Magnetism Magnetic Fields The source of all magnetism

Magnetism,

Magnetic Fields • The source of all magnetism is moving electric charges. • Iron is the element with the most magnetic properties due to its net electron spin of 4. • Magnetic field lines are vectors with a direction from North to South. • Magnetic field lines must not cross each other. • Magnetic fields are measured in Teslas and is represented with the symbol B.

Earth’s Magnetic Field The angle between the magnetic and geographic poles is called the magnetic variation or declination.

Compasses • Compass needles are magnetized and line up along magnetic field lines. • The North magnetic pole of a compass points to the geographic north. • Since opposites attract, the magnetic pole in the Northern Hemisphere is actually a South magnetic pole. • The North pole of a compass points in the direction of the field lines.

Magnetic Field around a straight current-carrying wire • A current moving through a wire creates a magnetic field around that wire. • The magnetic field forms concentric circles around the wire. • Right hand rule (see picture): thumb – points in direction of the current in the wire fingers curl – in direction of the magnetic field

Electromagnets • Electromagnets are temporary magnets formed by wrapping wire around an iron core. • The iron becomes magnetized when the current is flowing due to the magnetic field being concentrated inside the coil of wire. Right-hand Rule: Fingers curl – in direction of the current (+ to -) Thumb – points in direction of the North pole N S

Force of a magnetic field on a current-carrying wire • A conductor with a current flowing through it in a magnetic field will experience a force. F = force(N) I = current(A) B = magnetic field strength(T) l = length of wire in field (m), = angle between B & I

Right-hand Rule ( • Flat fingers – point in the direction of the magnetic field (B) • Thumb – points in the direction the current or moving charges (I) • Coming out of palm – direction of the force on the wire (F) • Remember: magnetic field is from N to S, current is from + to -

Magnitude of Force • If Ө is zero, F = 0. So there is no force on the wire if the wire moves parallel to the magnetic field. • If Ө is 90°, F = maximum. So there is a maximum force on the wire if the wire moves perpendicular to the magnetic field. • If Ө is between 0 and 90°, the force will between 0 and a maximum. For the right-hand rules we will assume 90°.

Force between 2 currentcarrying wires • When a current flows through a wire a magnetic field is produced around it. • When 2 wires carry current near each othere will be an interaction (force) between the magnetic fields produced by each individual wire. • If the current is in the same direction, the wires attract each other. • If the current is in opposite directions, the wires repel each other.

Force of a magnetic field on a charged particle • A charged particle moving through a magnetic field will experience a force that will cause it to move in a circular path. • The force is to both the velocity and the magnetic field direction. F = force(N), q = charge(C), v = velocity(m/s), B = mag. field strength(T), =angle between v & B

Right-hand Rule (F=Bvq) • Flat fingers – point in direction of the magnetic field (B) • Thumb – points in the direction the charged particle is moving (v) • Coming out of palm – direction of the force on the charged particle (F) • Note – This rule is for a positively charged particle. For a negatively charged particle, force is negative and so the direction of the force is in the opposite direction of the right-hand rule.

Magnitude of Force (F=Bvq) • If Ө is zero, F = 0. So there is no force on the charged particle if the particle moves parallel to the magnetic field. • If Ө is 90°, F = maximum. So there is a maximum force on the charged particle if the charged particle moves perpendicular to the magnetic field. • If Ө is between 0 and 90°, the force will between 0 and a maximum. For the right-hand rules we will assume 90°.

Motors vs. Generators • Motors – Electric current is changed to motion. – A coil of wire with a current through it will be forced to turn in a magnetic field. • Generators – Motion is changed to electric current. – Turning a coil in a magnetic field will induce an EMF (voltage), thus current is produced.

AC/DC • Alternating Current (AC) • Direct Current (DC) – current that switches – current that flows in direction of flow on only one direction regular time intervals through a circuit – 60 Hz in US – supplied by batteries or electrochemical cells – Supplied by wall outlet

Transformers • Changes the voltage. • An alternating current flows through the primary coil creating an alternating magnetic field. • This changing magnetic field induces an EMF (Voltage) in the secondary coil and thus current flows. • In an ideal transformer: Power in = Power out.

Step-up Transformer (picture on the left) • More turns on the secondary coil • Increases the voltage (VS > VP) • Used at the power plant to send over long distances (want minimum current so less heat loss – I 2 R)

Step-down Transformer (picture on the right) • More turns on the primary coil • Decreases the voltage (VS < VP) • Used near homes (so have enough current)