qq k d Chapter 16 Electric and Magnetic Forces Electric charge Electric charges Negative (electron) Positive (proton)

Transcription

1 Chapter 16 Electric and Magnetic Forces Electric charge Electric charges Negative (electron) Positive (proton) Electrons and protons in atoms/molecules Ions: atoms/molecules with excess of charge Ions can be positive (excess of protons) Ions can be negative (excess of electrons) Charging: Friction Contact Induction Matter is commonly neutral Charge conservation Charge is measured in Coulomb C Charge of electron = C Like charges repel Opposite charges attract Coulomb s law qq F= k d d = distance

2 k = N m 2 /C 2 Note the similarity to Newton s law of gravity mm F= G d G = 6.66 x N m 2 /kg 2 Gravitational force is always attractive Electric force is about times larger than gravitational force. Polarization of materials The centers of positive and negative charges are displaced (not in same place) Molecule is polarized => electric dipole Electric field around charge A small test charge will experience force Electric field= Force / test charge Direction of field: in direction of force experienced by positive test charge If the charges are in equilibrium, then the electric field is zero inside conductors Shielding

3 Magnetism Magnetic forces Poles are equivalent magnetic charge Like repel, opposites attract Poles always in pairs (N and S) Magnetic Field Needle in field experiences torque Aligns parallel to field Direction outside magnet is from N to S Cutting a magnet bar Isolated magnetic poles do not exist in nature Magnetizing Iron Magnetic domains Electric charges moving in a magnetic field Force experienced by positive charge Force is not in direction of magnetic field! Right hand rule Examples: Aurora Borealis (Northern Lights) Van Allen radiation belts Old CRT computer monitors and TV screens

4 Chapter 17 Electromagnetic Interactions Electric and magnetic phenomena are related Electromagnetic phenomena Batteries: Convert potential energy of electric charges to kinetic energy, when charges are allowed to move (e.g., move through a metal wire) Flow of charge is electric current or current Unit of current Ampere (A) = 1Coulomb/second Relation between electricity and magnetism: A. An electric current produces magnetic field Direction of magnetic field is not parallel to current Direction of magnetic field is determined by right hand rule Examples: Current through a straight wire coiled wire Electric motors Current carrying coil in a magnetic field B. A changing magnetic field produces an electric field Law of electromagnetic induction Magnetic field through loop (flux) Electric field is induced when flux changes Electric field can be detected by electric current in a wire

5 How to change flux: move loop in-out of field move field in-out of loop change magnetic field strength Faraday s law: Induced field in a coil is determined by product of # turns in loop, and rate of change of flux Lenz's law The direction of the induced current is such as to oppose the change that produces the current! Generators Transformers AC only Step-up (more turns in secondary) Step-down (more turns in primary) Summary Electric charges produce electric fields There are no magnetic monopoles An AC electric field is created whenever a magnetic field is changing An AC magnetic field is created whenever an electric filed is changing (Fields are at right angles to each other and The faster the change, the stronger the field produced) Maxwell's equations

6 Chapter 18 Electric Circuits Electric current I Measured in Amps (A) 1 A = 1 C /s To cause electric charges to move (gain kinetic energy) need to have a potential energy change (analog of ball rolling down inclined plane!) Potential energy change is provided by source (battery) Potential difference between end points of battery is called Voltage Voltage is measured in Volts (V) (acts as the pump in circuit) example is a battery Energy gained by charge q moving between 2 point of voltage V is q V (measured in J) Therefore, 1C 1V = 1J or 1 V = 1 J/ C Charges lose their energy by collisions (resistance) Voltage is required to overcome resistance to flow Ohms law: Current through conductor = Voltage across ends/resistance I = V / R Resistance in units of Ohms (Ω) = Volt/Ampere

7 Alternative form V = I R Electric Shock Motion of charges is what matters Body s resistance (100-1,000,000 Ω) Current through body = Voltage/50,000 Ω If voltage = 50 V, current = 0.001A If voltage = 500 V, current = 0.01 A (causes muscle spasm) Power: P = V I P measured in Watts (= J/s) Power loss (as heat) P = I V = V 2 / R or P = I V = I 2 R Direct Current (DC) and Alternating Current (AC) Frequency (60 Hz in USA) AC can be transformed using step-up or step-down transformers I s = (N p /N s ) I p V s = (N s /N p ) V p

8 N is number of turns in p(primary) or s(secondary) coil V is voltage across p (primary) or s (secondary) coil, and I is current through p (primary) or s (secondary) coil Series Connections (same current) Parallel Connections (current splits)

9 Chapter 19 Electromagnetic Spectrum From Maxwell's equations changing electric field causes magnetic field and changing magnetic field electric causes field Disturbance of electromagnetic (EM) fields travel together EM waves travel at speed of light c = 300,000 km/s or c = m/s Visible light is one kind of EM wave For waves speed = wavelength x frequency For EM waves c = λ f For visible light λ is between 400 and 700 nm ( 1 nm = 10-9 m)

10

11 Types of EM waves Radio Microwave Infrared (IR) Visible Ultraviolet (UV) X-rays Gamma Rays

12 Chapter 20 Optics Interaction of light with matter Light can be scattered (reflected) by matter Light can be absorbed by matter, and Light can be transmit through Scattering and Reflection When light hits the interface between 2 different materials (e.g., air-glass) If the interface is rough, we speak of diffuse scattering If the interface is smooth we speak of reflection Law of reflection: angle of incidence is equal to angle of reflection Application to mirrors plane mirrors concave mirrors convex mirrors What makes 2 media different? speed of light n material speed of light in matter is always less than c e.g., in glass it is about 200,000 km/s Index of refraction is the ratio: c divided by speed in material n = c / v m

13 E.g., for glass n = 1.5 Refraction When light transmits through the interface of 2 transparent materials the direction changes Light bends toward normal of interface when going from low to high n Light bends away from normal of interface when going from high to low n Large change in n between the 2 media, result in more bending The speed of light in the medium depends on wavelength (this is called: dispersion) As the speed depends on λ, the amount of bending depends on λ, i.e., difference colors bend by different amounts! Application of refraction: lenses converging diverging Colors Perception of colors superposition of colors color mixing additive color mixing subtractive color mixing (filters)

14