPeriodTSecondsPotential differenceVVoltVPowerPWattWResistanceROhmTemperatureTKelvinKTimetSeconds

Common Prefixes

PrefixNameMeaning (multiply by)Ttera1012Ggiga109Mmega106kkilo103mmilli10-3micro10-6nnano10-9ppico10-12

Electrical CircuitsElectric chargean amount of electrical energycan be positive or negativeElectric currenta flow of electrical charge, often a flow of electronsconventional current is in the opposite direction to a flow of electronsCurrent flow in a circuita sustained current needs a complete circuitalso requires a stimulus to cause the charge to flow

Electromotive force and potential differencethe stimulus that causes a current to flow is an e.m.f.this represents the energy introduced into the circuit by a battery or generatorthis results in an electric potential at each point in the circuitbetween any two points in the circuit there may exist a potential differenceboth e.m.f. and potential difference are measured in volts

A simple circuit

A water-based analogy

Voltage reference pointsall potentials within a circuit must be measured with respect to some other pointwe often measure voltages with respect to a zero volt reference called the ground or earth

Representing voltages in circuit diagramsconventions vary around the worldwe normally use an arrow, which is taken to represent the voltage on the head with respect to the taillabels represent voltages with respect to earth

Direct Current and Alternating CurrentCurrents in electrical circuits may be constant or may vary with timeWhen currents vary with time they may be unidirectional or alternatingWhen the current flowing in a conductor always flows in the same direction this is direct current (DC)When the direction of the current periodically changes this is alternating current (AC)

Resistors, Capacitors and InductorsResistors provide resistancethey oppose the flow of electricitymeasured in Ohms ()Capacitors provide capacitancethey store energy in an electric fieldmeasured in Farads (F)Inductors provide inductancethey store energy in a magnetic fieldmeasured in Henry (H)We will look at each component in later lectures

Ohms LawThe current flowing in a conductor is directly proportional to the applied voltage V and inversely proportional to its resistance R

V = IR

I = V/R

R = V/I

Kirchhoffs Current LawAt any instant the algebraic sum of the currents flowing into any junction in a circuit is zeroFor example I1 I2 I3 = 0 I2 = I1 I3 = 10 3 = 7 A

Kirchhoffs Voltage LawAt any instant the algebraic sum of the voltages around any loop in a circuit is zeroFor example E V1 V2 = 0 V1 = E V2 = 12 7 = 5 V

Power Dissipation in ResistorsThe instantaneous power dissipation P of a resistor is given by the product of the voltage across it and the current passing through it. Combining this result with Ohms law gives:

Resistors in Series and ParallelSeries

R = R1 + R2 + R3

Parallel

Resistive Potential DividersGeneral case

Example

Example

Sinusoidal QuantitiesLength of time between corresponding points in successive cycles is the period TNumber of cycles per second is the frequency ff = 1/T

Circuit Symbols

Key PointsUnderstanding the next few lectures of this course relies on understanding the various topics covered in this sessionA clear understanding of the concepts of voltage and current is essentialOhms Law and Kirchhoffs Laws are used extensively in later lecturesExperience shows that students have most problems with potential dividers a topic that is used widely in the next few lecturesYou are advised to make sure you are happy with this material now