During the next 4 weeks I will list the most misunderstood concept in each of the four sections of the current NSW HSC Physics syllabus. Back emf is the most misunderstood concept in the motors and generators section. Here are some tutorial points on back emf.
- Look at a demonstration motor and a generator. Draw diagrams of these labelling the parts.
- When the magnetic flux through a coil changes an emf is induced in the coil.
- Imagine a coil of wire (solenoid) carrying a constant current. As the current flows through the windings it makes a magnetic field that passes through the coil. If the current is turned off it does not drop to zero immediately. For a short time the current keeps flowing at a value that exponentially falls towards zero. During this time interval an emf is induced in the coil due to the changing magnetic flux through it causing the current to "keep going" for a short time, opposing the change that is happening. Sketch graphs showing (A) the current flowing in the coil versus time, (B) the potential difference across the coil versus time.
- As the coil of a motor rotates in a magnetic field the changing magnetic flux through it induces an emf in the coil in accordance with Lenz's law. This emf tends to make an induced current in the opposite direction to the current supplied to the coil and is called a back emf.
- When a motor is first turned on the current in the coils is large as there is no back emf. As the coils gain speed the back emf increases and the current in the coil decreases.