4.5 Electromagnetic effects
4.5.1 Electromagnetic induction
Core Concepts
1. What is Electromagnetic Induction?
- Electromagnetic induction is the process of generating an electromotive force (e.m.f.) in a conductor when:
- The conductor moves across a magnetic field, or
- There is a changing magnetic field around the conductor.
- This e.m.f. can cause a current to flow if the conductor is part of a complete circuit.
Figure 1: A Voltage is induced when the wire is moved up or down in the magnetic field
2. Experiment to Demonstrate Electromagnetic Induction
Figure 2: A Voltage is induced in the coil when the magnet is moved in or out
- Apparatus Needed: A coil of wire, a magnet, and a sensitive ammeter or galvanometer.
- Steps:
- Connect the coil to the ammeter to form a complete circuit.
- Move the magnet into the coil. Observe the ammeter needle deflecting, showing that a current is induced.
- Move the magnet out of the coil. The ammeter needle deflects in the opposite direction, showing that the induced current reverses.
- Hold the magnet still inside the coil. The ammeter shows no deflection, meaning no e.m.f. is induced.
- Conclusion: An e.m.f. is only induced when there is relative motion between the magnet and the coil (or when the magnetic field changes).
3. Factors Affecting the Magnitude of Induced E.M.F.
The size of the induced e.m.f. depends on:
- Speed of Movement: Faster movement of the magnet or conductor increases the e.m.f.
- Strength of the Magnetic Field: A stronger magnet produces a larger e.m.f.
- Number of Turns in the Coil: More turns in the coil increase the e.m.f.
Key Idea: To increase the induced e.m.f., move the magnet faster, use a stronger magnet, or add more coils to the wire.
Summary
- Electromagnetic induction happens when a conductor moves through a magnetic field or when the magnetic field around a conductor changes.
- You can demonstrate this by moving a magnet in and out of a coil connected to an ammeter.
- The induced e.m.f. depends on the speed of movement, magnetic field strength, number of coils, and the angle of movement.
Supplement Concepts
4. Direction of Induced E.M.F. (Lenz's Law)
Figure 3: Lenz's Law
- The direction of the induced e.m.f. (and the current it creates) always opposes the change that caused it.
- This is known as Lenz's Law.
- Example:
- If you push a magnet into a coil, the induced current creates a magnetic field that repels the magnet.
- If you pull the magnet out of the coil, the induced current creates a magnetic field that attracts the magnet.
- Why?: This happens because energy must be conserved. The work done to move the magnet is converted into electrical energy.
5. Relative Directions of Force, Field, and Induced Current
- The direction of the induced current, magnetic field, and force can be determined using Fleming's Right-Hand Rule.
Figure 4: Fleming's Right Hand Rule
- Fleming's Right-Hand Rule:
- Thumb: Direction of Force (Motion of the conductor).
- Index Finger: Direction of the Magnetic Field (North to South).
- Middle Finger: Direction of the Induced Current.
- How to Use:
- Point your index finger in the direction of the magnetic field (N to S).
- Point your thumb in the direction the conductor is moving.
- Your middle finger will point in the direction of the induced current.
Key Idea: Fleming's Right-Hand Rule helps you figure out the direction of the induced current when a conductor moves in a magnetic field.
Summary
- Lenz's Law: The induced e.m.f. always opposes the change causing it (e.g., movement of a magnet or change in magnetic field).
- Fleming's Right-Hand Rule: Used to determine the direction of the induced current, magnetic field, and force. Remember:
- Thumb = Force (Motion),
- Index Finger = Magnetic Field,
- Middle Finger = Induced Current.