REVISION ELECTROMAGNETISM. ELECTROMAGNETIC SPECTRUM (EMS)

REVISION

ELECTROMAGNETISM

ELECTROMAGNETIC SPECTRUM (EMS)

ELECTROMAGNETIC SPECTRUM (EMS)

• Accelerating charges produces a changing magnetic field (perpendicular to the movement of the charge) and the changing magnetic field produces a changing electric field (perpendicular to the magnetic field)

• EMS is light waves• It does NOT need a medium to propagate• There are 7 main groups

ELECTROMAGNETIC INDUCTIONWhen a conductor and a magnet move relative to

each other, it causes a changing magnetic field around the conductor. The changing magnetic field causes (induce) an emf and the emf causes charge to flow

FARADAY’S LAWThe induced emf in a conductor is directly proportional to the rate of change in die

magnetic flux in the conductor.

mf Nt

MAGNETIC FLUX

The number of magnetic field lines cutting through an area

cosBA

LENZ’S LAWThe direction of the induced current is such that it

withstands cause thereof.

RIGHT HAND RULE for CONDUCTOR

• Thumb points in the direction of current flow (positive to negative)

• Fingers point in the direction of the magnetic field

MAGNETIC FIELD IN A LOOP

RIGHT-HAND SOLENOID RULE

• Thumb points in direction of the North pole

• Fingers point in the direction of current flow in the loops

RIGHT-HAND SOLENOID RULE

ELECTROMAGNETS

ELECTROMAGNETS

mf Nt

Strengthen an electromagnet ...• More loops ... more loops

produces more field lines and that means a stronger field.

• Increase current ... because it is directly proportional to the EMF

• Type of material ... influence the magnetic flux

mf IR

WHY IS CURRENT INDUCED?• As the South pole of the magnet approaches the

solenoid, the coil tries to prevent its approach by producing its own South pole (that PUSHES the magnet away)

• The induced current in the coil opposes the change in magnetic field (caused by the magnet) by trying to prevent the magnet from coming closer.

WHY IS CURRENT INDUCED?• As the South pole of the magnet moves away from the

solenoid, the coil tries to prevent its departure by producing its own North pole (that PULLS the magnet back

• The induced current in the coil opposes the change in magnetic field (caused by the magnet) by trying to prevent the magnet from moving away.

WHY IS CURRENT INDUCED?• When the magnet comes

close to the coil, the magnetic field lines come in contact with the electrons in the coil.

• This induces a potential difference in the coil and the potential difference causes the electrons to flow (current)

• The coil now has its own magnetic field and this field opposes the applied magnetic field.

ELECTROMAGNETISM QUESTIONS

40

8

0

1.5

i

f

B emf

A N

I

R t

0 8 8

40 8

1.5213.3

f i Wb

N

t

V

ELECTROMAGNETISM QUESTIONS

2 2 2

2 25 5

9

5

5 o

6

6

3.80 10 2.85 10

2.256 10

4.75 10

cos

0 4.75 10 0.043 cos0

2.06 10

2.06 10233

0.0480.01

x y

i

B B B

B T

BA

Wb

Nt

V

22 20.235

0.0432

0

233

8

0

600.048

1250

o

i

f

B

A r m

I

R

emf

N

st s

rpm