This video is about induced electromotive force or induced EMF.
We'll start by learning high current is induced and then move on to discuss
how to use Fleming's right hand rule for induction effects.
Let's see high current is induced. Recall from subtopic 5.4 that when charge moves in a magnetic
field it experiences a force due to the field. The idea of an induced current is closely related
to this concept. Induced current is defined as the production of a current in a conductor
due to a relative change between the conductor and the magnetic field. By relative change we
often mean relative motion, so for example a conductor that moves in a magnetic field.
Other examples of changes are changes in magnetic field strength or magnetic field direction.
Later in this subtopic we'll learn about magnetic flux and magnetic flux density.
Both of these quantities are connected to induced current and induced EMF.
Next let's see an example of how induced current is produced. Let's assume that we have a magnetic
field that is pointing out of the page or screen. A conducting metal rod is moving in this magnetic
field horizontally to the right. The rod is moving at a constant speed. Let's apply Fleming's left
hand rule that we learned in subtopic 5.4. The rod contains positively and negatively charged
particles. For now let's focus on what happens to a positively charged particle
as the rod moves through the magnetic field. Since the positive charge together with the rod
is moving to the right, the direction of the conventional current is to the right,
so when we are using Fleming's left hand rule our middle finger should be pointing to the right.
The field direction is out of the page or screen, so our index finger should be pointing out of the
page. If you got the direction of these two fingers correctly, you will see that your thumb
is pointing downwards. Therefore the force due to the field that acts on the positive charge
is pointing downwards. Since the direction of the electromagnetic force that acts on a positive
charge is downwards, the direction of the electromagnetic force that acts on negative
charge is upwards. As a result electrons in the rod flow upwards, so the direction of the
conventional current is downwards. The current that is produced in the rod this way is called
induced current. It is induced by the relative change between the rod and the magnetic field.
You might be wondering how can there be a current in the rod when it is just a piece of metal and
it is not part of a closed circuit. An important concept here is that a current is induced in the
rod even though the rod is not part of a closed circuit. What eventually would happen is that
negative charge would accumulate at one end of the rod while positive charge would accumulate
at the other end. As a result after some time charge would stop moving. So in order to induce a
continuous current, the conductor must be part of a closed loop. Of course when the current is
induced electrons gain energy, so energy transfer takes place. Some form of energy is transferred
into the electrical energy of the electrons. Since energy is transferred into electrical
energy, we say that an EMF or Electromotive Force is induced in the conductor. Keep in mind that in
spite of what its name suggests, Electromotive Force is not a force but it is the electrical
energy per unit charge that is transferred to the electrons and it is measured in volts.
Through our example of the moving rod, we have seen how Fleming's left hand rule can be used
to find the direction of the induced current. Fleming also has a right hand rule that can
be used in this situation to determine the direction of the induced current. Let's see how this rule
works. Logically for this rule we'll use our right hand. Our thumb will point in the direction of
the conductor's motion. Our index finger in the direction of the magnetic field and our middle
finger will show the direction of the induced conventional current. Let's go back to the
moving rod in the magnetic field from our previous example and use Fleming's right hand rule to find
the direction of the induced conventional current. Position your right hand so that your thumb is
pointing in the direction of the conductor's motion so to the right your index finger should
point in the field direction so out of the page or screen and if you have done this correctly
your middle finger should point downwards showing you the direction of the induced conventional
current. This is the same direction that we found using Fleming's left hand rule in the previous video.
So when you work with the induced current you can use either of these rules to work out directions.
Let's summarize what we have learned in this video. We began by discussing induced current
and defined the meaning of this term. Then we examined a metal rod moving through a magnetic field
and explained step by step how a current is induced in the rod. We added that during the
induction process energy transfer takes place so we say that an EMF is induced in the conductor.
In the second part of the video we learned about Fleming's right hand rule
which we can use to find the direction of the induced current. When using this rule we should
point our thumb in the direction of the conductor's motion align our index finger with the field
direction and if we do this correctly our middle finger will show the direction of the induced
conventional current. We concluded that when working with induced current we can use either
Fleming's left hand or right hand rule. This completes our discussion of induced
electromotive force or induced EMF. In the next video we'll learn about Lentz's law.