Why do eddy currents waste energy in a transformer?

The currents flow through the resistance of the core, so electrical energy is dissipated as heat that is never delivered to the secondary.

How does laminating a transformer core reduce eddy currents?

The core is built from thin sheets separated by an insulating layer, which breaks the conducting loops across the core and so greatly increases the resistance of the eddy current paths.

In which direction do eddy currents flow?

They flow in whichever sense produces a magnetic field that opposes the change of flux causing them, as Lenz's law requires.

OCR A-Level Physics B H557

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Electromagnetism

Eddy currents and real transformers

3 things to know
Components 01, 02 and 03

Overview

Recall questions on eddy currents in transformer cores and the energy they waste, laminated cores, their direction, electromagnetic braking, induction hobs and metal detectors, copper loss, flux leakage and hysteresis loss, transformer efficiency and cooling, and the jumping aluminium ring.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Explain how eddy currents are used in electromagnetic braking.

Key term

What are eddy currents?

Key term

Why do eddy currents arise in a solid iron transformer core?

Questions from this subtopic

Each one with the model answer underneath.

1
What are eddy currents?
Model answer
Eddy currents are circulating currents induced in the body of a conductor when the flux through it changes.
2
Why do eddy currents arise in a solid iron transformer core?
Model answer
The iron is a conductor sitting in a changing flux, so e.m.f.s are induced in closed loops within the metal and currents circulate.
3
Why do eddy currents waste energy in a transformer?
Model answer
The currents flow through the resistance of the core, so electrical energy is dissipated as heat that is never delivered to the secondary.
4
How does laminating a transformer core reduce eddy currents?
Model answer
The core is built from thin sheets separated by an insulating layer, which breaks the conducting loops across the core and so greatly increases the resistance of the eddy current paths.
5
In which direction do eddy currents flow?
Model answer
They flow in whichever sense produces a magnetic field that opposes the change of flux causing them, as Lenz's law requires.
6
Explain how eddy currents are used in electromagnetic braking.
Model answer
A metal disc moving through a magnetic field has eddy currents induced in it, and the force on those currents opposes the motion, so the disc slows without any physical contact.

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