22.2.7 Eddy Current
1 Current induced in conducting plate and block
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Figure 22.43 (a) Plat logam ditempatkan dalam medan magnet yang bertambah, menyebabkan fluks magnet meningkat dan menginduksi emf. (b) Arus eddy mengalir dalam arah tertentu untuk menentang peningkatan fluks, menghasilkan medan magnet yang berlawanan dengan medan luar sesuai Hukum Lenz. |
(a) Suppose a metal plate is placed in a magnetic field, with the surface perpendicular to the field, as shown in Figure 22.43(a). The strength of the field increases with time. This means that the magnetic flux through the plate increases with time. As a result, an emf is induced in the plate.
(b) An induced current flows in the plate. It has to flow in such a direction that it opposes the change producing it, in accordance with Lenz’s law. The change is the increase in magnetic flux.
(c) The current produces a magnetic field. This field must point in a direction which is opposite to that of the applied field. In this direction the induced field will oppose the increase in strength of the applied field. The induced field pointing in the direction stated can be produced only when the current flows in the direction as shown in Figure 22.43(b).
(d) If the applied field still points in the same direction but with its strength decreasing, then the induced current will flow in the opposite direction.
(e) The induced current is known as eddy current.
(f) If we replace the plate with a metal block, eddy current is also produced in the block.
2 Current induced in rotating metal core
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Figure 22.44 (a) Batang logam yang berputar dalam medan magnet menghasilkan arus eddy yang menimbulkan gaya pengerem berlawanan arah putaran. (b) Penggunaan inti berlaminasi mengurangi arus eddy sehingga mengurangi gaya pengerem. |
(b) Since the eddy current is found inside the magnetic field, the current produces a retarding magnetic force which acts in the direction which is opposite to the direction of rotation, as shown in Figure 22.44(a).
(c) To reduce the eddy current, hence also the retarding force, we need to use a laminated core, as shown in Figure 22.44(b).
22.2.8 Energy Loss Due to Eddy Current
1. Eddy current loss
Eddy current flowing in a metal plate or block produces heat. In general, energy in heat form is dissipated in a system where eddy current flows through it. Hence, the system loses energy due to eddy current. Loss of energy by a system due to heat produced by eddy current flowing through the system is known as eddy current loss.
2. Reducing eddy current
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Figure 22.45 Plat logam dengan celah (slot) digunakan untuk mengurangi arus eddy yang mengalir tegak lurus terhadap medan magnet. Celah ini meningkatkan hambatan sehingga memperkecil arus eddy. |
Eddy current flows on the plane of the metal plate which is perpendicular to the applied magnetic field, as indicated by the arrow shown in Figure 22.45. In order to reduce this current, we use a plate with slots. This is equivalent to increasing the resistance of a circuit.
EXAMPLE 22.23
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| Figure 22.46 A flat rectangular copper plate attached to a rod swings as a compound pendulum about an axis at the free end of the rod. |
A flat rectangular copper plate is attached to a copper rod so that the composite bodies can swing like a compound pendulum about an axis which passes through the free end of the rod, as shown in Figure 22.46. The period T1 of oscillation of the metal plate in air without any magnetic field is longer than the period T2 in air with a uniform magnetic field whose direction is perpendicular to the surface of the plate. Explain why. Suggest one way which can be used to increase T2.
Answer
1. When the suspended pendulum which is initially outside the magnetic field is released, the component of its weight pushes the pendulum downwards so that the copper plate swings into the magnetic field.
2. At the moment the copper plate swings into the magnetic field, magnetic flux passes through the plate and it increases with time.
3. According to Faraday’s law of electromagnetic induction, a current is induced in the plate since there is an increase in magnetic flux. This current is the eddy current.
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| Figure 22.47 (a) Eddy currents produce a retarding magnetic force that opposes the motion of the plate when entering and leaving the magnetic field, reducing its speed. (b) A slotted (laminated) plate reduces eddy currents, thus decreasing the retarding magnetic force. |
4. The current flows in such a direction that, according to Lenz’ law, it opposes the change that produces it. Hence, the eddy current produces a magnetic force which acts in the direction opposite to the forward direction of motion of the plate, as shown in Figure 22.47(a). This opposing force will slow down the forward motion of the plate.
5. When the plate is completely inside the magnetic field, the magnetic flux does not change anymore. The magnetic force disappears and the forces that act on the plate now are the gravitational force and air friction force.
6. When the plate swings out of the magnetic field the magnetic flux through the plate decreases with time. Once again, an eddy current is produced in the plate. A magnetic force is produced which opposes the forward motion of the plate.
7. As a result, the motion is further slowed down. However, the magnetic force disappears when the plate emerges completely out of the applied magnetic field.
8. Hence, each time when the plate enters the applied magnetic field and also when it swings out of the field, it is retarded by a magnetic force. In one complete oscillation, the motion of the plate is retarded four times by the magnetic force.
9. Consequently, the period T2 of oscillation of the plate in and out of the applied magnetic field is shortened by the retarding magnetic force which is produced by the eddy current.
10. Without the applied magnetic field the plate is retarded only by air resistance. This friction force is much smaller than the retarding magnetic force. Hence, the period T1 is longer than T2.
One way which can be used to reduce the magnitude of the retarding magnetic force is to reduce the eddy current. This can be achieved if we use a laminated plate.
slotted plate instead of a complete plate, as shown in Figure 22.47(b). The slotted plate offers greater electrical resistance to the eddy current, hence reducing its magnitude, which in turn reduces the retarding magnetic force.
Applications of Eddy Current
| No | Application | Explanation |
|---|---|---|
| 1 | Induction Heating | Eddy currents produce heat in metals, used in induction furnaces and cooktops. |
| 2 | Magnetic Braking | Eddy currents create opposing magnetic forces to slow down moving objects like trains. |
| 3 | Metal Detectors | Detect metals by changes in eddy currents induced in conductive objects. |
| 4 | Speedometers | Used in vehicles where eddy currents help measure rotational speed. |
| 5 | Transformers (Core Loss) | Eddy currents cause energy loss, reduced using laminated cores. |
| 6 | Nondestructive Testing | Used to detect cracks or defects in metal structures. |
Conclusion of Eddy Current Concept
Eddy current is an electric current induced in a conductor when it is exposed to a changing magnetic field. It flows in closed loops within the material and follows Lenz’s law, opposing the change that produces it.
This phenomenon can be useful in many applications such as induction heating, magnetic braking, and metal detection. However, it can also cause unwanted energy losses in electrical devices.
Therefore, eddy currents must be carefully controlled. Techniques such as using laminated cores or slotted plates are commonly applied to reduce their effects and improve efficiency.
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