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Magnetic Fields

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Magnetic Fields

It is a Field Force

AP Physics

- For many years it was believed electricity and magnetism were separate and unrelated.

- In chemistry you learned protons and electrons have charges equal in magnitude but opposite in charge.
- Therefore atoms and molecules can have charge and areas of partial charge (δ).
- These electrical charges can attract (if opposite) or repel (if the same) one another.

qA

qB

What does this equation look like?

F

=

K

r2

Gravitational Force!!

qA

qB

Charge (q) is measured in coulombs!

F

=

K

r2

The charge on one electron is -1.60× 10-19 coulombs. This is the elementary charge!

qA

qB

F

=

K

r2

qA and qB is the charges on the objects, measured in coulombs(C).

r is the distance between the objects in meters(m).

K is the constant in 9.0 × 109 N∙m2/C2.

F is force in Newtons (N)

- Some objects are magnetized, have North and South polar ends.
- Using iron filings, this is the magnetic field of a simple polarized piece of metal.
- Metals such as Aluminum, Nickel, Cobalt, and Neodymium readily create magnetic fields.

- The poles are at opposite ends of a magnet.
- Opposite poles attract one another, like poles repel one another.
- The upper picture shows the repulsion of two like poles.
- The Earth itself also has a magnetic field, which is why compasses work!

- Magnetic fields are caused by d-orbital electrons spinning in the same direction.
- The more the fields of the atoms are aligned, the stronger the magnet.
- Magnetic Fields travel from the North to South pole!

In 1820 it was first observed that electrical current flowing through a wire caused a magnet to turn at right angles to the current.

- Faraday concluded that as current (I) flowed through a metal it created a magnetic field around it.
- A compass always pointed at right angles to the electrical flow because it was in the magnetic field.

- A coil of wire with electrical current running through it is called a solenoid.
- The magnetic field of each loop adds to the magnetic field of other loops.
- The South Pole is where current flows in.
- The North Pole is where the current flows out.

- Grip a solenoid by the right hand with fingers curling in the same direction of the current.
- This way the thumb points to the north pole of the magnetic field.

is called the magnetic permeability of a vacuum.

N is the number of turns in the coil.

ℓ is the length of the solenoid.

I is the current.

- Electromagnets exert a magnetic force . The direction of the force is dependent on the direction of the current.

Iis the electrical current measured in amperes(A).

ℓis the length of the wire measured in meters(m).

Bis the magnetic field strength measured in teslas(T).

T= 1N/A∙m

Force is still in Newtons (N).

Angle θ is the angle between the current and the magnetic field.

qis the charge on the particle in coulombs(C).

vis the velocity of the particle measured in meters per second(m/s).

Bis the magnetic field strength measured in teslas(T).

Force is still in Newtons (N).

Angle θ is the angle between the direction of the velocity and the magnetic field.

Why does the particle have to be charged??

Neutral particles, such as neutrons, are not deflected by magnetic fields!!

is called the magnetic permeability of a vacuum.

This was discovered by H.C. Ørsted and is often referred to as Ørsted’s Equation.

Electromagnetic Induction

Faraday’s brilliant insight!!

- Oersted was the first to observe that an electric current through a wire creates an effect on a compass.
- Faraday, in order to prove a magnetic field was created by electrical current, decided to see if the reverse was true.
- Faraday was able to get a magnetic field to create an electrical current, proving electricity and magnetism were a unified force.

Moving a magnet through an electrical coil causes an electrical current to be induced!

- Electrical motors and electrical generators which power our modern world.
- Even modern electrical generators powered by nuclear or hydroelectric stations use magnets to generate electrical current.