Goal: To understand Magnetic Induction

1. Goal: To understand Magnetic Induction Objectives: Understanding how to Induce a Current Understanding how to Induce a magnetic field To Learn more about transformers To Understand how to Generate an AC current

2. Inducing a current • There are two ways to have fun with magnetic fields. • The first involves a changing magnetic field (in terms of sign or magnitude). • Any time a magnetic field changes all electrons that are around it that are moved. • This creates a current in any metal object – such as a wire. • So, the changing magnetic field creates a current. • This is called an induced current.

3. Induced currents • So, a magnet traveling through a looped wire will induce a current. • The size of the current will depend on only two things, the rate of change of the magnetic field (so how quickly the magnetic field changes) and the area of the loop. • The bigger the loop the bigger the current and the more quickly the magnetic field changes the bigger the current.

4. Induced magnetic fields • Turns out it works both ways. • A current can induce a magnetic field! • Yes, that means that the current that the changing magnetic field creates will create a magnetic field of its own! • In fact this induced magnetic field will be the opposite in sign to the change in the magnetic field. • So, if the magnetic field that induces the current increases, then the magnetic field induced from the induced current will be negative (positive if the original field decreases). • THEREFORE, I clearly cannot choose the glass in front of you!

5. No, I have not even started yet! • Wait, I have started, but have not finished yet… • Each loop will have its own current. • If you have 10 loops, you get 10 currents. • If they are all the same size then you get the same current in each, and each current is the same in that loop as it would be if there was only 1 loop.

6. If all else is equal • If all loops have the same amount of resistance, since V = IR, then every loop has the same amount of VOLTAGE! • Note: When you build something that is near either a changing magnetic field or something with a current, you need to watch out for induced currents, and be sure you have resistance - otherwise….

7. Transformers revisited • For a transformer, you have two sides with different amounts of loops. • Each loop has the same amount of voltage! • So, the more loops, the more voltage you have on that side. • That is why the ratio of voltages was the ratio of the # of loops!

8. AC only! • However, since transformers work because of induced currents, therefore transformers ONLY work using AC current. • That is a big part of why we use AC current.

9. Generating an AC current • So, how do you generate an alternating current? • Way #1 is move a magnet back and forth. • As it goes in you will increase the magnetic field and generate a current. • When it comes back you will decrease a magnetic field and generate a current which flows in the opposite direction. • This gives you a current which alternates from positive to negative.

10. 2nd way • The 2nd way is to leave the magnetic but rotate the loop. • As you rotate the loop, the area the magnet sees changes (drops to 0 when it is flat) and then will be opposite when the loops is upside down. • This is the most common way to generate electricity – all you need is a way to turn the loop. • Ways to do this are to use steam or flowing liquid water.

11. Conclusion • We have learned about induction both from induced magnetic fields and induced magnetic currents. • We have seen why transformers work. • We have learned how to create an AC circuit.