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- 1. Current flow versus ElectronflowConventionalcurrent flowsthis way.Electronsflow thisway.
- 2. What formula relatesCharge, Current and Time?A current of 1 Ampere is flowing when 1 Coulomb of chargeflows past a point in a circuit in 1 second.Charge = current x time(C) (A) (s)If a current of 5 A is flowing then 5 C of charge pass a pointin 1 second.In general, if a steady current I (amperes) flows for time t(seconds) the charge Q (coulombs) passing any point isgiven byQ = I x t
- 3. Worked exampleA current of 150 mA flows around a circuit for1minute. How much electrical charge flows past apoint in the circuit in this time?SolutionSubstituting into Q = Itgives Q = 0.15 A x 60 s= 12 C
- 4. 1. Convert the following currents into amperes:a) 400 mA b) 1500 mA.Ans. = a) 400 mA = 0.4 A b) 1500 mA = 1.5 A2. What charge is delivered if a current of 6A flows for10 seconds?Ans. = 60 C3. What charge is delivered if a current of 300 mA flowsfor 1 minute(60 seconds)?Ans. = 18 CFor you to do!!
- 5. What is Ohm’s Law?The voltage dropped across a resistor is directlyproportional to the current flowing through it,provided the temperature remains constant.Voltage (V) = Current (A) x resistance (Ω)V = I x RWhat is the formula for Ohm’s law?
- 6. Worked example on Ohm’s Law2 A 8 ΩV = ?V IxR== 2A x 8= 16 VΩ
- 7. Ammeters and VoltmetersAmmeters measure current and are placed in seriesin a circuit.Voltmeters measure voltage and are placedVoltmeters measure voltage and are placedin parallel in a circuit.in parallel in a circuit.AV
- 8. Rules forResistors in SERIESRTotal = + +R R R1 2 3
- 9. Examples on Resistors in Series6 Ω 9 ΩAns. = 15 Ω4Ω 6 Ω 3 ΩAns. = 13 ΩNo. 1No. 2
- 10. Rules forResistors in PARALLEL1 1 1R1R R RThis formula is shortened toRR RR RoductSumTotal 1 2 3Total1 21 2= + +=+=Pr
- 11. Examples on Resistors in Parallel6 Ω12 ΩAns. = 3 Ω6 ΩAns. = 6 Ω12 ΩNo. 1No. 2
- 12. For you to do!!!!16 Ω6 Ω16 ΩAns. = 14 ΩNo. 3
- 13. Ans. = 9 ΩAns. = 6 Ω6 Ω6 Ω12 Ω10 Ω3 Ω10 Ω 2 Ω2 ΩNo. 4No. 5
- 14. Rules for SERIES CIRCUITS• Same current but ……• split voltage between them.
- 15. 18 V6 V 6 V 6 V?Equal resistors share thevoltage between them!!
- 16. Rules for PARALLELCIRCUITS• Same voltage but ……• split current between them.
- 17. ? A? A4 A? AEqualresistorsWhat will be the currents flowingthrough each ammeter?
- 18. Electrical PowerE.g. A study lamp is rated at 60 W, 240 V.How much current is the bulb carrying?Solution60 W = 240 V * Current60 WCurrent = ----------- = 0.25 A240 VElectricalElectrical Power = Potential difference * currentPower = Potential difference * currentWatts Volts AmpsWatts Volts Amps
- 19. A transformer is a device for increasing or decreasingan a.c. voltage.
- 20. Structure of Transformer
- 21. Circuit Symbol for Transformer
- 22. How Transformer worksLaminated softiron corePrimary coil Secondary coilInput voltage(a.c.)Output voltage(a.c.)
- 23. All transformers have three parts:1. Primary coil – the incoming voltage Vp(voltage across primary coil) is connectedacross this coil.2. Secondary coil – this provides the outputvoltage Vs(voltage across the secondary coil)to the external circuit.3. Laminated iron core – this links the two coilsmagnetically.Notice that there is no electrical connection between the two coils,which are constructed using insulated wire.
- 24. Two Types of TransformerA step-up transformer increases the voltage -there are more turns on the secondary than on theprimary.A step-down transformer decreases the voltage- there are fewer turns on the secondary than onthe primary.To step up the voltage by a factor of 10, theremust be 10 times as many turns on the secondarycoil as on the primary. The turns ratio tells usthe factor by which the voltage will be changed.
- 25. Formula for Transformervoltage across the primary coilvoltage across the secondary coilnumber of turns on primarynumber of turns on secondaryVVNNpsps==Where Vp= primary voltageVs = secondary voltageNp= Number of turns in primary coilNs= Number of turns in a secondary coil.
- 26. Worked example No. 1The diagram shows a transformer. Calculate thevoltage across the secondary coil of this transformer.Step-up transformer!
- 27. SolutionVVNNSubstituting12V180540Crossmultiplying180.V 12 x 540V12 x 540180V 36 VPSPSSSSS===∴ =∴ =
- 28. Worked example No. 2A transformer which has 1380 turns in its primary coil is to be used toconvert the mains voltage of 230 V to operate a 6 V bulb. How manyturns should the secondary coil of this transformer have?VP = 230 VNP = 1380VS = 6 VNS = ?Obviously, a Step-down transformer!!
- 29. SolutionVVNNSubstituting23061380NCrossmultiplying2300.N 6 x 13800N6 x 1380230N 36 turnsPSPSSSSS===∴ =∴ =

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