Electric and magnetic fields, microwave therapy

Transcription

1 Electric and magnetic fields, microwave therapy November, 2010 Dubravka Krilov Sanja Dolanski Babić

2 Electric field the region around the charged body E electric field strength = electric force per unit charge

3 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) E F / q scalar field equipotential surfaces in each point of the space the potential energy of the test charge is called electric potential W / q is called strength of electric field the electric force changes the energy of the body

4 Electric field of the point charge inhomogeneous field Strength of electric field Principle of superposition r 0 r Q r E ) ( i i i i i r r Q r E r E i0 i 2 ) ( ) (

5 Potential energy in the electric field charge Q 2 is in the electric field of charge Q 1, at distance r A from Q 1. The potential energy is: Q 1 E pot ( r A ) 1 Q1Q 4 r 2 A W A Q 2 - potential energy of unit charge at point A is electric potential: ( E r pot A 1 ra ) q 4 ra 1 Q E pot ( ra ) Q2 ( ra ) - points of equal potential build the equipotential surface

6 Charge in homogeneous field electric field of capacitor E U d F E q

7 Charge in homogeneous field E kin W Q W U electric field always act by electric force to charge and always change amount of speed

8 Cell membrane as capacitor Intracellular E U d 3 60mV 6010 V 7 E 10 V / m 9 6nm 610 m Extracellular

9 Electric dipole activity of the heart model for electrical - electric dipole is a pair of opposite charges q at distance l; it has a dipole moment p (p ~ C m) ql p q i li i - for more dipoles the net dipole moment is vector sum of individual contributions - situation in the heart

10 Note: for point charge ~ 1/r; E ~ 1/r 2 potential of a dipole l +q - q d r r + = r r - = r + d d = l cos a r >> l electric field of a dipole ( q ) ( q ) ( q 4r q 4 ( r l cos a ) r l cos a << r 2 1 p r 4 r 0 r, a) 2 1 p r 4 r 0 E 3

11 Electric dipole in the external electric field M d F d M M l sina q E p E torque of couple of forces rotates the dipole in external electric field

12 Magnetic field the region around moving charge or electric current Bar magnets Electromagnets

13 Magnetic induction B 0 r 2 I r B IN 0 r 4 l

14 Magnetic induction - B magnetic field is the vector field - the field lines are closed curves force between two wires which carry electric currents: I1 F I2 2 d magnetic induction is force on unit current element (I 2 l =1 Am): I B 2 d

15 Point charge in electric and magnetic field F q v B qe Lorentz force

16 Earth B ~ 50 T= 0.5 G Model of dynamo

17 Biomagnetism Some organisms use the Earth's geomagnetic field for the orientation and navigation. Magnetotactic bacteria produce tiny magnetite particles (Fe 3 O 4 ), the clearest in nature.

18 Source Magnetic induction ( T ) Human brain ~ Urban noise ~ 10-7 Earth s field Bar magnet Electromagnet < 16

19 Electromagnetic induction If the flux of magnetic field through the coil is time dependent, the voltage will be generated in the coil. Faraday law: U N t U U0 sint I I sin t 0

20 Demonstration of electromagnetic induction If we insert the magnet into the coil it will cause the change of magnetic field and current flow due 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.

21 Electromagnetic field energy density of electric and magnetic field the source of field is oscillating charge E pot V E B 2 2 electromagnetic wave is transversal direction of propagation is perpendicular to the planes of oscillation of both fields oscillating fields are mutually perpendicular E B E B 0 0 sin t sint wave velocity depends on substance's properties v 1 c 1 0 0

22 Microwave therapy equipment technique of heating biological tissue by electromagnetic radiation high frequency oscillations generated in magnetron types of aerials microwave frequencies 1-3 GHz emitted by dipole aerial

23 Microwave therapy Intensity of entranced radiation Beer s law: I t I 0 e a x Intensity of transmitted radiation absorption coefficient, a the best absorption of microwaves occurs in muscle tissue standing waves increase the heating at tissue boundaries