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Before we move on to applications of electromagnetic induction, we need to understand more the basic idea we first heard last week about induction.

There are three components to understand

• The direction of induced current/emf/E field . The essential idea is encapsulated into Lenz's law. Essentially, the system is trying to oppose the change in magnetic field and counteract it.
• The magnitude of the induced EMF. This is Faraday's law and there we can be quantitative using magnetic flux as defined last week.
• Then finally we can go more abstract and write Faraday's law in terms of electric field and it then looks very similar to Ampere's law and it can be used in similar ways.

In the following two videos Prof. Leblond discusses induced EMF with an emphasis on understanding the direction of the induced current.

Reading Guide

Lenz's law takes practice. Make sure you differentiate the induced B field from the source B field. Fig 30.21 gives you all 6 possible configurations.

Examples 30.6-30.10 are all super important.

• We will come back to Maxwell's E&M wave discussion but you can have quick peak at the end of section 30.6.
• for section 30.7, you should understand how a generator work but the equations are not important (and neither is example 30.11). Same with the application to metal detectors.

Application, the Transformer

Next week we will discuss alternating current(AC) and how circuit elements such as R, C and the inductor L that we will see next responds to alternating current.

In the early beginning of the century there was a debate whether the electric grid should use AC current or direct current DC. The debate was led by Edison (DC team) versus Tesla (AC team).

Well AC won and the main reason is the transformer. You can see transformer on electric poles almost everywhere. To minimize loss in power when we transport electricity over large distances it is better to use high voltage (i.e., low current).

But in your house, high voltage would be dangerous and unusable. Fortunately, it is easy to change the voltage from high to low (or the reverse) using a transformer. The ratio of high to low voltage is exactly the same as the ratio of the number of wraps of wire around the transformer (just like the strength of an electromagnet depends on the number of wire wraps).

The transformer only works with AC current and this is ultimately why this is the current we get in our houses!