1. Magnetism, Electromagnetism, & Electromagnetic Induction

2. 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.

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

4. 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.

5. 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

6. 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

7. 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) l = length of wire in field (m), B = magnetic field strength(T)  = angle between B & I

8. 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) Out of palm – direction of the force on the wire (F) Remember: magnetic field is from N to S, current is from + to -

9. Force between 2 current- carrying wires When a current flows through a wire a magnetic field is produced around it. When 2 wires carry current near each other there 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.

10. 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

11. Right-hand Rule (F=qvB) 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.

12. Induced EMF (Voltage) A conductor in a changing magnetic field will have an EMF (voltage) induced. Either the conductor can be moving across field lines or the magnetic field can itself be changing. EMF = electromotive force, voltage(V), B = magnetic field strength (T), v = velocity(m/s), = angle between B & v EMF

13. Right-Hand Rule (EMF = Blv) Flat Fingers – point in direction of Magnetic Field (B) Thumb – points in the direction the wire is being moved (v) Coming out of palm – direction of the EMF (Voltage and current also in same direction as the EMF)

14. Induced Current When a EMF (voltage, or potential difference) is present in a closed loop of conducting material, current will flow. I=current(A), EMF = V = Voltage(V), R = resistance

15. Lenz’s Law The voltage (and thus current) induced when a wire or conductor is moved through a magnetic field is induced in such a way that the magnetic field created by the induced current opposes the original magnetic field that induced the voltage or current.

16. AC Generator As the loop of wire is turned in the magnetic field, one side is moving up while the other is moving down, therefore a current is induced in opposite directions in the different sections of the loop. As the loop continues to turn, the sections of wire change places and so the current switches direction. This causes the current to change constantly as shown in the graph.

17. 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.

18. AC/DC Alternating Current (AC) –current that switches direction of flow on regular time intervals –60 Hz in US –created by EMF induced in a coil of wire turning in a magnetic field Direct Current (DC) –current that flows in only one direction through a circuit –supplied by batteries or electrochemical cells –created by a chemical reaction that produces a potential difference (voltage) between the two electrodes (terminals)

19. Effective vs. Maximum with AC Current Unlike DC circuits, AC circuits do not get the effect of the maximum current and voltage produced so we use effective current and voltage for AC. The power equivalent of AC to DC voltage is half.

20. When using Ohm’s Law for AC, be consistent:

21. Transformers 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. In a step-up transformer, there are more turns on the secondary coil and thus the secondary voltage is greater than the primary voltage.

22. To solve Transformer Problems The ratio of voltages on the two coils is equal to the ratio of the number of turns in the coils. Ideally, power in (primary power) is equal to power out (secondary power)