MAGNETISM

Magnetic fields (B FIELDS) occur whenever charge moves. The charge may move in the following ways:

  • An ion (positive or negative) is accelerated from rest by an E field. Facts to remember:  V = Ed for parallel plates and  q D v = D Kinetic Energy

  • In a long current carrying wire. Facts to remember--Current (I) flows from high potential or positive terminal to low potential or negative terminal. A current-carrying wire generates a magnet field around the wire which encircles the wire. V=IR

  • In a long tightly wound coil --also called a solenoid--the magnetic field is very strong and parallel to the axis of the coil inside the core. It is very, very weak outside the core....  Facts to remember: B = monI for a solenoid where n = number of turns of wire per length of solenoid, mo = 4p x 10^-7 and i is current.

 

This image illustrates the way the magnetic field encircles a long straight current carrying wire. If your thumb of your right hand points with i, your fingers encircle with B.

 

OTHER IMPORTANT CONCEPTS:

  • The magnetic field lines (B) always flow from North to South!

  • Like poles repel and unlike poles attract.

  • Magnetic fields exert forces on charge (either positive or negative) under two conditions ..........1. The charge MUST be moving AND....... 2. The velocity of the charge must have some component that is perpendicular to the magnetic field B (Bperpendicluar = B sine q where q is the angle between B and I or v).

  • When the moving charge is in a wire then F= B I L sin q or iL x B .

  • When charge is free from a wire / not contained then F = qvB sin q or qv x B.

  • Magnetic field forces always act at right angles to velocity or to current and so create a circular path when the charge is free to move and not trapped in a wire.

  • F = mv^2/R whenever circular motion occurs, so qvB=mv^2/R. This simplifies to qB=mv/R !

  • Magnetic fields do ZERO work because they always act perpendicular to the path!

 

EQUATIONS CONNECTING TO CONCEPTS:

The magnitude of the magnetic field at a point in space is defined by the force exerted on a moving charged particle. The force is at a maximum when the charge moves perpendicular to the direction of the magnetic field and ZERO when it moves parallel to the magnetic field.

F = qvB sine q

The SI unit of the magnetic field is the TESLA

(Look at the flying frog!)

1T = 1 N s/ C m

1 Gauss= 10^-4 Telsa

For a current carrying wire, the force bends the wire or qv & the right hand rule applies! The right hand rule selects the thumb to point in the direction of "I" or "qv", the fingers in the direction of "B", and the palm will face the direction of "F".

 

 

Torque may occur from this force on a loop or a Number..N... of loops of wire. That torque will equal m x B where m = NIA. N is the number of coils on the frame, i is the current in the wire, and A is the face area of the frame.

Look at this applet of the forces acting on a motor.

 

INDUCED CURRENTS FROM MAGNETIC FIELDS

Moving charge produces a magnetic field that encircles the path of the charge. Therefore a change in the position of a charged object (that means the charge has a velocity) creates a magnetic field. What if an external magnetic field changes around a fixed charge? Does that affect the charge? And how does it affect the charge? Let's look at Lenz's Law to see.

 

LENZ'S LAW

The induced emf (in volts) in a loop of wire acts to oppose the change in magnetic flux through that loop. Change in Magnetic flux is Df = B DA or DB A.

  • Unlike poles fight a rise in F.

  • Like poles fight a drop in F.

 

FARADAY'S LAW

emf (IN VOLTS) = - N DF/ Dt

N= number of turns in the coil. F = B A. Dt = time change. Change in F (DF) would be caused by a DB or DA or both!

 

Sometimes, the MOTIONAL emf equation is written as e = Bvd

 

 

AMPERE'S LAW