Magnets – Magnetic field – EN resolution
MAGNETS – MAGNETIC FIELD – RESOLUTION
Commented resolution of entrance exam exercises on
Magnets – Magnetic field
01- a ) The pole of the magnetic compass needle that points to Roraima (M) is its North pole and the one that points to Paraná (N) is its South pole.
b) Observe in the figure that N attracts P and, as N is South, P is North and Q is South.
02 – R- A — See theory
03 – R- A — remember that a magnet attracts iron regardless of polarity and that poles with the same name repel each other and poles with opposite names attract each other
04 – R- C — remember that a magnet attracts iron regardless of polarity and that poles with the same name repel each other and poles with opposite names attract each other
05 – R- E — Magnetic forces obey the principle of action and reaction (they always have the same intensity, same direction but opposite directions) and always appear in pairs.
06- R- D — see theory
07 – R- C — see theory
08 – R- D — see theory
09- Right hand to the sunrise C (east), in front B (north), behind D (south) and to the left A, (west) — R- D
10 – The end L of the piece of iron is attracted by the pole K of the magnet (the magnet attracts iron regardless of
polarity) — one of the possibilities is the one in the figure — R- B
11- It can be attracted or repelled depending on which poles are close — R – E
12- a) It is bar N because its poles, where the magnetic properties are more intense, attract any position of the iron bar, which is M.
b) Bar M would fall, since the central region of the magnet is neutral, unlike the poles where it is maximum.
13 – The magnet attracts the two nails, causing them to attract each other, determining their polarities.
as shown in the figure above — R- B
14- R- A — see theory
15- Look at the figure below:
16- Observe the polarities in the figures below:
R- And
17- a) Observe in the figure below that the resulting forces at P and Q make magnet B rotate clockwise around G.
b) Observe in the figure below that the resulting forces at P and Q make magnet B rotate counterclockwise around G.
18- Observe the sequence below:
Thus, the right part of the magnetized nail (head) is the south pole and the left part (tip) is the north pole.
19- The most coherent alternative is C, assuming the forces that the needle exchanges with the Earth and with the magnet are comparable.
R-C
20 – 01- Correct — see theory.
02- False — if there is no magnetic field, the compass will not orient itself.
04- Correct — the compass always orients itself in the geographic or magnetic north-south direction.
08- Correct — both are demagnetized.
16- Correct — if the oven temperature is high enough, it will cause an intense oscillatory movement of the elementary particles, disorienting them.
32- False — this is not one of the magnetization processes.
Correct: 01; 04; 08 and 16
21- Since the intensity of the force is inversely proportional to the square of the distance x, doubling x will make the force 4 times smaller — A- A 
22- a)
b) Vertical balance — F at =P=mg=0.02.10 — F at =0.2N — F at = μF N — 0.2=0.5.F N — F N =0.4N — horizontal balance — F mag =F N — F mag =0.4N
23- R- B — see theory
24- R- B — see theory
25- Note that horizontally, the two south poles cancel out the orientation of the compass needle, which aligns vertically according to alternative A.
A- A
26- R- A — see theory
27- It is impossible to obtain isolated poles — R- D
28- The permanent magnet magnetizes the iron bar, transforming end A into the south pole and end B into the north pole — R- C
29- The north and south poles are swapped — R- C
30- Poles of opposite names attract each other — R- B
31- The forces of attraction that the iron exchanges with the magnet have the same intensity (for action and reaction) — F(i)=F(f) — F(i)=ma(i) — F(f)=2ma(f) — ma(i)=2ma(f) — a(i)=2a(f) — R- B
32- R- A — see theory
33 – R- D — see theory
34- Note in the diagram below that, as the bar is in equilibrium, the resultant of the forces acting on it is zero, that is,
F din =P bar + F mag — R- D
35- If it were broken in the α or β directions you would have two opposite poles in the direction of the cut and they would be attracted and fit together.
due to the magnetic force — so, so that they do not fit together, the distribution can only be according to the π plane, as shown in the figure
R-C
36- a) The magnetic needles will have, at each point, their north pole indicating the direction of . 
b) The magnetic induction vector 
at each point is tangent to the induction lines with the same direction as the induction lines.
37- Observe the figure below where the induction lines of the magnet on the left (1) and right (2) are drawn, passing through point P.
The magnetic induction vector 
that is tangent to point P has a vertical direction and the direction of the induction line, that is, downwards — the magnetic induction vector 
that is tangent to point P has a vertical direction and the direction of the induction line, that is, upwards — note that these two vectors cancel each other out at point P — R-E
38- Note that the vector sum of the Earth’s magnetic field with the horizontal magnetic field provides the magnetic field 
resultant 
of the compass — R- E
39- The intensity of the magnetic field is lower in the region of the magnet where there is a lower concentration of induction lines, that is, at point P 1 — R- A
40- R- C — see theory
41- 01- Correct — see theory
02- False — the magnetic north pole is located close to the geographic south pole
04- Correct — see theory
08- Correct — see theory
16- False — as you move away from the Earth’s surface, the intensity of the Earth’s magnetic field decreases
Sum – (01 + 04 + 08)=13
42- a) Observe the completed table:
b) Bacteria from sample A are attracted to the bottom of lakes located near magnetic north. Bacteria from sample B will orient themselves towards the bottom of lakes located in magnetic south. Since there is no magnetic polarity at the equator, bacteria from sample C will be at this location.
43- Note that in figure 1 the interactions between the poles are attractive (opposite names) and in figure 2, repulsive (same name) —
R-D
44- The north pole of the magnetic needle is tangent at each point to the lines of induction that leave the north pole and reach the south pole and have the direction of (see figure below)
A- A
45- The north poles of compasses are always oriented in the direction of the induction lines (they leave the north pole and reach the south pole of the compass).
magnet) — R- B46- Placing at point 0 all the intensity vectors B, which move away from the north poles and reach the south poles
R- And
47- The north pole of the compass always points to the geographic north pole — note in the figure and also note that — in Havana
the compass indicates the north pole and is tilted downwards — in Natal, on the equator, the compass indicates the north pole and is not tilted — in Punta Arenas´, the compass indicates the north pole and is tilted upwards — R- D
48- Assuming the upper part is a north pole and the lower part a south pole and separating them, you will get —
R-C
49- Look at the figure below:
1 full turn + 1 full turn = 2 full turns — R- D
50- The induction lines are closed lines that leave the north pole and reach the south pole — A- A
51- (01) Wrong. The poles of a magnet are inseparable, there is always a north and a south.
(02) Correct. Analyzing the behavior of two nearby magnets, William Gilbert noticed that poles with the same name never point in the same direction, concluding that the Earth behaves like a large magnet.
(04) Wrong. The interaction between certain minerals and pieces of iron is magnetic in nature.
(08) Correct. In 1820, Oersted carried out his important experiment, showing that an electric current is capable of producing deflections in a magnetic needle.
R- (02 + 08)=10
52- If the rupture occurred in the direction of planes a or b (horizontal), it could be repaired, since poles with opposite names would appear in the rupture region, generating forces of attraction. In the direction of plane p (vertical), the ends of the two magnets formed with the rupture would have to be aligned by joining poles with the same name, which is impossible, since they repel each other. The figure below illustrates the ruptures in both directions.
R-C
53- a) Correct.
b) Wrong — the electric field only exerts force on electrified particles — as the statement itself states, nanoparticles must be conducted by magnets, that is, by a magnetic field.
c) Wrong — the magnetic field of a wire is proportional to the inverse of the distance.
d) Wrong — only ferromagnetic metallic substances can be used.
e) Wrong — artificial magnets can also be used
A- A
54- Analyzing the statements:
I – Wrong. The magnetic south pole points to the geographic south.
II – Correct.
III – Correct.
IV – Wrong. In the absence of external magnetic fields, the needle is oriented in the north-south direction of the Earth.
R-B
55- 1. False — the needle’s magnetic field exists, however, for this needle to undergo any deflection, it has to be influenced by another magnetic field.
2. Correct — the compass needle does not undergo deflection because it is not in the presence of any magnetic field, and therefore the magnetic field on the Moon is null.
R-B
56- I- Correct — The uniform magnetic field is created by two poles and the direction and sense of the infinite points of the same is 
from the north pole to the south pole (see figure) — the north pole of the magnet is subject to a force of the same direction and sense as the magnetic field, while the south pole is subject to a force of the same direction as the magnetic field, but in the opposite direction — these two forces have the same intensity, therefore the resultant of the forces on the magnet is zero, and consequently the magnet does not translate, but the distance between the lines of action of these forces is different from zero, therefore the body is subject to torque and can therefore rotate.
II. Correct — the process of producing artificial magnets consists of causing the alignment of atomic magnets with the field
magnetic field existing in the region — (see figure above).
III. False — it is not possible to separate the poles of a magnet.
IV. False — the magnetic compass needle indicates the Earth’s geographic north which is close to the Earth’s magnetic south pole.
A- A
57 – a) The forces acting on the magnet-clamp assembly are — 
– weight force, vertical and downward, applied to the assembly
by the Earth — 
– tensile force on the wire, vertical and downward, applied to the assembly by the hanging mass M — 
– friction force, exchanged between the assembly and the refrigerator wall, vertical and upward, contrary to the movement or its tendency — 
– magnetic force, horizontal and which attracts the assembly to the left, is magnetic attraction between the magnet and the refrigerator wall — 
– normal force, horizontal and to the right, reaction of the refrigerator wall on the assembly.
b) If the set must not fall, the resulting force on it both vertically and horizontally must be zero — vertical equilibrium —
F at =P + T — P=m o g — T=Mg — F at = μ e N — μ e N = m o g + Mg (I) — horizontal equilibrium — F M =N (II) — (II0 in (I) — μ e F M = m o g + Mg — Mg = μ e F M – m o g — M= ( μ e F M – m o g)/g or M= μ e F M /g – m o — for this value of M the set is on the verge of falling — for any value of M less than this, the set does not fall.
58- Initially you must remember that the lines of force of a magnetic field are closed lines and of an electrostatic field are open lines — Figure I shows the lines of force (field) of an electric (electrostatic) field created by a flat plate, very extensive and uniformly electrified by positive electric charges (removal fields) — Figure II suggests the electrostatic field originated by two point charges electrified with positive charges (removal field), placed close to each other, but it can also represent the magnetic field originated by two north poles of different magnets (induction lines come out of the north poles), when placed close together (see figure below), only that, in this case the
lines are closed (they leave the north pole of each magnet and reach the south pole of the same magnet) — figure III represents a magnetic field, as they are closed lines — it can represent the lines of
induction of a coil traversed by electric current or by two straight wires, traversed by electric currents in opposite directions (see figures above) — figure IV also shows a magnetic field (closed lines) — this magnetic field is generated by a straight wire, perpendicular to the plane of this sheet, with the electric current coming out of it.
A- A or E
59- A- A — read the information below — the Earth behaves like a large magnet where the magnetic South pole is approximately located at the geographic North pole and vice versa — if you hang a bar magnet by its center or observe the magnetic needle of a compass you will see that its poles are always aligned in the north-south direction.
The pole that indicates the geographic north pole is called the north pole and will be indicating the Earth’s magnetic south pole — the pole that indicates the geographic south pole is called the south pole and will be indicating the Earth’s magnetic north pole. All this occurs because poles of the same name attract each other.
Note: The geographic North Pole does not coincide exactly with the magnetic South Pole, being approximately 1,900 km apart — the geographic poles are fixed, based on the
geographic coordinates — the geographic North Pole is located in the Arctic Ocean and the geographic South Pole is in the center of Antarctica — the magnetic poles have variable locations and are in constant movement, which occurs due to the liquid metals in the Earth’s core
if they are, together with the Earth, in rotational motion — according to researchers, this movement occurs at a speed of more than 60 km/year.