Practical Electricity and Safety
Chapter 23 MCQs
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
1
A straight wire carries a current in a magnetic field. The wire experiences a force.
Which condition is needed for the force to be maximum?
A current parallel to magnetic field
B current perpendicular to magnetic field
C current zero
D magnetic field zero
2
A current-carrying wire is placed between the poles of a magnet. The current is parallel to the magnetic field.
What happens to the wire?
A it experiences maximum force
B it experiences no magnetic force
C it rotates continuously
D it becomes permanently magnetised
3
A current-carrying wire is placed in a magnetic field. The current is reversed.
What happens to the direction of the force on the wire?
A it remains unchanged
B it reverses
C it becomes zero permanently
D it becomes parallel to the current
4
A current-carrying wire is placed in a magnetic field. The magnetic field direction is reversed, but the current direction is unchanged.
What happens to the force on the wire?
A it reverses direction
B it remains unchanged
C it becomes zero every time
D it changes into an electric field
5
A current-carrying wire is placed in a magnetic field. Both the current direction and the magnetic field direction are reversed.
What happens to the force direction?
A it reverses
B it remains the same
C it becomes zero
D it becomes random
6
Which rule is used to find the direction of force on a current-carrying conductor in a magnetic field?
A Fleming’s left-hand rule
B Fleming’s right-hand rule
C right-hand grip rule only
D corkscrew rule for screw motion only
7
In Fleming’s left-hand rule, the first finger represents:
A force
B magnetic field
C current
D voltage
8
In Fleming’s left-hand rule, the second finger represents:
A motion/force
B magnetic field
C conventional current
D resistance
9
In Fleming’s left-hand rule, the thumb represents:
A magnetic field
B conventional current
C force/motion
D electron flow
10
A wire carries current into the page. The magnetic field is from left to right.
Using Fleming’s left-hand rule, what is the direction of the force on the wire?
A upwards
B downwards
C left
D right
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
11
A wire carries current out of the page. The magnetic field is from left to right.
What is the direction of the force on the wire?
A upwards
B downwards
C into the page
D out of the page
12
A wire carries current from left to right. The magnetic field is vertically downwards.
What is the direction of the force on the wire?
A into the page
B out of the page
C left
D right
13
A wire carries current upwards. The magnetic field is into the page.
What is the direction of the force on the wire?
A left
B right
C upwards
D downwards
14
A conductor of length 0.20 m carries a current of 3.0 A at right angles to a magnetic field of flux density 0.50 T.
What is the force on the conductor?
A 0.030 N
B 0.30 N
C 3.3 N
D 30 N
15
A wire experiences a force of 0.48 N when carrying a current of 4.0 A at right angles to a magnetic field. The length of wire in the field is 0.30 m.
What is the magnetic flux density?
A 0.040 T
B 0.40 T
C 1.6 T
D 5.8 T
16
A wire of length 0.50 m is in a magnetic field of flux density 0.20 T. The force on the wire is 0.80 N.
The wire is perpendicular to the field.
What current flows in the wire?
A 0.080 A
B 0.20 A
C 4.0 A
D 8.0 A
17
A student wants to increase the force on a current-carrying wire in a magnetic field.
Which change would not increase the force?
A increase the current
B use a stronger magnetic field
C increase the length of wire in the field
D turn the wire so current is parallel to the field
18
A wire carrying current is placed in a magnetic field and moves upwards.
Which change makes the wire move downwards?
A increase current only
B reverse current only
C increase magnetic field strength only
D increase length of wire only
19
A current-carrying coil is placed between the poles of a magnet. The two vertical sides of the coil experience forces in opposite directions.
What is produced?
A a turning effect
B a heating effect only
C no movement because forces cancel completely
D a sound wave only
20
In a simple d.c. motor, the split-ring commutator is used to:
A reverse the current in the coil every half-turn
B increase the resistance to infinity
C stop the coil after one half-turn
D change direct current into sound
21
Why does the current in a d.c. motor coil need to reverse every half-turn?
A to keep the turning effect in the same rotational direction
B to make the coil stop at 90°
C to remove the magnetic field
D to make the battery recharge
22
Which pair of parts in a simple d.c. motor supplies current to the rotating coil?
A soft iron core and permanent magnet
B brushes and split-ring commutator
C axle and magnetic field lines
D north pole and south pole only
23
A simple d.c. motor rotates too slowly.
Which change would increase its speed or turning effect?
A reduce current in the coil
B use weaker magnets
C increase number of turns on the coil
D remove the split-ring commutator
24
A d.c. motor coil is exactly vertical between magnetic poles. The forces on its two vertical sides are equal and opposite.
Why does the coil continue rotating past this position?
A the split-ring commutator disconnects the battery forever
B inertia carries the coil past the vertical position
C the magnetic field becomes zero
D the coil becomes non-conducting
25
A loudspeaker uses the motor effect.
What happens when an alternating current flows through the loudspeaker coil?
A the coil vibrates because the force reverses repeatedly
B the coil moves once and then remains fixed
C the coil becomes a permanent magnet only
D the cone absorbs all sound waves
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
26
A conductor is moved through a magnetic field and a potential difference is induced.
Which condition is necessary?
A conductor cuts magnetic field lines
B conductor is stationary parallel to the field
C conductor is made of plastic
D magnetic field strength is zero
27
A straight wire is moved upwards through a horizontal magnetic field. A current is induced in the wire.
Which change reverses the direction of the induced current?
A move the wire faster in the same direction
B move the wire in the opposite direction
C use a stronger magnetic field only
D use a longer wire only
28
A wire is moved through a magnetic field. The induced potential difference increases.
Which change could cause this?
A move the wire more slowly
B use a weaker magnetic field
C move the wire faster
D keep the wire stationary
29
A magnet is pushed into a coil connected to a sensitive galvanometer. The galvanometer deflects.
What happens when the magnet is held stationary inside the coil?
A the deflection becomes maximum
B the deflection returns to zero
C the deflection reverses continuously
D the coil melts
30
A magnet is pushed into a coil and then pulled out again.
What happens to the galvanometer deflection?
A same direction both times
B opposite directions for pushing in and pulling out
C no deflection in either case
D only deflects when the magnet is outside the coil
31
A magnet is pushed into a coil faster.
What happens to the induced e.m.f.?
A it decreases
B it increases
C it remains unchanged
D it becomes zero
32
A coil has 200 turns. A magnet is moved into it and produces an induced e.m.f.
Which change increases the induced e.m.f.?
A use 100 turns instead
B move the magnet more slowly
C use a stronger magnet
D keep the magnet stationary inside the coil
33
An e.m.f. is induced in a coil when:
A magnetic flux linkage changes
B the coil has no turns
C the magnet and coil are both stationary relative to each other
D the coil is made of plastic
34
Lenz’s law states that the induced current:
A always helps the change that produces it
B always opposes the change that produces it
C always flows clockwise
D has no magnetic effect
35
A north pole of a magnet is pushed towards a coil. The induced current makes the near end of the coil become a north pole.
Why?
A to attract the incoming north pole
B to oppose the approach of the north pole
C to remove all resistance from the coil
D to stop magnetic fields existing
36
A north pole of a magnet is pulled away from a coil. What pole is induced at the near end of the coil?
A north, to repel the magnet
B south, to attract the magnet
C no pole because the magnet is moving
D both poles at the same end
37
A coil connected to a galvanometer is rotated in a magnetic field.
Which change increases the maximum induced e.m.f.?
A rotate the coil more slowly
B reduce the magnetic field strength
C increase the number of turns on the coil
D keep the coil stationary
38
In a simple a.c. generator, the output changes direction because:
A the coil cuts magnetic field lines in opposite directions every half-turn
B the battery reverses every half-turn
C the magnets change from north to south every second
D the slip rings disconnect the coil permanently
39
Which part of a simple a.c. generator allows the coil to rotate while maintaining connection to the external circuit?
A split-ring commutator
B slip rings and brushes
C fuse and earth wire
D soft iron keeper
40
A simple a.c. generator is rotated faster.
What happens to the output?
A peak e.m.f. increases and frequency increases
B peak e.m.f. decreases and frequency decreases
C peak e.m.f. unchanged and frequency decreases
D output becomes direct current
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
41
A simple generator has a coil rotating in a magnetic field. The induced e.m.f. is zero at a certain instant.
Which condition is most likely at that instant?
A the rate of cutting magnetic field lines is zero
B the coil is cutting field lines at maximum rate
C the magnetic field has disappeared
D the coil has infinite turns
42
A transformer has 200 turns on the primary coil and 1000 turns on the secondary coil.
What type of transformer is it?
A step-down because secondary has more turns
B step-up because secondary has more turns
C step-up because secondary has fewer turns
D step-down because primary has fewer turns
43
A transformer has 800 turns on the primary coil and 200 turns on the secondary coil. The primary voltage is 240 V.
What is the secondary voltage?
A 30 V
B 60 V
C 960 V
D 3840 V
44
A transformer has 150 turns on the primary and 900 turns on the secondary. The primary voltage is 12 V.
What is the secondary voltage?
A 2.0 V
B 72 V
C 162 V
D 7200 V
45
An ideal transformer has primary voltage 240 V and secondary voltage 12 V. The secondary current is 5.0 A.
What is the primary current?
A 0.25 A
B 4.8 A
C 100 A
D 250 A
46
An ideal transformer has primary voltage 20 V and primary current 3.0 A. The secondary voltage is 120 V.
What is the secondary current?
A 0.50 A
B 3.0 A
C 18 A
D 720 A
47
A transformer is used in power transmission to step up voltage before electricity travels along cables.
Why does this reduce energy loss?
A higher voltage gives lower current for the same power, reducing heating loss in cables
B higher voltage increases cable resistance to infinity
C higher voltage makes current zero always
D higher voltage changes a.c. into d.c.
48
Why does a transformer only work with alternating current in the primary coil?
A alternating current produces a changing magnetic field
B direct current has no charge
C alternating current has no magnetic field
D direct current melts the secondary coil every time
49
A transformer core is made of laminated soft iron.
Why is soft iron used?
A it is easily magnetised and demagnetised
B it has infinite resistance
C it is non-magnetic
D it prevents magnetic flux from linking the coils
50
Why is the iron core of a transformer laminated?
A to reduce eddy currents and heating losses
B to make the transformer work only with d.c.
C to increase the mass of the transformer only
D to stop magnetic flux changing
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Chapter 23 Answer Key
| Q | Ans | Q | Ans | Q | Ans | Q | Ans | Q | Ans |
|---|---|---|---|---|---|---|---|---|---|
| 1 | B | 11 | A | 21 | A | 31 | B | 41 | A |
| 2 | B | 12 | A | 22 | B | 32 | C | 42 | B |
| 3 | B | 13 | A | 23 | C | 33 | A | 43 | B |
| 4 | A | 14 | B | 24 | B | 34 | B | 44 | B |
| 5 | B | 15 | B | 25 | A | 35 | B | 45 | A |
| 6 | A | 16 | D | 26 | A | 36 | B | 46 | A |
| 7 | B | 17 | D | 27 | B | 37 | C | 47 | A |
| 8 | C | 18 | B | 28 | C | 38 | A | 48 | A |
| 9 | C | 19 | A | 29 | B | 39 | B | 49 | A |
| 10 | B | 20 | A | 30 | B | 40 | A | 50 | A |
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Detailed Explanations
1. B
-
Force on a current-carrying wire is maximum when current is perpendicular to the magnetic field.
-
If current is parallel to the field, force is zero.
-
Maximum force condition:
-
current at 90° to magnetic field.
-
2. B
-
If current is parallel to the magnetic field, there is no sideways motor-effect force.
-
Force is zero.
-
The wire does not experience maximum force.
3. B
-
Reversing the current reverses the direction of force.
-
This is a direct application of Fleming’s left-hand rule.
-
Same field, opposite current = opposite force.
4. A
-
Reversing the magnetic field reverses the force.
-
Current direction is unchanged.
-
One of the two inputs is reversed, so the output force reverses.
5. B
-
If both current and magnetic field are reversed, the force direction remains the same.
-
Reversing one reverses force.
-
Reversing both reverses it twice, so it returns to the original direction.
6. A
-
Fleming’s left-hand rule is used for the motor effect.
-
It gives the direction of force on a current-carrying conductor in a magnetic field.
7. B
-
Fleming’s left-hand rule:
-
First finger = magnetic field
-
seCond finger = current
-
thuMb = motion/force
-
8. C
-
The second finger represents conventional current.
-
Conventional current flows from positive to negative.
9. C
-
The thumb represents force or motion.
-
This is the direction the wire moves.
10. B
-
Current is into the page.
-
Magnetic field is left to right.
-
Using Fleming’s left-hand rule, the force is downwards.
-
Answer = B
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
11. A
-
Current is out of the page.
-
Magnetic field is left to right.
-
Fleming’s left-hand rule gives force upwards.
-
Answer = A
12. A
-
Current is left to right.
-
Magnetic field is vertically downwards.
-
Fleming’s left-hand rule gives force into the page.
-
Answer = A
13. A
-
Current is upwards.
-
Magnetic field is into the page.
-
Fleming’s left-hand rule gives force to the left.
-
Answer = A
14. B
-
Force on a current-carrying conductor:
-
F = BIL
-
-
F = 0.50 × 3.0 × 0.20
-
F = 0.30 N
15. B
-
F = BIL
-
B = F / IL
-
B = 0.48 / (4.0 × 0.30)
-
B = 0.48 / 1.20
-
B = 0.40 T
16. D
-
F = BIL
-
I = F / BL
-
I = 0.80 / (0.20 × 0.50)
-
I = 0.80 / 0.10
-
I = 8.0 A
17. D
-
Force increases when:
-
current increases
-
magnetic field strength increases
-
length of wire in field increases
-
-
Turning the wire so the current is parallel to the field makes force zero.
-
So it would not increase the force.
18. B
-
To reverse the direction of movement, reverse either:
-
current direction
-
magnetic field direction
-
-
Reversing current only makes the wire move downwards.
19. A
-
The two sides of the coil experience forces in opposite directions.
-
These forces form a couple.
-
A couple produces a turning effect.
-
This is the basic action of a motor.
20. A
-
The split-ring commutator reverses the current in the coil every half-turn.
-
This keeps the turning effect in the same rotational direction.
-
Without it, the coil would not continue rotating properly.
21. A
-
After half a turn, the two sides of the coil swap positions.
-
If current did not reverse, the torque would reverse and stop continuous rotation.
-
Reversing current every half-turn keeps the coil turning in the same direction.
22. B
-
Brushes press against the split-ring commutator.
-
Together, they transfer current from the external circuit to the rotating coil.
-
The split-ring also reverses current every half-turn.
23. C
-
A stronger motor effect is produced by:
-
more turns on the coil
-
larger current
-
stronger magnets
-
soft iron core
-
-
Increasing number of turns increases turning effect.
24. B
-
When the coil is vertical, the turning effect may be zero for an instant.
-
The coil continues rotating because of inertia.
-
Then the commutator reverses current and torque continues in the same direction.
25. A
-
A loudspeaker coil sits in a magnetic field.
-
Alternating current reverses direction repeatedly.
-
The force on the coil reverses repeatedly.
-
The coil and cone vibrate, producing sound.
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
26. A
-
Electromagnetic induction occurs when a conductor cuts magnetic field lines.
-
There must be relative motion between the conductor and magnetic field.
-
Stationary conductor in a steady field gives no induced e.m.f.
27. B
-
The direction of induced current depends on direction of motion and magnetic field direction.
-
Moving the wire in the opposite direction reverses the induced current.
-
Moving faster only increases the size of the induced e.m.f.
28. C
-
Induced potential difference increases when:
-
conductor moves faster
-
magnetic field is stronger
-
wire is longer in the field
-
more turns are used
-
-
Moving the wire faster increases the rate of cutting field lines.
29. B
-
When the magnet is moving, magnetic flux linkage changes.
-
When held stationary inside the coil, flux linkage is constant.
-
No induced e.m.f. is produced.
-
Galvanometer deflection returns to zero.
30. B
-
Pushing the magnet in and pulling it out produce opposite changes in magnetic flux.
-
Therefore the induced current reverses.
-
The galvanometer deflects in opposite directions.
31. B
-
Faster motion means greater rate of change of magnetic flux linkage.
-
Greater rate of change produces larger induced e.m.f.
-
So the induced e.m.f. increases.
32. C
-
A stronger magnet gives a stronger magnetic field.
-
This increases the change in flux linkage.
-
Therefore induced e.m.f. increases.
33. A
-
An e.m.f. is induced when magnetic flux linkage changes.
-
No change in flux linkage means no induced e.m.f.
-
This is the main idea behind generators.
34. B
-
Lenz’s law:
-
induced current flows in a direction that opposes the change producing it.
-
-
It does not always flow clockwise.
-
It depends on the direction of the change.
35. B
-
A north pole is pushed towards the coil.
-
The coil induces a north pole at the near end.
-
Like poles repel.
-
This opposes the approach of the magnet.
36. B
-
A north pole is pulled away from the coil.
-
The coil tries to oppose the removal.
-
It induces a south pole at the near end to attract the north pole back.
-
Answer = south
37. C
-
Maximum induced e.m.f. increases when:
-
coil rotates faster
-
magnetic field is stronger
-
coil has more turns
-
coil area is larger
-
-
Increasing the number of turns increases induced e.m.f.
38. A
-
In an a.c. generator, the coil rotates in a magnetic field.
-
Every half-turn, the sides of the coil cut field lines in opposite directions.
-
This reverses the induced e.m.f.
-
So the output is alternating.
39. B
-
A simple a.c. generator uses slip rings and brushes.
-
These allow the rotating coil to stay connected to the external circuit.
-
Split-ring commutator is for a d.c. motor/generator, not a simple a.c. generator.
40. A
-
Rotating faster increases:
-
rate of change of magnetic flux
-
peak induced e.m.f.
-
number of cycles per second
-
-
Therefore both peak e.m.f. and frequency increase.
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
41. A
-
Induced e.m.f. depends on the rate of cutting magnetic field lines.
-
If the rate of cutting is zero, induced e.m.f. is zero.
-
This happens at certain positions during rotation.
42. B
-
Secondary turns = 1000
-
Primary turns = 200
-
Secondary has more turns than primary.
-
Therefore it is a step-up transformer.
43. B
-
Transformer equation:
-
Vs / Vp = Ns / Np
-
-
Vs / 240 = 200 / 800
-
Vs = 240 × 1/4
-
Vs = 60 V
44. B
-
Vs / Vp = Ns / Np
-
Vs / 12 = 900 / 150
-
Vs / 12 = 6
-
Vs = 72 V
45. A
-
Ideal transformer:
-
input power = output power
-
VpIp = VsIs
-
-
240 × Ip = 12 × 5.0
-
240Ip = 60
-
Ip = 0.25 A
46. A
-
Ideal transformer:
-
VpIp = VsIs
-
-
20 × 3.0 = 120 × Is
-
60 = 120Is
-
Is = 0.50 A
47. A
-
For the same power:
-
P = VI
-
-
If voltage is increased, current decreases.
-
Cable heating loss depends on current:
-
power loss = I²R
-
-
Lower current means much smaller heating losses.
48. A
-
A transformer works by electromagnetic induction.
-
The primary coil must create a changing magnetic field.
-
Alternating current creates a changing magnetic field.
-
Steady direct current does not produce continuous induction in the secondary.
49. A
-
Soft iron is easily magnetised and demagnetised.
-
This allows the transformer core to follow the changing magnetic field efficiently.
-
It also helps link magnetic flux between primary and secondary coils.
50. A
-
Laminating the core means it is made of thin insulated layers.
-
This reduces eddy currents in the core.
-
Smaller eddy currents mean less heating loss.
-
The transformer becomes more efficient.
For Full Scale Course: Written and Compiled By Sir Hunain Zia (AYLOTI), World Record Holder With 154 Total Personal A Grades, 11 World Records and 7 Distinctions, Educate A Change.
Common Traps From This Chapter
| Trap | Correct Rule |
|---|---|
| Motor effect maximum force | current perpendicular to magnetic field |
| Current parallel to field | no magnetic force |
| Reverse current | force reverses |
| Reverse magnetic field | force reverses |
| Reverse both current and field | force direction unchanged |
| Motor direction rule | Fleming’s left-hand rule |
| First finger | field |
| Second finger | conventional current |
| Thumb | force/motion |
| Force formula | F = BIL |
| D.C. motor commutator | reverses current every half-turn |
| Motor continuous rotation | current reversal keeps torque same direction |
| Loudspeaker | alternating current causes vibration |
| Induction condition | changing magnetic flux linkage |
| Stationary magnet inside coil | no induced e.m.f. |
| Faster motion | larger induced e.m.f. |
| Stronger magnet | larger induced e.m.f. |
| More coil turns | larger induced e.m.f. |
| Lenz’s law | induced current opposes the change |
| A.C. generator | uses slip rings |
| D.C. motor | uses split-ring commutator |
| Faster generator rotation | larger peak e.m.f. and higher frequency |
| Transformer equation | Vs/Vp = Ns/Np |
| Step-up transformer | more secondary turns |
| Step-down transformer | fewer secondary turns |
| Ideal transformer power | VpIp = VsIs |
| Power transmission | high voltage, low current, less I²R loss |
| Transformer primary | needs a.c. for changing magnetic field |
| Soft iron core | easily magnetised and demagnetised |
| Laminated core | reduces eddy current heating losses |