Electric and magnetic fields, microwave therapy · Electric and magnetic fields, microwave therapy...
Transcript of Electric and magnetic fields, microwave therapy · Electric and magnetic fields, microwave therapy...
Electric and magnetic fields,
microwave therapy
Dubravka Krilov
Sanja Dolanski BabićNovember, 2010
Electric field
E – electric field strength = electric force per unit charge
the region around the charged body
Presentation of electric field
vector field – field lines
in each point of the region,
the magnitude and direction
of force acting at unit positive
charge (test charge)
is called strength of electric
field
the electric force changes
the energy of the body
scalar field –
equipotential surfaces
in each point of the
space the potential
energy of the test charge
is called electric
potential
qFE /
qW /
Electric field of the point charge
inhomogeneous field Strength of electric field
Principle of superposition
0rr
QrE
24
1)(
i i i
ii r
r
QrErE i0i
2
)()(
Potential energy in the electric field
- potential energy of unit charge at point A is electric potential:
A
A
Apot Wr
QQrE 21
4
1)(
A
Apot
Ar
Q
q
rEr 1
4
1)(
Q1
Q2
charge Q2 is in the electric field of charge Q1, at distance rA from Q1. The potential energy is:
)()( 2 AApot rQrE
- points of equal potential build
the equipotential surface
Charge in homogeneous field
d
UE
qEF
electric field of capacitor
electric field always act by electric force to charge and
always change amount of speed
UQW
WEkin
http://magnetizam.ifs.hr/
Charge in homogeneous field
Cell membrane as capacitor
d
UE
mVm
V
nm
mVE /10
106
1060
6
60 7
9
3
Intracellular
Extracellular
Electric dipole model for electrical
activity of the heart
i
i
i lqp
lqp
(p ~ 10-30 C m)
- electric dipole is a pair of
opposite charges q at distance ℓ;
it has a dipole moment
- for more dipoles the net dipole
moment is vector sum of
individual contributions -
situation in the heart
potential of a dipole
r
d
l
r+ = r
r- = r + d
d = l cos a
)coslr(
q
r
q
)q()q(
a
44
r >> l24
1),(
r
rpr 0
a
34
1
r
rpE 0
r l cos a << r2
Note: for point charge ~ 1/r; E ~ 1/r2
- q
+q
electric field of a dipole
Electric dipole in the external
electric field
d
EpM
EqlM
FdM
asin
torque of couple of forces rotates
the dipole in external electric field
Electromagnets
Bar magnets
the region around moving charge or electric current
Magnetic field
r
IB
r 20
l
INB
r
40
Magnetic induction
Magnetic induction - B
magnetic field is the vector field - the field lines are closed curves
force between two wires which carry electric currents:
magnetic induction is force on unit current element (I2 ℓ =1 Am):
21
2I
d
IF
d
IB
2
Point charge in electric and
magnetic field
EqBqF
v Lorentz force
Earth B ~ 50 T= 0.5 G
Model of dynamo
Biomagnetism
Some organisms use the Earth's
geomagnetic field for the
orientation and navigation.
Magnetotactic bacteria produce tiny
magnetite particles (Fe3O4 ), the
clearest in nature.
SourceMagnetic induction
( T )
Human brain ~ 10 -13
Urban noise ~ 10 -7
Earth’s field 0.5· 10 -4
Bar magnet 0.001-1
Electromagnet < 16
Electromagnetic induction
tUU sin0
tNU
tII sin0
If the flux of magnetic field
through the coil is time
dependent, the voltage will be
generated in the coil.
Faraday law:
Demonstration of electromagnetic induction
The magnitude of the
current depends on speed
of magnet motion in or
out of the coil, and the
number of loops in the
coil.
If we insert the magnet into
the coil it will cause the
change of magnetic field and
current flow due
electromagnetic induction.
Electromagnetic field
energy density of electric and magnetic field
electromagnetic wave is transversal – direction ofpropagation is perpendicular
to the planes of oscillation of
both fields
wave velocity depends on substance's properties
the source of field is oscillating charge
oscillating fields are mutually perpendicular
22
2
1
2
1 BE
V
E pot
tsinBB
tsinEE
0
0
00
11
cv
microwave frequencies 1- 3
GHz emitted by dipole aerialtypes of aerials
Microwave therapy
technique of heating
biological tissue by
electromagnetic radiation
high frequency oscillations
generated in magnetron
equipment
absorption coefficient, a
the best absorption of microwaves
occurs in muscle tissue
standing waves increase the heating at
tissue boundaries
x
t eIIa
0 Beer’s law:
Intensity of transmitted radiation
Intensity of entranced radiationMicrowave therapy