workbook - eeng (part a) marking guide

8/14/2019 Workbook - EEng (Part a) Marking Guide

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Index

1 ATOMIC STRUCTURE ................................................................. 1-1

2 STATIC ELECTRICITY ................................................................. 2-3

3 ELECTRICAL TERMINOLOGY .................................................... 3-1

4 PRODUCTION OF ELECTRICITY ................................................ 4-1

5 CELLS & BATTERIES.................................................................. 5-3

6 OHMS LAW .................................................................................. 6-1

7 ELECTRICAL MEASURING INSTRUMENTS .............................. 7-1

8 RESISTANCE & RESISITORS ..................................................... 8-1

9 RESISTORS IN DC CIRCUITS ..................................................... 9-1

10 THE WHEATSTONE BRIDGE ...................................................... 10-1

11 ENERGY & POWER IN DC CIRCUITS ........................................ 11-1

12 CAPACITANCE & CAPACITORS ................................................ 12-1

13 CAPACITORS IN DC CIRCUITS .................................................. 13-1

14 MAGNETISM ................................................................................ 14-1

15 ELECTROMAGNETISM ............................................................... 15-1

16 INDUCTION & INDUCTORS ........................................................ 16-1

17 INDUCTORS IN DC CIRCUITS .................................................... 17-1

Issue 1 - 20 March 2001 Page 1-1


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1 ATOMIC STRUCTURE

Answer the following questions in your own words.

Q1. What is the difference between an element, a compound and a mixture ?

Notes page 1-1.

Q2. Describe the structure of an atom. What is the significance of the chargeson the fundamental particles ?

Notes page 1-3.

Q3. What are ions and how are they formed ?

Notes page 1-5.

Q4. What is the significance of the number of electrons in the valance shell ?

Notes page 1-7.

Q5. Locate a table of elements that shows the distribution of electrons within theshells and determine how many electrons there are in the valence shell ofthe following material:

Aluminium

Silver Copper

Gold

Germanium

Q6. Are there any elements that have more than 8 electrons in their outer shell ?(shell K,L,M,N,O,P or Q, not sub-shells)

Palladium (10) and Iridium (9).

Q7. Each shell is limited as to the number of electrons it can hold. Is there anyway of working out the maximum number of electrons that each shell canhold ?

2n2where n is the shell number.



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2 STATIC ELECTRICITY

Q1. What is static electricity ?

Q2. Why must maintenance engineers consider static electricity when workingon aircraft:

i) electronic equipment

ii) structures

Q3. What is the Triboelectric series ?

Q4. What sort of potentials can be created by somebody walking across acarpet and does humidity have any effect on the value ?

Up to 35 000 volts. As humidity increases, static charge decreases.

Q5. As two charged bodies are moved apart, the force of attraction or repulsionbetween them decreases by the square of the distance (i.e. if the distancebetween them is increased from 1cm to 2 cm, the force decreases by afactor of 4. If the distance is increased to 3cm, the force decreases by afactor of 9), why ?

Draw right angled triangle base andheight 1 unit long. Double base length to 2units and calculate new height it has alsodoubled. The same reasoning can be usedalong the horizontal.

If both sides double in length, from one unitto two, the area increases from one squareunit to 4 square units. That is the square ofthe distance.

If the distance is increased by a further 1unit, the area will increase to 9 square units(the square of the distance - 3)



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3 ELECTRICAL TERMINOLOGY

Q1. Define the following electrical terms in your own words:

i) Potential (notes page 3-1)

ii) Potential difference (notes page 3-1)

iii) Electromotive Force (notes page 3-2)

iv) Voltage (notes page 3-1)

v) Current (notes page 3-2)

vi) Resistance (notes page 3-2)

Q2. What is the difference between conventional current flow and electron flow ?

Q3. The following potential differences were measured in a circuit:

A is +4 volts with respect to (wrt) B

B is +3 volts wrt C

C is +2 volts wrt D

D is +1 volts wrt to ground

What is the potential difference of:

i) A wrt ground (+10v)

ii) B wrt ground (+6v)

iii) A wrt C (+7v)

iv) B wrt D (+5v)

Q4. The following potential differences were measured in a circuit:

E is at the same potential as ground and 25 volts wrt D

D is 15 volts wrt C

C is 10 volts wrt B

B is 30 volts wrt A

i) What are the potentials at B, C and D wrt ground. (+50v, +40v, +25v)

ii) What is the potential difference of A wrt C. (+40v)

iii) What is the potential difference of C wrt A. (-40v)

iv) What is the potential difference of ground wrt B (-50v)



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Q5. The Following potentials were measured in a circuit:

E is at ground potential and A is +100 volts wrt ground

D is +50 volts wrt E

C is +15 volts wrt D

A is +20 volts wrt B

i) What are the potentials at B, C and D with respect to ground.(+80v, +65v, +50v)

ii) What is the potential at C wrt B and D. (-15v, +15v)

iii) What is the potential at B wrt E. (+80v)

Q6. What is the difference between an emf and a potential difference ?

Q7. What factors affect resistance and how do they affect it ?

Resistivity (directly proportional, depends on material)Length (directly proportional)Area (inversely proportional)Temperature (depends on material +ve or ve co-efficient)

Q8. What is conductance and conductivity ?

Conductance reciprocal of resistanceConductivity reciprocal of resistivity


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4 PRODUCTION OF ELECTRICITY

Q1. Explain how a thermocouple produces electricity.

Notes page 4-2.

Q2. What happens when a force is applied to a piece of quartz crystal ?

Notes page 4-1.

Q3. Describe one aircraft related use of quartz crystals.

Accelerometers for acceleration or vibration measurement.

Q4. Investigate the construction and operation of a common primary cell andwrite a brief report on your findings.

This information should come from the internet or other reference source.



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5 CELLS & BATTERIES

Q1. What is the difference between primary and secondary cells?

Primary not re-chargeable / re-useable.Secondary re-chargeable / re-useable.

Q2. A 25 A-h at the 10 hour rate battery can supply what maximum currentcontinuously and for how long ?

25 amps for 10 hours.

Q3. A 35 A-h at the 1 hour rate battery can supply what maximum current and

for how long ?35 amps for 1 hour.

Q4. If a 40 A-h at the 10 hour battery was discharged at 8 Amps how long wouldyou expect it to be able to provide this current ?

Answer cannot be calculated, but it will be less 5 hours. Battery should be able toprovide 4 amps continuously for 10 hours, but will not be able to provide 8 ampsfor 4 hours because its capacity decreases with increase in load.

Q5. When carrying out a capacity test on a 20 A-h battery, it only achieved 80%,what is the actual battery capacity ?

Q6. A battery was discharged at 10 Amps for 10 hours in order to test itscapacity. What was the batterys rating ?

Q7. If four 2 volt, 1 ampere-hour cells, each with an internal resistance of 2ohms are connected in series to form a battery, what is the battery voltage,capacity and internal resistance ?

Q8. How can a 4 volt, 6 A-h supply be obtained from six 2 volt, 2 A-h cells ?

Q9. Two 12 volt, lead acid, aircraft batteries are connected in parallel to give agreater capacity. Draw a circuit diagram depicting the individual cells ofthese two interconnected batteries.

Q10. What happens if a 4 volt battery is connected in parallel with a 2 voltbattery?

Q11. When 3 identical cells are connected in series across a filament, in whichorder do they discharge?

Q12. In your own words, describe the construction of a typical lead acid aircraftbattery.



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Q13. In your own words, describe the construction of a typical Nickel Cadmiumbattery aircraft battery.

Q14. State some typical A-h ratings for lead acid and Ni-cad aircraft batteries.

Q15. What is thermal runaway and what might cause it occur ?


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6 OHMS LAW

Q1. How are current, voltage and resistance related.

Q2. Complete the table below.

Voltage Current Resistance

12 volts 05 amps 24

24 volts 1/3 amp 720 Ohms

940 volts 200mA 4K7

240 volts 12 Amps 200

12 volts 5 mA 2400 Ohms

68 volts 001 Amps 6.8 k

6 volts 03 amps 20

1020 volts 150A 68M

200mV 50mA 4

500mV 20A 25k200 volts 10 Amps 20

540 volts 200A 2.7M

15 volts 15 A 1

3000 volts 05 A 6k

Q3. If the voltage across a component is doubled what affect will it have on thecurrent flowing through it ? (doubled)

Q4. If the value of a resistor in a circuit is doubled, but the current is keptconstant at its original value, what affect will it have on the voltage acrossthe resistor ? (doubled)

Laboratory Tasks

1. Complete the Proving Ohms Law by measurement experiment.



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8 RESISTANCE & RESISITORS

Q1. If the length of a conductor is doubled and its cross-sectional area halved,what affect will this have on its resistance ?

Q2. If the length of a conductor is doubled and its cross sectional area doubled,what affect will it have on its resistance ?

Q3. Do the following have positive or negative temperature co-efficient ofresistance:

i) Battery electrolyte (-ve)

ii) Thermistors (-ve)

iii) Copper wire (+ve)

iv) Aluminium wire (+ve)

v) Carbon (-ve)

Q4. For what purpose would Eureka wire be used ?

Calibration resistor almost no change in resistance with changes intemperature.

Q5. Use the colour codes to work out the values of the following resistors:i) Red Orange Yellow Gold (230 000 +-5%)

ii) Orange Orange Orange Red (33 000 +-2%)

iii) Violet Green Blue silver (75M+-10%)

iv) Green Green Black Gold (55+-5%)

v) Brown Brown Black Brown (11 +-1%)

vi) Red Green Gold (2.5)

vii) Brown Grey Gold (1.8)

viii) Green Blue Silver (056)

ix) Blue Yellow White Yellow Red (649M+-2%)

x) Orange Brown Yellow Red Red (31400 +-2%)



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Q6. Assuming you want to use resistors with a 10% tolerance, what are the

closest preferred values for the following resistance values:

i) 13M Ohms (12M)

ii) 3.0K Ohms (27k)

iii) 4.9K Ohms (47kor 33k)

iv) 54K Ohms (56k)

v) 14.5K Ohms (15k)

Q7. For the preferred values selected in answer to Q6. Above. What are themaximum and minimum values one could expect to find ?

Q8. What is the difference between a potentiometer and a rheostat ?

Q9. A resistor is connected across a potentiometer and the slider is increasedfrom minimum resistance to maximum (see diagram in notes p.8-7). Whataffect will this have on the voltage across the resistor and the currentflowing through it ?

As wiper moved from zero to max resistance (L to R on the diagram), thepotential across the load increases, the increased potential leads to an increasein load current.

Q10. A resistor is connected in series with a rheostat and the slider is moved fromminimum to maximum resistance (see diagram in note p.8-7). What affectwill this have on the voltage across the resistor and the current flowingthrough it ?

As the wiper is moved from zero to max resistance (R to L on diagram), thecurrent through the load decreases, therefore the voltage across the load alsodecreases.

Q11. When current flows through a resistor, what affect is it likely to have on itsvalue of resistance ? Explain your answer.

The resistance is likely to increase due to the increase in temperature.

Q12. Explain how can a thermistor be used to stabilise the current flowingthrough a coil of wire that tends to get hot when used.

Attach thermistor to coil. Thermistor has ve temp co-efficient of resistance asthe temperature increases, the resistance of the coil increases, but the resistanceof the thermistor decreases. So the overall resistance and therefore currentremains constant.


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9 RESISTORS IN DC CIRCUITS

Q1. Explain Kirchoffs second law.

Q2. Using Ohms law and Kirchoffs second law, derive the formula for the totalresistance of resistors connected in series.

Q3. A 12kresistor is connected in series with a 24kresistor across an 18volt supply. What is the circuit current and the voltage drop across eachresistor ?

RTotal= 36k Current = 18/3600 = 5mA V12k6volts v24k12 volts

Q4. Two 4.7kare connected in series across a 12 volt battery. What is thecircuit current and the voltage drop across each resistor ?

12/94k = 13mA voltage drop = 6v and 6v

Q5. When a 1.2kresistor is connected in series with a 3.6kacross a powersupply, the current is 150mA. What are the voltage drops across theresistors ?

Q6. If three resistors of equal value are connected in series across a powersupply, what proportion of the supply voltage will be dropped across each

resistor.

Figure 1

Q7. Calculate the current flowing in the circuitof Figure 1.

Q8. Calculate the voltage drop across each ofthe resistors in Figure 1.

Q9. Calculate the voltage drops across each of the

resistors in Figure 2.

Q10. Calculate the current flowing in the circuit ofFigure 2

Q11. Calculate the voltage at point B wrt point A inFigure 2.

Q12. Assuming a ground is placed at point C in Figure2, what will be the potential at point B wrt C ?

Q13. Assuming a ground is placed at point B in Figure2, what will be the potential at point B wrt C ? Figure 2



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Q14. Assuming two resistors R1 and R2 are connected in series across VSupply,

complete the following table.

VSupply R1 R2 RTOT ISupply VR1 VR2

100V 1.2k 2.8k 4k 25mA 30V 70V

12V 800 1600 2400 5mA 4V 8V

24V 2666 5333 80 300mA 8V 16V

95V 20 30 50 190mA 3.8V 5.7V

36V 3K6 7K2 10k8 333A 12V 24V

9V 60 120 180 50mA 3V 6V

120V 20 40 60 2A 40V 80V

1000V 44M 12M 56M 1786mA 786V 214V

2080V 1k 300 13k 1.6A 1600V 480V

24V 300 700 1000 24mA 72V 168V

100V 33k 1700 5k 20mA 66V 34V

182V 400 1000 14k 13mA 5.2V 13V

Q15. The off-load voltage of a power supply is 100 volts. When the supply is

connected to a 200resistor the terminal voltage falls to 90 volts. Calculatethe internal resistance of the power supply.

90/200 = 045mA 10/045 = 222

Q16. The off-load voltage of a power supply unit is 28 volts, the on-load voltage is

26 volts. If the load is 600, calculate the internal resistance of the powersupply unit.

26/600 = 43mA 2V/43mA = 46

Q17. The internal resistance of a 30 volt power supply is 60, calculate the

terminal voltage when it is connected to a 240load.

Ratio of the voltage drop across RInt: RLoadis 1:4. Therefore voltage drops are 6volts to 24 volts. Ans. 24V



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10 THE WHEATSTONE BRIDGE

Refer to Figure 1 to answer the following questions

Q1. If R1=R2=R3=R4=100and E=24 volts:

A. Calculate the current flowing throughR1 and R2. (120mA)

B. Calculate the potential differencebetween points A and B. (12V:12Vtherefore 0 volts)

C. If E was decreased to 20 volts, what

affect would it have on the potentialdifference between points A and B ?No effect.

Figure 1Q2. If R1= 200, R2= 600, R3= 1kand E=16 volts:

A. What value must R4be to balance the bridge.

B. What will be the current flowing in R2.

C. What will be the potential at A.

Q3. If R1= 2k2, R2= 32k, R4= 80kand E=21 volts:

A. What value of R3will balance the bridge.

B. If R3has a value of 4k4, what will be the potential difference betweenpoints A and B.

C. If R3has a value of 1k1, what will be the potential difference betweenpoints A and B.

Q4. On connecting the Wheatstone Bridge circuit shown, the meter indicated fullscale deflection. Discuss possible fault conditions that would exhibit thesesymptoms.

Q5. On connecting the Wheatstone Bridge circuit shown, the meter needleabutted against the zero end-stop. Discuss possible fault conditions that

would exhibit these symptoms.

Laboratory Tasks

1. Complete the Electronics Workbench Wheatstone Bridge tasks

2. Complete the Wheatstone Bridge practical task. (investigating the effects ofsupply voltage changes on the operation of a Wheatstone Bridge circuit).


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11 ENERGY & POWER IN DC CIRCUITS

Q1. How much work is done if 3 coulombs of charge is moved through apotential of 20 volts. (60 Joules)

Q2. How much work is done when 10 amps flows through a potential of 6 voltsfor 2 minutes. (7200 Joules)

Q3. How much energy is used when 3 amps flows through a potential of 5 voltsfor 10 minutes. (9 kJoules)

Q4. The voltage across a 3k2resistor in an electrical circuit is 6 volts. Howmuch energy is used in pushing the current through this resistor for 1

minute. (0675 Joules)Q5. 6 amps flows through a 12 volt filament in a circuit. Calculate the:

A. work done by the circuit if the filament is on for 2 hours.

B. energy used by the filament in 2 hours.

C. rate at which work is being done by the filament.

Q6. Assuming the circuit comprises a single resistor connected across a voltagesource, complete the following table.

Voltage

(v)

Resistance

()Current

(I)

Power

(W)

Work Done in

10 mins (J)

10 Volts 600

24 Volts 1200 Watts

2 Amps 400 Joules

24 Volts 320 Joules

200 40mAmps

28 Volts 300 Watts

120 Volts 3k6

Q7. If a 2.5kW kettle is left switched on for 10 minutes, how much energy isused in kWhs.

Q8. If 1kWh costs 7 pence, how much will it cost to keep a 100 Watt light on for6 hours per day for 7 days.

Q9. What is the maximum voltage and current that can be applied to a 300resistor with a 20 Watt power rating.

Q10. What is the maximum voltage and current that can be applied to a 4k7resistor with a 30 Watt power rating.


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Q11. A 28 volt power supply with an internal resistance of 100is connected to a

100load. Calculate the:A. circuit current.

B. power dissipated in the load.

C. power supply terminal voltage when connected to the load.

Q12. A light bulb, rated at 100 Watts, is connected across a 240 volt supply.Calculate the circuit current and the resistance of the filament when the lightis on.

Q13. A light bulb, rated at 100 Watts is connected across a 28 volt supply.Calculate the circuit current and the resistance of the filament when the light

is on.Q14. How much power will be dissipated by a 3k6resistor with 6 amps of

current flowing in it.



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12 CAPACITANCE & CAPACITORS

Q1. A simple parallel plate capacitor has plates with an area of 10cm2, adistance of 001mm between the plates and a dielectric material betweenthe plates with a relative permitivity of 40:

A. Calculate the capacitance of the capacitor.

B. What would have to be done to the distance between the plates todouble the capacitance of the capacitor.

C. What would have to be done to the cross sectional area of the plates tohalf the capacitance of the capacitor.

D. If the dielectric is replaced by a material with twice the relative

permitivity, what affect will it have on the capacitance of the capacitor.

Q2. Calculate the capacitance of a simple parallel plate capacitor which hasplates of cross sectional area of 100cm2, a distance of 0015mm betweenthe plates and a dielectric material between the plates with an absolutepermitivity of 8x10-9Fm-9.

Q3. Calculate the capacitance of a simple parallel plate capacitor which hasplates of cross sectional area of 200cm2, a distance of 0025mm betweenthe plates and a dielectric material between the plates with a relativepermitivity of 300.

Q4. A simple parallel plate capacitor has plates with an area of 150cm

2

, adistance of 002mm between the plates and a dielectric material betweenthe plates with a relative permitivity of 350. Calculate the:

A. Capacitance of the capacitor.

B. Charge on the capacitor if it is fully charged on a 28 volt supply.

C. Energy stored in the capacitor when it is fully charged.

Q5. A multi-plate capacitor has 9 plates each with a cross sectional area of4cm2. The distance between the plates is 015mm and the dielectricmaterial between the plates has a relative permitivity of 50. Calculate the:

A. Capacitance of the capacitor. (944nF)


C. Energy stored in the capacitor when it is fully charged to 28 volts.

Q5. A multi-plate capacitor has 3 plates each with a cross sectional area of500cm2. The distance between the plates is 10mm and the dielectricmaterial between the plates has a relative permitivity of 4. Calculate the:

A. Capacitance of the capacitor. (354nF)


C. Energy stored in the capacitor when it is fully charged to 28 volts.


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13 CAPACITORS IN DC CIRCUITS

Q1. A simple parallel plate capacitor has plates with an area of 12cm2, adistance of 001mm between the plates and a dielectric material betweenthe plates with a relative permitivity of 40:

A. Calculate the capacitance of the capacitor. (423nF)

B. Calculate the electrical energy stored in the capacitor if it is connected

across a 28 volt supply. (1665J)

C. Calculate the charge on the capacitor if the potential is across it is

increased to 100 volts. (4248C)

Q2. Complete the following table.

Capacitor 1 Capacitor 2 Capacitor 3 ConnectionTotal

Capacitance

100F 200F none Series 6666F

200F 100F none Parallel 300F

100F 300F 200F Series 5454F



30F 470F 100F Parallel 600F

200F 47F 150F Series 304F

200F 100F 200F Series 50F

30F 47F 100F Parallel 177F

200F 400F 400F Series 100F

Q3. A 400F capacitor is connected in series with a 200F capacitor across a

120 volts supply. Assuming the capacitors are fully charged, calculate the:A. Voltage across each capacitor. (40V, 80V)

B. Energy stored in each capacitor. (320mJ, 640mJ)

Q4. A 47F capacitor is connected in series with a 100F capacitor across a 28volts supply. Assuming the capacitors are fully charged, calculate the:

A. Charge on each capacitor. (896C)

B. Voltage across each capacitor. (19V, 9V)

C. Energy stored in each capacitor. (85mJ, 4mJ)

D. Total charge on both capacitors. (series connected so same as A

above)


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Q5. C1=100F, C2=200F and VSUPPLY= 60 volts. Calculate the voltage drop

across each capacitor (C1 40 volts, C2 20 volts)Q6. C1=100F, C2=200F, C3=400and VSUPPLY= 120 volts. Calculate the:

A. Total capacitance.

B. Voltage drop across each capacitor.(V1 - 6857 volts, V2 3428 volts, V3 - 1714 volts)

C. Charge on each capacitor. (69mC series)

D. Energy stored in each capacitor. (235mJ, 118mJ, 59mJ)

Q7. C1=120F, C2=170F, C3=90F and VSUPPLY= 100 volts. Calculate the:

A. Total capacitance. (395F)


C. Charge on each capacitor. (65mJ, 46mJ, 87mJ)

D. Energy stored in each capacitor.

Q8. C1=10F, C2=100F, C3=170F and VSUPPLY= 100 volts. Calculate the:

A. Total capacitance. (863F)


C. Charge on each capacitor. (863C series)

D. Energy stored in each capacitor. (37mJ, 36mJ, 212mJ)



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Refer to Figure 1 to answer the following questions

Q7. A 100F capacitor is connected in series with a 900resistor across a 20volt power supply.

A. Calculate the circuit timeconstant.

B. Calculate how long will it takefor the circuit current to fall tozero when the switch is movedto position A.

C. Calculate the voltage acrossthe resistor after 1 time

constant.

Figure 1

D. Calculate the maximum circuit current.

E. Calculate the energy stored in the electric field when the capacitor isfully charged.

F. Sketch a graph to show the circuit current and the voltage across thecapacitor with respect to time when the switch is moved to position A.



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14 MAGNETISM

Q1. Describe the domain theory of magnetism.

Q2. Explain how lines of flux can be demonstrated to a group of students.

Q3. Describe the Earths magnetic field and explain the terms dip and variation.

Q4. Describe one use for a hard iron material and one use for a soft ironmaterial. Explain how the materials magnetic properties are exploited.

Q5. Explain how components can be shielded from magnetic fields.

Q6. Explain why might it be necessary to shield components from magneticfields.


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15 ELECTROMAGNETISM

Q1. Describe the field around two adjacent current carrying conductors if thecurrents are flowing in opposite direction.

Q2. Explain how the right hand gripping rule and corkscrew rule can be used todetermine the magnetic north end of a coil

Q3. A coil of 200 turns is wound uniformly in the form of a toroid with an air core.The mean circumference is 60cm and the uniform cross-sectional are is5cm2. If the current flowing in the coil is 4 amps, calculate the:

i) Magnetising force (1333 At/m)ii) Flux density (1.68 mT)

iii) Total flux (084 Wb)

Q4. The airgap in a certain magnetic circuit is 01cm long and has a cross-sectional area of 30 cm2. If 600 At are available to produce flux, calculatethe flux density in the airgap. (0755T)

Q5. An iron ring has a mean diameter of 50 cm and a cross-sectional area of10cm2. 500 turns of wire are uniformly wound on the ring and a flux of125mWb is produced by a current of 2 amps. Calculate the relative

permeability of the iron. (1560)Q6. An airgap in a magnetic circuit is 012cm long and has a cross-sectional

area of 25cm2. Calculate the reluctance of the airgap, the number of

ampere-turns to send a flux of 800Wb across the gap and the magnetisingforce H in the gap. (382000 At/Wb, 306 At, 255000 At/m)

Q7. A coil of 300 turns is wound on an iron ring having a mean circumference of

40cm and a cross-sectional area of 5cm2. The coil has a resistance of 8

and is connected to a 20V d.c. supply. Taking r= 900, calculate:

i) mmf (750 At)

ii) magnetising force (1875 At/m)

iii) total flux (106 mWb)

iv) Flux density (212 T)

v) Reluctance of the ring (707000 At/Wb)


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Q8. An iron ring of mean circumference 50cm has an airgap of 01cm and is

wound with a coil of 120 turns, If the relative permeability for the iron is 350when a current of 25 amps flows in the coil, calculate the flux density.(254mT)

Q9. A steel ring has a mean diameter of 20cm and a cross-sectional area of

1.5cm2broken by a parallel sided airgap of length 05cm. Taking r= 500,calculate the current necessary in 3000 turns of wire wound on the ring toproduce a flux density of 05 Tesla in the airgap and the total flux in the steelring. ( )

Q10. A steel ring having a mean diameter of 35cm and a cross-sectional area of24cm2is broken by a parallel sided airgap of length 12cm. Short pole

pieces of negligible reluctance extend the effective cross-sectional area ofthe airgap to 12cm2. Taking r= 700 and neglecting leakage, determine thecurrent necessary in 300 turns of wire wound on the ring to produce a fluxdensity in the airgap of 025 Tesla. (13.17 Amps)

Q11. A steel ring having a mean diameter of 25cm and a cross-sectional area of12cm2is broken by a parallel sided airgap of length 16cm. Short polepieces of negligible reluctance extend the effective cross-sectional area of

the airgap to 10cm2. Taking r= 600 and neglecting leakage, determine thecurrent necessary in 3000 turns of wire wound on the ring to produce a fluxdensity in the airgap of 05 Tesla. ( Amps)



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Q4. A 15mH inductor is connected in series with a 470resistor across a 12

volt power supply, calculate the:A. circuit time constant.

B. circuit current after 1 time constant.

C. voltage across the resistor after 1 time constant.

D. energy stored in the magnetic field when the current has reached itsmaximum value.

Q5. When an inductor of unknown value was connected across a 12 volt powersupply, the current settled at 300mA after 200mS. Calculate the:

A. resistance of the inductor.

B. circuit time constant.C. voltage across the inductor after 1 time constant.

Q6. If an ideal capacitor was fully charged on a power supply and thenconnected across an ideal inductor. Explain what would happen. (an idealcapacitor has no inductance or resistance and an ideal inductor has nocapacitance or resistance)

Q7. A real capacitor is fully charged on a power supply and then connectedacross an actual inductor. Explain what happens.

Q8. Explain how a coil, connected into 24 volt d.c. circuit, could produce a back

emf of 2000 volts.

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