【People's Education Press】High School Physics - Required Course, Volume 3: Oersted's Discovery and Determining Magnetic Field Direction

The Serendipitous Moment That Launched Electromagnetism

In April 1820, during a lecture, Oersted accidentally placed a wire above a compass and passed a current through it. At that moment,the magnetic needle deflected noticeably, which was exactly what Oersted had been anticipating. Before this, people generally believed electricity and magnetism were entirely unrelated. This discovery proved that 'moving electricity' can generate 'magnetism,' not only revealing the intrinsic connection between electricity and magnetism but also shattering long-standing prejudices in physics.

Core Physical Reality: Magnetic Fields and Magnetic Field Lines

Magnetic Field (Magnetic Field): A physical field generated by current in its surrounding space. It is not imaginary—it is a form of matter that exerts force on magnets placed within it.
Magnetic Induction Line (Magnetic Field Line): To visualize the magnetic field, wedraw curves along fine iron filings in the magnetic field. The density of magnetic field lines indicates field strength, while the tangent direction at any point represents the direction of the magnetic field there.
Spatial Topology Determination: The relationship between the direction of a straight current and the direction of its magnetic field lines can be determined usingAmpère's Rulethis rule.

💡 Historical Insight

Oersted's experiment was not just the discovery of 'electricity generating magnetism'; it also marked the beginning of electromagnetic unification. Faraday was directly inspired by this breakthrough and, ten years later, discovered electromagnetic induction, achieving a symmetric leap from 'magnetism generating electricity'.

QUESTION 1

[Visual: Figure 13.1-13] A small compass near a straight current-carrying wire is shown in Figure 13.1-13, with its north pole pointing northeast. Indicate the direction of the current in the wire.

Out of the page, perpendicular to it

From bottom to top

From top to bottom

Horizontally to the left

QUESTION 2

[Visual: Figure 13.1-14] As shown in Figure 13.1-14, when current flows counterclockwise through a circular wire loop, in which direction will the north pole of a small compass at the center eventually point?

Into the plane of the paper

Out of the plane of the paper

Radially outward

Tangentially clockwise

QUESTION 3

Which of the following statements about the magnetic field of a current-carrying solenoid is correct?

The magnetic field at the ends is stronger than inside

The internal magnetic field is stronger than outside the ends because the magnetic field lines are most densely packed inside

There is no magnetic field inside the solenoid

Magnetic field lines inside the solenoid go from the N pole to the S pole

QUESTION 4

In Oersted's experiment, when a wire is placed directly above a small compass along the north-south direction and energized, what causes the compass needle to rotate?

Charges in the wire exerted an electrostatic force on the compass needle

The magnetic field produced around the energized wire exerted a magnetic force on the compass needle

The wire heating changed air density

Earth's magnetic field suddenly disappeared

QUESTION 5

According to Ampère's hypothesis on Earth's magnetism, Earth's magnetic field is caused by a circular current flowing around an axis passing through the Earth's center. If the magnetic S pole is near the geographic North Pole, what should be the direction of this circular current?

From west to east

From east to west

From south to north

From north to south

Integrated Application: Quantifying Magnetic Fields and Energy Conversion

Deep analysis combining magnetic field definitions with new energy power systems

This case study explores the fundamental understanding of magnetic flux density, identifying induced currents in electromagnetic induction, and quantitative calculation of wind-to-electricity conversion. These issues form a technical chain from basic magnetism to macro-scale energy applications.

1. In a certain region, wind speed is $14 \text{ m/s}$, air density $\rho = 1.3 \text{ kg/m}^3$. If the wind turbine has a cross-sectional area $A = 400 \text{ m}^2$, and 20% of the wind energy can be converted into electricity, calculate its power output.

Answer:
Using the wind power formula: $P_{total} = \frac{1}{2} \rho A v^3$. Substituting values: $P_{total} = 0.5 \times 1.3 \times 400 \times 14^3 = 713,440 \text{ W}$. Final power output $P = P_{total} \times 20\% \approx 1.43 \times 10^5 \text{ W}$ (or $143 \text{ kW}$).

2. Analyze the statement: Some believe that according to $B = \frac{F}{Il}$, magnetic flux density $B$ is directly proportional to the magnetic force $F$ and inversely proportional to the product of current $I$ and length $l$. Is this view correct? Why?

Answer:
This view is incorrect. Magnetic flux density $B$ is an inherent property of the magnetic field, determined solely by the source (such as permanent magnets or current sources), independent of the test current ($I, l$) or force ($F$) involved. This formula is merely a definition of magnetic flux density, not a determining equation.

3. As shown in Figure 13.3-12, coils A and B are wound around core P. If the current $i$ in coil A varies with time $t$. During the $t_1 \sim t_2$ period, if current $i$ is constant (as in Figure C), will there be an induced current in coil B? What if it increases linearly (as in Figure A)?

Answer:
If the current is constant (Figure C), the magnetic flux does not change, so no induced current appears in coil B. If the current changes over time (e.g., linear increase in Figure A, decrease in Figure B, or nonlinear change in Figure D), the resulting magnetic field changes accordingly, altering the magnetic flux through coil B, thus inducing a current in B.

4. Calculate magnetic flux: The coil has area $S$ and is perpendicular to a uniform magnetic field of flux density $B$. Find the magnetic flux at initial position, after rotating $60^{\circ}$ about axis $OO'$, and after rotating $90^{\circ}$.

Answer:
Initial flux $\Phi_1 = BS$; after rotating $60^{\circ}$, the effective projected area halves, $\Phi_2 = BS \cos 60^{\circ} = 0.5 BS$; after rotating $90^{\circ}$, the coil is parallel to the field, so no field lines pass through, $\Phi_3 = 0$.