Heinrich Lenz
stated a law to determine the exact polarity of an induced voltage. In doing
that, Lenz reasoned that the process of induction must conform to the well know
principle in physics that states that reaction is equal and opposite to action.
Of several ways of stating Lenz’s law, we shall use the following form;
stated a law to determine the exact polarity of an induced voltage. In doing
that, Lenz reasoned that the process of induction must conform to the well know
principle in physics that states that reaction is equal and opposite to action.
Of several ways of stating Lenz’s law, we shall use the following form;
Lenz law: the
polarity of the induced voltage must be such that the current resulting from it
will develop a flux which tends to oppose any change in the original flux.
polarity of the induced voltage must be such that the current resulting from it
will develop a flux which tends to oppose any change in the original flux.
When we first
close the switch in the mutual induction demonstration of figure 1a, the
primary flux will increase from zero. Since a resistor is connected across the
secondary winding, according to Lenz’s law, the flux produced by this secondary
current must try to oppose the increase in primary flux. Applying the Right
Hand Rule to the primary winding, we find that the primary flux will have a
clockwise direction around the core. To oppose an increase in this clockwise
primary flux, the flux produced by the secondary current must have an
anticlockwise direction.
close the switch in the mutual induction demonstration of figure 1a, the
primary flux will increase from zero. Since a resistor is connected across the
secondary winding, according to Lenz’s law, the flux produced by this secondary
current must try to oppose the increase in primary flux. Applying the Right
Hand Rule to the primary winding, we find that the primary flux will have a
clockwise direction around the core. To oppose an increase in this clockwise
primary flux, the flux produced by the secondary current must have an
anticlockwise direction.
Again applying
the Right Hand Rule, this time to the secondary winding, we can establish the
direction in which the secondary current must flow through the resistor, and
thus determine the polarity of the induced voltage in terms of the polarity of
the voltage drop across the resistor.
the Right Hand Rule, this time to the secondary winding, we can establish the
direction in which the secondary current must flow through the resistor, and
thus determine the polarity of the induced voltage in terms of the polarity of
the voltage drop across the resistor.
When we open
the switch, as in figure 1b above, the primary current stops flowing and the
primary flux collapse, according to Faraday’s law, this reduction in flux will
induce a voltage into the secondary, and according to Lenz’s law, the secondary
current must try to oppose the collapse of the primary flux. To help sustain
the primary flux, which has a clockwise direction in the core, the secondary
current must also produce a clockwise flux direction. Applying the right Hand
Rule, we obtain the polarity shown in Figure 1b. The polarity of the induced
voltage when the primary current is decreasing is, therefore, opposite to that
induced when the primary current is increasing.
the switch, as in figure 1b above, the primary current stops flowing and the
primary flux collapse, according to Faraday’s law, this reduction in flux will
induce a voltage into the secondary, and according to Lenz’s law, the secondary
current must try to oppose the collapse of the primary flux. To help sustain
the primary flux, which has a clockwise direction in the core, the secondary
current must also produce a clockwise flux direction. Applying the right Hand
Rule, we obtain the polarity shown in Figure 1b. The polarity of the induced
voltage when the primary current is decreasing is, therefore, opposite to that
induced when the primary current is increasing.
We can also
use Lenz’s law to determine the direction of induced voltage of a current
carrying conductor in a magnetic field. If we consider two magnetic fields
separately the field of a stationary magnet has the form as shown in figure 2a
below
use Lenz’s law to determine the direction of induced voltage of a current
carrying conductor in a magnetic field. If we consider two magnetic fields
separately the field of a stationary magnet has the form as shown in figure 2a
below
Figure 2b:
Component magnetic field for the demonstration of Electromagnetic induction
Component magnetic field for the demonstration of Electromagnetic induction
If we assume
at the moment that the environmental current direction in the conductor is out
of the page, the magnetic field around the conductor has the form as shown in
figure 2b above. Before we place the conductor between the magnetic poles in
the sketch and superimpose the two magnetic fields, we must recall that
magnetic lines of force can never intersect. Therefore, the two composite
magnetic field patterns, as shown in figure 3 below
at the moment that the environmental current direction in the conductor is out
of the page, the magnetic field around the conductor has the form as shown in
figure 2b above. Before we place the conductor between the magnetic poles in
the sketch and superimpose the two magnetic fields, we must recall that
magnetic lines of force can never intersect. Therefore, the two composite
magnetic field patterns, as shown in figure 3 below
Above the
conductor (figure 2b) the direction of the magnetic lines of force is opposite
to that of the magnetic field (figure 2a). Consequently, there is a cancelling
effect which bends the resultant magnetic field away from the conductor as
shown in figure 3 above. Below the conductor, the direction of its magnetic
foiled is the same as that of the permanent magnetic field. As a result, the
flux density is increased below the conductor in the composite flux pattern of
figure 3.
conductor (figure 2b) the direction of the magnetic lines of force is opposite
to that of the magnetic field (figure 2a). Consequently, there is a cancelling
effect which bends the resultant magnetic field away from the conductor as
shown in figure 3 above. Below the conductor, the direction of its magnetic
foiled is the same as that of the permanent magnetic field. As a result, the
flux density is increased below the conductor in the composite flux pattern of
figure 3.
A notable
characteristic of magnetic lines of force is that they tend to become as short
as possible and tends to repel one another. Therefore, the lines of force in
the composite magnetic fields of figure 3 will attempt to straighten out and to
space themselves uniformly, in order to regain the flux pattern of figure 2a.
To accomplish this, the magnetic field will attempt to force the current
carrying conductor to move upward, according to the principles stated in Lenz’s
law, this force will oppose the actual motion of the conductor through the
stationary field. Consequently, when the electric conductor moves downward
through the stationary magnetic field, the voltage induced into the conductor
has polarity such as to cause the current in the close loop to have a direction
out of the page.
characteristic of magnetic lines of force is that they tend to become as short
as possible and tends to repel one another. Therefore, the lines of force in
the composite magnetic fields of figure 3 will attempt to straighten out and to
space themselves uniformly, in order to regain the flux pattern of figure 2a.
To accomplish this, the magnetic field will attempt to force the current
carrying conductor to move upward, according to the principles stated in Lenz’s
law, this force will oppose the actual motion of the conductor through the
stationary field. Consequently, when the electric conductor moves downward
through the stationary magnetic field, the voltage induced into the conductor
has polarity such as to cause the current in the close loop to have a direction
out of the page.
