Physics·Electricity and magnetism · NSSCO 5.7.2
Electromagnetic induction, a.c. generator & transformer
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Move a magnet near a coil, or move a wire through a magnetic field, and electricity appears out of nothing but movement — this is electromagnetic induction. In this lesson we induce an e.m.f. with a magnet and a coil, find the three things that make it bigger, meet Lenz's law for its direction, build a spinning a.c. generator with its slip rings and sine-wave output, and finally build a transformer, use Vp/Vs = Np/Ns for step-up and step-down with worked calculations, and see why high-voltage transmission carries electricity across Namibia with so little waste.
What you'll learn in this lesson
By the end you should be able to (NSSCO Physics 5.7.2):
- Describe an experiment showing that a changing magnetic field, or a wire moving through a field, induces an e.m.f.
- State the factors that increase the induced e.m.f. — moving faster, a stronger magnetic field, and more turns on the coil
- State that the induced current opposes the change that causes it (Lenz's law)
- Describe a simple a.c. generator (rotating coil, slip rings and brushes) and sketch its voltage-time output, including the effect of spinning faster
- Describe the structure and operation of a simple iron-cored transformer and recall and use Vp/Vs = Np/Ns for step-up and step-down
- Recall and use Vp Ip = Vs Is (100% efficiency) and state why high-voltage transmission reduces energy loss
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