Detection of a magnetic field by its effect on an electric current. left hand rule

Thanks to today's video tutorial, we will learn how a magnetic field is detected by its effect on an electric current. Remember the left hand rule. Through experience we learn how a magnetic field is detected by its effect on another electric current. Let's learn what the left hand rule is.

In this lesson, we will discuss the issue related to the detection of a magnetic field by its effect on an electric current, and get acquainted with the left hand rule.

Let's turn to experience. The first such experiment to study the interaction of currents was carried out by the French scientist Ampère in 1820. The experiment was as follows: an electric current was passed through parallel conductors in one direction, then the interaction of these conductors was observed in different directions.

Rice. 1. Ampère's experience. Conductors that carry current in the same direction attract, and opposite directions repel.

If we take two parallel conductors, through which an electric current passes in the same direction, then in this case the conductors will be attracted to each other. When electric current flows in different directions in the same conductors, the conductors repel each other. Thus, we observe the force effect of a magnetic field on an electric current. So, we can say the following: a magnetic field is created by an electric current and is detected by its action on another electric current (Ampère force).

When a large number of similar experiments were carried out, a rule was obtained that relates the direction of the magnetic lines, the direction of the electric current and the force action of the magnetic field. This rule is called left hand rule. Definition: the left hand must be positioned so that the magnetic lines enter the palm, four outstretched fingers indicate the direction of the electric current - then the bent thumb will indicate the direction of the magnetic field.

Rice. 2. Left hand rule

Please note: we cannot say that where the magnetic line is directed, the magnetic field acts there. Here the relationship between the quantities is somewhat more complicated, so we use left hand rule.

Recall that electric current is the directed movement of electric charges. This means that the magnetic field acts on the moving charge. And we can use in this case also the rule of the left hand to determine the direction of this action.

Take a look at the figure that shows different uses of the left hand rule and analyze each one for yourself.

Rice. 3. Various Applications of the Left Hand Rule

Finally, one more important fact. If the electric current or the speed of a charged particle is directed along the lines of the magnetic field, then there will be no effect of the magnetic field on these objects.

List of additional literature:

Aslamazov L.G. Motion of charged particles in electric and magnetic fields // Kvant. - 1984. - No. 4. - S. 24-25. Myakishev G.Ya. How does an electric motor work? // Quantum. - 1987. - No. 5. - S. 39-41. Elementary textbook of physics. Ed. G.S. Landsberg. T. 2. - M., 1974. Yavorsky B.M., Pinsky A.A. Fundamentals of physics. T.2. - M.: Fizmatlit, 2003.

From the 8th grade physics course, you know that for any current-carrying conductor placed in a magnetic field and not coinciding with its magnetic lines, this field acts with some force.

The presence of such a force can be shown using the installation shown in the figure. A three-sided frame ABCD, made of copper wire, is hung on hooks so that it can freely deviate from the vertical. The BC side is located in the region of the strongest magnetic field of the arcuate magnet, located between its poles (Fig. a). The frame is connected to a current source in series with a rheostat and a key.

Rice. The action of a magnetic field on a current-carrying conductor

When the key is closed, an electric current appears in the circuit, and the BC side is drawn into the space between the poles (Fig. b).

If you remove the magnet, then when the circuit is closed, the conductor BC will not move. This means that from the side of the magnetic field, a certain force acts on the current-carrying conductor, deflecting it from its original position.

The action of a magnetic field on a current-carrying conductor can be used to detect a magnetic field in a given region of space.

Of course, finding a magnetic field is easier with a compass. But the effect of a magnetic field on the magnetic needle of a compass located in it, in essence, also comes down to the effect of the field on elementary electric currents circulating in the molecules and atoms of the magnetic substance from which the arrow is made.

Thus, a magnetic field is created by an electric current and is detected by its effect on the electric current.

Let's change the direction of the current in the circuit by swapping the wires in the sockets of the insulating rod (Fig.). In this case, the direction of movement of the conductor BC will also change, and hence the direction of the force acting on it.

Rice. The direction of the force acting in a magnetic field on a current-carrying conductor depends on the direction of the current

The direction of the force will also change if, without changing the direction of the current, the poles of the magnet are interchanged (i.e., change the direction of the magnetic field lines). Therefore, the direction of the current in the conductor, the direction of the lines of the magnetic field and the direction of the force acting on the conductor are related.

The direction of the force acting on a current-carrying conductor in a magnetic field can be determined using the left hand rule.

In the simplest case, when the conductor is located in a plane perpendicular to the lines of the magnetic field, this rule is as follows: if the left hand is positioned so that the lines of the magnetic field enter the palm perpendicular to it, and four fingers are directed along the current, then set aside on The 90° thumb will show the direction of the force acting on the conductor (fig.).

Rice. Applying the left-hand rule to a current-carrying conductor

Using the rule of the left hand, it should be remembered that the direction of current in an electrical circuit is taken from the positive pole of the current source to the negative. In other words, the four fingers of the left hand should be directed against the movement of electrons in the electrical circuit. In conductive media such as electrolyte solutions, where an electric current is created by the movement of charges of both signs, the direction of the current, and hence the direction of the four fingers of the left hand, coincides with the direction of movement of positively charged particles.

Using the left-hand rule, one can determine the direction of the force with which a magnetic field acts on individual particles moving in it, both positively and negatively charged.

For the simplest case, when the particle moves in a plane perpendicular to the magnetic lines, this rule is formulated as follows: if the left hand is placed so that the magnetic field lines enter the palm perpendicular to it, and four fingers are directed along the movement of a positively charged particle (or against the movement of a negatively charged one), then the thumb set aside by 90 ° will show the direction of the force acting on the particle (Fig.).

Rice. Applying the Left Hand Rule to Charged Particles Moving in a Magnetic Field

According to the left hand rule, you can also determine the direction of the current (if we know how the magnetic field lines and the force acting on the conductor are directed), the direction of the magnetic lines (if the directions of the current and force are known), the sign of the charge of a moving particle (in the direction of the magnetic lines, force and speed particle motion), etc.

It should be noted that the force of the magnetic field on a conductor with current or a moving charged particle is zero if the direction of the current in the conductor or the speed of the particle coincides with the line of magnetic induction or is parallel to it (Fig.).

Rice. The magnetic field does not act in cases where a rectilinear current-carrying conductor or the speed of a moving charged particle is paparallel to or coincident with magnetic field lines

Homework.

Task 1. Answer the questions.

What experience allows you to detect the presence of a force acting on a conductor with current in a magnetic field?
How is a magnetic field detected?
What determines the direction of the force acting on a current-carrying conductor in a magnetic field?
Formulate the rule of the left hand for a current-carrying conductor in a magnetic field; for a charged particle moving in this field.
What can be determined using the left hand rule?
In what case is the force of the magnetic field on a current-carrying conductor or a moving charged particle equal to zero?

Task 2. Solve the rebus.

The file "It's interesting!" is attached to the lesson. You can download the file at any time convenient for you.

Used sources:

http://www.tepka.ru/fizika_9/36.html

Questions.

1. How to experimentally detect the presence of a force acting on a current-carrying conductor in a magnetic field?

It is necessary to place a conductor with current between the poles of the magnet so that the direction of the current is perpendicular to the lines of the magnetic field, and the mount allows the conductor to move. When a current is passed, the conductor will deflect, but this will not happen if the magnet is removed.

2. How is the magnetic field detected?

A magnetic field can be detected by its effect on a magnetic needle or on a current-carrying conductor.

3. What determines the direction of the force acting on a current-carrying conductor in a magnetic field?

From the direction of the current and the direction of the magnetic lines.

4. How is the left hand rule read for a conductor with current in a magnetic field? for a charged particle moving in this field?

If the left hand is positioned so that the lines of magnetic induction enter the palm perpendicular to it, and the outstretched four fingers indicate the direction of the current (the direction of movement of a positively charged particle), then the thumb set aside by 90 ° will show the direction of the force acting on the conductor.

5. What is taken as the direction of the current in the external part of the electrical circuit?

This is the direction from the positive pole to the negative.

6. What can be determined using the left hand rule?

The direction of the force acting on the conductor, knowing the direction of the current and the lines of the magnetic field. The direction of the current, knowing the direction of the force and magnetic lines. The direction of the magnetic field lines, knowing the direction of the current and the force acting on the conductor.

7. In what case is the force of the magnetic field on a current-carrying conductor or a moving charged particle equal to zero?

In the case when the direction of current movement or the direction of particle velocity coincides with the direction of magnetic induction lines, the force of the magnetic field is equal to zero.

Exercises.

1. In which direction will the light aluminum tube roll when the circuit is closed (Fig. 112)?

According to the rule of the left hand, we determine what is right.

2. Figure 113 shows two bare conductors connected to a current source and a light aluminum tube AB. The entire installation is in a magnetic field. Determine the direction of the current in the tube AB if, as a result of the interaction of this current with a magnetic field, the tube rolls along the conductors in the direction indicated in the figure. Which pole of the current source is positive and which is negative?

According to the rule of the left hand, the current moves from point A to B, therefore, the upper pole of the current source is positive, and the lower one is negative.

3. Between the poles of the magnets (Fig. 114) there are four conductors with current. Determine in which direction each of them is moving.

Left - up, down. Right - down, up.

4. Figure 115 shows a negatively charged particle. moving at a speed v in a magnetic field. Make the same drawing in your notebook and indicate with an arrow the direction of the force with which the field acts on the particle.

5. A magnetic field acts with a force F on a particle moving at a speed v (Fig. 116). Determine the sign of the charge of the particle.

The sign of the particle charge is negative (we apply the left hand rule).

Detection of a magnetic field by its effect on an electric current. left hand rule
Electromagnetic Phenomena