A2 PHYSICS: G485 - PIGUY
ELECTRIC AND
MAGNETIC FIELDS
>>REVISION>>
ELECTRIC
FIELDS
ANY ELECTRIC FIELD
• created by electric charges
-1
• electric field strength (NC ) is force
F
___
E=
Q
Key
E - electric field strength
F - force
Q / q - charge
ε0 - permitivity of free
space
D / r - distance
V - voltage
per unit positive charge
• field lines point direction a positive
POINT CHARGES
charge would travel
-
• charges moving perpendicular to
uniform fields act like projectiles:
- acceleration constant in one plane
- zero acceleration in the other.
gravitational
electric
similarities
fields; strength inversely
differences
only attractive;
much weaker
also repels;
much stronger
+
Qq
_____
F=
4πε r2
0
-
UNIFORM FIELD’S
V
___
E=
D
-
+
+
MAGNETIC
FIELDS
Key
B - magnetic flux density
ANY MAGNETIC FIELD
F - force
I - current
L - length of wire
Q - charge
v - velocity
E - electric field strength
m - mass
r - radius of orbit
F = BILsinθ
• created by moving charges
• magnetic flux density (Tesla) is force per
unit current & length
MOVING CHARGES
• flemmings left-hand rule:
- first finger field
- second finger current
- thumb motion / force
in
X
out
• charges moving perpendicular to uniform
fields goes in circular orbit
• mass spectrometer:
- electric field accelerates ions
- electric & magnetic field selects velocity
- magnetic field deflects ions
F = BQv
mv
___
r=
BQ
X
v
X
X
X
X
X
r
X
-
B
X
BQv
___
m
-
VELOCITY SELECTOR
E
__
v=
X
+
v
+
ELECTROMAGNETISM
• magnetic flux (Weber) - number of field
MAGNETIC FLUX
ɸ = BAcosθ
lines in an area perpendicular to them
Key
ɸ - magnetic flux
B - magnetic flux density
A - area
V - voltage
I - current
N - turns of coil
• magnetic flux linkage - magnetic flux for a
coil - N times magnetic flux
F A R A D AY ’ S L A W
AC GENERATOR
• Lenz’s Law - direction of induced e.m.f.
opposes change in magnetic flux causing it.
• how transformers work:
1. ac current gives changing flux
dɸ
e.m.f. = -N __
dt
2. coils are magnetically linked, hence e.m.f.
S
TURN
N
3. low resistance allows current to flow
• reduce energy loss in transformers:
- lamination minimises eddy currents
- soft iron core lowers magnetic hysteresis
- low resistance (thick) wires
TRANSFORMERS
Vs
Vp
__
__
=
Np
Ns
VpIp =* VsIs
*if 100% efficient