EM-3 Electromagnetism Notes
Forcing a wire through a magnetic field (or vice-versa) produces or generates
a current - Generators
Electromagnetic induction (induced current) - a current is produced in a wire
by passing a magnet near it
(The magnetic field lines (flux) crossing the wire will cause the electrons in the
wire to move, which in essence is what electricity or current is, WITHOUT
touching it)
This is the principle generators are based on. Generators force or spin a bundle of
wire attached to a motor through magnets causing a current or electricity in to be
Magnetic flux ( Φ ) - the number of magnetic field lines passing through a given
(The greatest concentration of flux is near the poles)
Φ - magnetic flux ( N m / a )
B - magnetic field strength ( N / a m )
A - area ( m2 ) formula : length x width (π r2 for a circle)
A current carrying wire placed in a magnetic field experiences a force Electric Motors
(A current in a wire produces a magnetic field circling the wire - 1st right hand
rule, that magnetic field produced by the current in the wire will interact with the
magnetic field from the magnet causing either attracting or repelling which is the
1st right hand rule
- thumb points in the direction of the positive current in a wire
- fingers CURL in the direction of the magnetic field around the wire
2nd right hand rule
- thumb points in the direction of the positive current
- fingers are STRAIGHT point in the direction of the magnetic field
- force is directed out of the palm
(all are perpendicular to each other)
The variables that determine the strength of the force are:
- amount of current ( I = amps (a))
- magnetic field strength
- length of wire
F = force (N)
B = magnetic field strength (N/am)
I = current (a)
L = length of wire (m)
X = current (I)
X = into the page
= out of the
Voltage – energy needed to move a charge (electric potential)
Relative motion - the movement of one object with respects to another
Lenz’s Law - the current induced in a wire is in such a direction that its magnetic
field is opposite the magnetic field that induced it
Faraday’s Law - Voltage (emf) induced in a wire is proportional to the rate of
magnetic flux (magnetic field lines) crossing the wire
EMF - Potential difference produced by electromagnetic induction (Voltage
created by forcing a wire through a magnetic field)
Transformers – A device to increase (step-up transformer) or decrease (stepdown transformer) voltage
N1 = V1
N2 V2
Iron Core
10 turns
5 turns
If the primary coil having 5 turns of wire contains 20 volts, what is the voltage in
the secondary coil having 10 turns?
5 turns = 20 V
10 turns X
5 X = 200
X = 40 V

EM-3 tv notes 09 - Moline High School