The Earth’s magnetic field resembles that of a bar magnet.

North magnetic is close to south geographicSouth magnetic is close to north geographic

What is the source of the Earth’s magnetic field?

Unknown, but there are some theories.1. Iron Core acts as a bar magnet2. Motion of magma, which contains

ionized particles3. Combination of 1 and 2

What is the purpose of the Earth’s magnetic field?Shields us from high energy charged particles – “cosmic rays”.• Charged particles are directed along magnetic field lines

towards the magnetic poles. Charged particles that collide with our atmosphere cause light to be emitted.• Aurora Borealis• Aurora Australis

• Charged particles become trapped in toroidal clouds surrounding the equator called the Van Allen Belts

Cosmic rays (atomic nuclei stripped bare of their electrons) would continuously bombard Earth’s surface if most of them were not deflected by Earth’s magnetic field. Given thatEarth is, to an excellent approximation, a magnetic dipole, the intensity of cosmic rays bombarding its surface is greatest at the

1. poles.2. mid-latitudes.3. equator.

CH 29: Electromagnetic Induction

If current can generate a magnetic field is it possible to generate a current from a magnetic field? Yes, but not from a constant magnetic field!If you connect a ammeter to a coil of wire you will be able to detect any current in that wire. What happens if you hold a magnet in front of the coil?

Nothing – The ammeter will read zero current.What happens if you move the magnet into the coil?

A current will be generated in the coil.When the magnet is moved into the coil the number of magnetic field lines that pass through the coil is changed. This change in the magnetic field strength causes the current.

A changing magnetic field, or more specifically a changing magnetic flux, will cause an induced emf, which in turn will cause a current to be induced in the coil.

Bd

dt

e – Emf induced by changing flux [V]FB – Magnetic flux [Wb]

B B dA

The coil resists the changing fluxThe induced emf depend on changing the flux, which is either changing the magnetic field strength, changing the area, changing the angle between B and A or some combination of the three.

cosBdA

In figure (a), a solenoid produces a magnetic field whose strength increases into the plane of the page. An induced emf is established in a conducting loop surrounding thesolenoid, and this emf lights bulbs A and B. In figure (b), points P and Q are shorted. After the short is inserted,

1. bulb A goes out; bulb B gets brighter.2. bulb B goes out; bulb A gets brighter.3. bulb A goes out; bulb B gets dimmer.4. bulb B goes out; bulb A gets dimmer.5. both bulbs go out.6. none of the above

Explanation

Example: A solenoid connected to a variable power supply generates a current that increases as a function of time I(t) = 5t2 + 3t. This solenoid has 200 turns, is 10 cm long and has a diameter of 2 cm. A second solenoid with 1000 turns, 10 cm long and 2 cm in diameter is placed near one end of the first solenoid such that long axis of both solenoids coincide. How much current flows in the second solenoid when t = 5s if it the resistance of the solenoid is 100 W.

xxxxx

I

A

B

Bd

dt

BdNdt

dN B dAdt

2 1 2

dN B Adt

B A

B doesn’t depend on the area of the loops that it passes through.

12 2 0 1

1

NdN A I

dt

2

1

B for solenoid

22 2 0 1

1

5 3N A N d

t tdt

2 2 0 1

1

10 3N A N

t

0.042V I

R

0.42mA