Content
In the last lesson we looked at the magnetic field of a straight conductor carrying current. What will happen if this conductor has a different shape?
This question becomes most interesting if we are talking about a coil .
A coil is a conductor wound on a non-metallic (most often wooden) frame.
Typically, the coil has a large number of turns located close to each other (Figure 1). It turns out that when passing through these wires, the current flows in a spiral.
Figure 1. Coil
In this lesson you will learn what kind of magnetic field arises when current passes through a coil, what interesting properties it has and what applications it has.
Rationale for Lenz's rule
To explain Lenz's rule, it is enough to recall the law of conservation of energy.
The current arising in the circuit, passing through the resistance of the circuit, does work, which is spent on heating the coil wire. The energy for this is precisely what arises when the magnet moves. And, since the magnet must perform positive mechanical work, the magnetic field of the coil must be directed against the field of the magnet itself, no matter in which direction it moves.
Only in this case will the magnet perform positive work, the energy of which will move charges inside the circuit, generating an induction current, and the induction current, in turn, will do work to heat the coil wire (and deflect the galvanometer needle).
Rice. 3. Direction of induction current.
Magnetic field of the coil
If current flows through the coil, a magnetic field appears around it. It can be seen by conducting an experiment with iron filings, similar to what we did for a straight conductor with current in the last lesson.
Figure 3 shows a schematic representation of magnetic lines for a current-carrying coil.
Figure 3. Magnetic lines of a coil with current
As you can see, magnetic lines are closed curves . It is generally accepted that they are directed from the north pole of the coil to the south .
Lenz's rule
The interaction of a current circuit and a magnetic field was studied by the Russian physicist E. Lenz.
Rice. 2. E. H. Lenz.
He established a rule that was later named after him:
The induction current arising in the circuit is always directed in such a way as to interfere with the cause that generated it.
And indeed, in accordance with this rule, when a magnet is introduced into a coil, the current arising in the coil creates a magnetic field that resists the introduction of the magnet. And vice versa - when a magnet is removed from the coil, an induced current appears in it in such a direction as to prevent the magnet from being removed.
Right hand rule for current coil
You know that the direction of current and the direction of magnetic lines are related. Using the right-hand rule for a straight current-carrying conductor, we can find the direction of the current if we know the direction of the magnetic lines. Or, conversely, with a known direction of the current in the conductor, we can determine the direction of the magnetic lines.
For a coil with current, this rule also applies, but takes a slightly different form (Figure 4).
Right hand rule for a coil with current: if you take the coil in your right hand so that four fingers are facing in the direction of current flow, then the extended thumb will point to the north pole of the coil and coincide with the direction of the magnetic lines.
Figure 4. Right-hand rule for a current coil
§ 44. Direction of current and direction of lines of its magnetic field -
Questions. 1. How can you experimentally show the connection between the direction of the current in a conductor and the direction of its magnetic field line? If you change the direction of the current in a conductor to the opposite, all the magnetic needles located in the magnetic field created by this conductor will also rotate 180°. 2. Formulate the gimlet rule. If the direction of translational movement of the gimlet coincides with the direction of the current in the conductor, then the direction of rotation of the gimlet handle coincides with the direction of the magnetic field lines created by this current. 3. What can be determined using the gimlet rule? Using the gimlet rule, you can determine the direction of the magnetic field lines, knowing the direction of the current or vice versa. 4. State the right hand rule for the solenoid. If we imagine that the right hand is a solenoid, and position it so that the current comes out of the fingertips, then the thumb will indicate the direction of the magnetic induction lines. 5. What can be determined using the right hand rule? Using the right hand rule, you can determine the direction of magnetic lines, knowing the direction of the current and vice versa.
Exercises. 1. Figure 99 shows a wire rectangle, the direction of the current in it is shown by arrows. Draw the drawing in a notebook and, using the gimlet rule, draw one magnetic line around each of its four sides, indicating its direction with an arrow.
2. Figure 100 shows the magnetic field lines around current-carrying conductors. Conductors are depicted as circles. Draw the drawing in a notebook and use symbols to indicate the directions of currents in the conductors, using the gimlet rule for this.
3. A current of the indicated direction is passed through a coil, inside of which there is a steel rod (Fig. 101).
Determine the poles of the resulting electromagnet. How can you change the position of the poles of this electromagnet? According to the right-hand rule, we find that the electromagnet shown in Figure 101 has the south pole S on the left, and the north pole N on the right. To change the position of the poles to the opposite, you need to make sure that the current flows in the opposite direction.
4. Determine the direction of the current in the coil and the poles at the current source (Fig. 102), if, when current passes through the coil, the magnetic poles indicated in the figure appear.
In the coil, the current flows from right to left, from plus to minus.
5. The direction of the current in the turns of the winding of a horseshoe-shaped electromagnet is shown by arrows (Fig. 103).
Identify the poles of an electromagnet. If the horseshoe magnet is located with a cut towards us, then S will be on the left, N on the right, if the cut is away from us, then vice versa.
6. Parallel wires carrying currents in the same direction attract, and parallel beams of electrons moving in the same direction repel.
In which of these cases is the interaction caused by electric forces, and in which by magnetic forces? Why do you think so? Since charges of the same sign always repel, the repulsion of electron beams is due to electric (Coulomb) forces, and the attraction of conductors is due to magnetic forces.
Changing the magnetic action of the coil
Since current-carrying coils have two poles, they are often used in technology as magnets . Why not just use a regular magnet then?
The fact is that the magnetic action of the coil can be changed (strengthened or weakened). Now we will look at how this can be done.
Let's carry out a simple experiment (Figure 5). Let's add fine iron filings and turn on the current. The coil will begin to attract them.
Figure 5. Attraction of iron filings onto a current coil
And now, without changing the current strength, let's take a coil with a greater number of turns than the previous one. You will see that the amount of sawdust attracted has increased noticeably.
The greater the number of turns in it, the stronger the magnetic effect of a current-carrying coil.
a rheostat to our electrical circuit (Figure 6). It will allow you to change the current strength.
With the help of such changes in current strength, we will see that at different current values, the coil attracts different numbers of iron objects.
Figure 6. The effect of the magnetic field of the coil when the current changes.
As the current increases, the effect of the magnetic field of the current coil increases, and as the current decreases, it weakens.
Is it possible to increase the magnetic effect of a current-carrying coil without changing the number of turns and the current strength? Can! To do this, you need to insert an iron rod (Figure 7). Such rods are called cores.
Figure 7. Effect of the magnetic field of the coil when adding a core
§ 10. Direction of induction current. Lenz's rule
Chapter 2. Electromagnetic inductionBy connecting the coil in which the induced current occurs to a galvanometer, you can find that the direction of this current depends on whether the magnet is approaching the coil (for example, with the north pole) or moving away from it (see Fig. 2.2, b).
The resulting induction current of one direction or another somehow interacts with the magnet (attracts or repels it). A coil with current passing through it is like a magnet with two poles - north and south. The direction of the induction current determines which end of the coil acts as the north pole (the magnetic induction lines come out of it). Based on the law of conservation of energy, it is possible to predict in which cases the coil will attract a magnet and in which cases it will repel it.
Interaction of induction current with a magnet. If the magnet is brought closer to the coil, then an induction current appears in it in such a direction that the magnet is necessarily repelled. To bring the magnet and coil closer together, positive work must be done. The coil becomes like a magnet, with its pole of the same name facing the magnet approaching it. Poles of the same name repel each other.
When the magnet is removed, on the contrary, a current appears in the coil in such a direction that a force attracting the magnet appears.
What is the difference between the two experiments: bringing a magnet closer to the coil and moving it away? In the first case, the number of lines of magnetic induction penetrating the turns of the coil, or, what is the same, the magnetic flux, increases (Fig. 2.5, a), and in the second case it decreases (Fig. 2.5, b). Moreover, in the first case, the induction lines of the magnetic field created by the induced current generated in the coil come out of the upper end of the coil, since the coil repels the magnet, and in the second case, on the contrary, they enter this end. These magnetic induction lines are shown in black in Figure 2.5. In case a, the coil with current is similar to a magnet, the north pole of which is located at the top, and in case b, at the bottom.
Similar conclusions can be drawn using the experiment shown in Figure 2.6. At the ends of the rod, which can rotate freely around a vertical axis, two conductive aluminum rings are fixed. One of them has a cut.
The magnet does not interact with the cut ring, since the cut prevents the occurrence of induction current in the ring. Whether a magnet repels or attracts a coil depends on the direction of the induction current in it. Therefore, the law of conservation of energy allows us to formulate a rule that determines the direction of the induction current.
Lenz's rule. Now we come to the main thing: with an increase in the magnetic flux through the turns of the coil, the induced current has such a direction that the magnetic field it creates prevents the increase in the magnetic flux through the turns of the coil. After all, the induction lines of this field are directed against the induction lines of the field, a change in which generates an electric current. If the magnetic flux through the coil weakens, then the induced current creates a magnetic field with induction 'increasing the magnetic flux through the turns of the coil.
This is the essence of the general rule for determining the direction of the induction current, which is applicable in all cases. This rule was established by the Russian physicist E. H. Lenz.
According to Lenz's rule, the induced current arising in a closed loop with its magnetic field counteracts the change in the magnetic flux that causes it. More briefly, this rule can be formulated as follows: the induced current is directed so as to interfere with the cause that causes it.
To apply Lenz's rule to find the direction of the induction current in the circuit, it is necessary to do this:
1. Determine the direction of the magnetic induction lines of the external magnetic field.
2. Find out whether the flux of the magnetic induction vector of this field through the surface bounded by the contour increases (ΔФ > 0) or decreases (ΔФ < 0).
3. Set the direction of the magnetic induction lines ' magnetic field of the induced current. According to Lenz's rule, these lines must be directed opposite to the lines of magnetic induction B at ΔФ > 0 and have the same direction as them at ΔФ <0.
4. Knowing the direction of the magnetic induction lines ', find the direction of the induction current using the gimlet rule.
The direction of the induction current is determined using the law of conservation of energy. In all cases, the induced current is directed in such a way that its magnetic field prevents the change in the magnetic flux that causes this induced current.
Questions for the paragraph
1. How is the direction of the induction current determined?
2. Will an electric field appear in a ring with a cut if a magnet is brought near it?
Electromagnet
Adding cores to current-carrying coils is a simple way to greatly enhance their magnetic effect. Therefore, such designs are widely used. They are called electromagnets .
An electromagnet is a coil with an iron core inside.
Electromagnets are a fundamental part of many devices. They have several extremely useful properties:
- They quickly demagnetize when the current is turned off
- During operation, you can change the current strength in the coil and in this way change the magnetic effect of the electromagnet
- Electromagnets are easily manufactured in a wide variety of sizes.
Application of electromagnets
Let's look at a few examples of the use of electromagnets.
Figure 8 shows an arc-shaped electromagnet. It holds an iron plate (anchor) with a suspended weight.
Figure 8. Arc-shaped electromagnet
Such installations are widely used in factories for moving various metal products and collecting metal shavings.
Figure 9 shows a cross-section of a magnetic grain separator.
Figure 9. Magnetic grain separator
The principle of its operation is very simple. Very fine iron filings are added to the collected grain. They do not stick to smooth grains of cereals, but they do stick to grains of weeds.
From hopper 1, grains and sawdust are poured onto a rotating drum 2. Inside it there is a powerful electromagnet 5. It attracts iron filings, and with them weed grains. This is how the separator cleans the grain.
Electromagnets are also used in many other devices. We will look at some of them below in this lesson in the “Tasks” section.
Exercises
Exercise No. 1 You need to build an electromagnet, the lifting force of which can be adjusted without changing the design. How to do it?
The lifting force will depend on the magnetic action of the electromagnet. We know three ways to do this: change the number of turns, add a core, or change the current.
The first method is not suitable for us, since it involves changing the design. The second one is not suitable, since we already have a coil with an inserted core (electromagnet).
What remains is the change in current strength . In order for us to be able to do this, it is necessary to include a rheostat . By changing the current strength with its help, we will reduce or increase the magnetic effect of the electromagnet and change its lifting force.
Exercise No. 2 What needs to be done to change the magnetic poles of the coil with current to the opposite ones?
You already know that you can use the right hand rule to determine the poles of a coil. Using it, we clasp the coil so that our four fingers coincide with the direction of the current in the turns. Then our thumb points to the north pole of the coil.
This means that the direction of the current and the location of the poles of the coil are related to each other.
What can be done to make the North Pole appear on the other side? Reverse the direction of current .
Exercise No. 3 How to build a strong electromagnet if the designer is given the condition that the current in the electromagnet be relatively small?
If we cannot strengthen the magnetic effect of an electromagnet by increasing the current, then all that remains is to increase the number of turns in the coil .
We also cannot insert an additional iron core, since an electromagnet is already a coil with a core.
Exercise No. 4 The electromagnets used in the crane have enormous power. The electromagnets used to remove stray iron filings from the eyes are very weak. In what ways is this difference achieved?
To increase power, increase the number of turns in the coil, increase the current strength, and leave an iron core in the coil. To reduce power, you can reduce the number of turns, reduce the current, and remove the core.
