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Chapter 8 Light & Electromagnetism

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1 Chapter 8 Light & Electromagnetism

2 8.1 Waves: Something Else that travels
Waves: A disturbance that travels through a medium and transfers energy without transferring matter. Medium: material through which a disturbance travels

3 In quantitative terms:
Wavelength (λ): Distance between corresponding points on a wave pattern. Crest to crest or trough to trough Frequency (ƒ): Number of vibrations/waves per second ƒ=1/T [1/s- HZ] *Hertz (HZ)- Unit for ƒ Amplitude: Width of a disturbance; distance of maximum height/distance Wave Speed: Speed at which the disturbance moves through the medium Wavelength (λ), wave speed (s), and frequency (ƒ) are related: v = [m/s] = λ/t [m/s] = λƒ [m· 1/s = m/s] v = λƒ

4 8.2 Interference: A behavior unique to waves
Wave Interference: Effects that occur when 2 waves of the same type are present at the same time. Interference is constructive or destructive. Destructive interference: Equal waves of opposite directions interfere, cancelling each other out Constructive Interference: Equal waves of the same orientation interfere, reinforcing each other Before During After

5 8.3 Light: A particle or wave?
1801→Thomas Young gives evidence that light is a wave Double-Slit Expt. →Confirms wave theory of light →Light is a wave but has no medium

6 8.4 Electricity: Part of Electromagnetic Force
Light is electromagnetic wave, traveling through electromagnetic force Electric Force: Electric part of electromagnetic force Electric Force Law: Electrically charged objects exert forces on each other, even when they are some distance apart. Objects may be charged positively or negatively. →Like charges repel →Unlike charges attract

7 8.5 Magnetism: The Other Part
Magnet’s ends are North and South magnetic poles Like → Repel Unlike → Attract Magnetism of a bar magnet is permanent Magnetic Force Law: Charged objects that are moving exert and feel an additional force beyond the electric force that exists when they are at rest. This force is magnetic. →All magnetic forces are caused by the motion of charged Objects Electromagnetic Force: The total (Electric and Magnetic) force between charges

8 8.6 The Electric Atom Planetary Model Atom is → almost entirely empty
→ Divisible → made up of many parts Nucleus→ tiny center, made up of protons and neutrons, surrounded by electrons Electrons e- = -1.6 x 10-19C me= 9.11 x 10-31kg Protons e+= +1.6 x 10-19C mp= 1.67 x 10-27kg Neutrons No Charge mn= 1.67 x 10-27kg Excess of Electrons → Negative Charge Deficit “ “ → Positive Charge

9 8.8 Force Fields: Disturbance of Space
Gravitational Field: Exists throughout any region of space where an object would feel a gravitational force if an object were placed there. → “Possibility” of a g-force Electromagnetic Field: Exists everywhere any other charged object would (if it were present) feel EM forces → “Possibility” of EM-force Electric Field: Wherever any charged object would feel an electric force Magnetic Field: Exists wherever any moving charged object would feel a magnetic force Electric Field Lines → Point outward from +q’s → “ inward from – q’s

10 8.8 Force Fields +q -q # of lines indicates amount of charge
E-Field strength is greater where lines are closer together Direction of E-field lines shows direction of force on a +q

11 8.8 Force Fields Maxwell’s Equations
Charges produce E-Fields (Gauss’s law) Moving charges produce M-fields (law for magnetism) Changing M-fields produce changing E-fields (Faraday’s Law) Changing E-fields produce changing M-field Electric Force Law-In terms of fields: An E-field surrounds every charged object and charged objects feel forces due to the E-fields of other objects Magnetic Force Law-in terms of fields An M-field surrounds every moving charged object and charged objects feel forces due to the M-field of other moving charges

12 Concept Quiz 1. Which of the following is a true wave?
(a) A row of falling dominoes (b) Ripples on the surface of a pond, extending outward from a pebble dropped in the water (c) A large water wave coming into the beach, one that surfers can ride on (d) Water rushing downstream (a) and (c)

13 2. Which wave has the larger or “higher” frequency, and which carries more energy?
The top wave has higher frequency and carries more energy The top wave has higher frequency but the bottom wave carries more energy The bottom wave has higher frequency and carries more energy The bottom wave has higher frequency but the top wave carries more energy (c)

14 3. Give an example of (a) a one-dimensional medium, (b) a two-dimensional medium, (c) a three-dimensional medium. (a) Undisturbed Rope (b) Surface of a lake (c) Space in a room 4. Describe the experimental evidence supporting that claim that light is a wave. In Young’s double-slit experiment, a single light source was sent through two very small and narrow parallel slits, which acted as two light sources with synchronized vibrations. Young demonstrated the wave nature of light by the interference pattern that was produced on the viewing screen

15 5. How do we know that the medium for light waves is not air?
We know that light does not use air as its medium for travel because we receive light from the sun and there is no air in space between Earth and the sun. 6. Give two reasons that the force between bar magnets cannot be the electric force. (1) bar magnets do not interact with charged objects and (2) bar magnets always have a pair of poles, both north and south; whereas, charged objects are either only positively or negatively charged. Not both types of charge on each end. 7. What is meant by an electric current? An electric current is a flow of charged particles.

16 8. Name and briefly describe the three kinds of subatomic particles found in atoms. The three subatomic particles found in atoms are the proton, neutron, and electron. The proton has a positive charge and is located in the nucleus of the atom. Neutrons have no charge and are also in the nucleus. Electrons are negatively charged and orbit the nucleus. 9. Which force fields would be felt by a stationary charged object: gravitational, electric, or magnetic? By a moving uncharged object? By a moving charged object? All three objects would feel an imperceptible gravitational force due to their mass. Both the stationary and moving charged objects would feel an electric force due to their charge. Only the moving charge would feel a magnetic force due to the magnetic properties of moving charges

17 10. What does Faraday’s law say about magnets and wires
10. What does Faraday’s law say about magnets and wires? What does it say about magnetic fields and electric fields? Faraday’s law says that when either a magnet or loop of wire is in motion with respect to the other object, a current is induced in the loop of wire. Stated in terms of fields: A changing magnetic field creates an electric field. 11. Find the wavelength of an AM radio wave whose frequency is 1000 kHz (1 kHz = 1000 Hz). 12. Two small electrically charged objects are placed a certain distance apart, where they exert an electric force of 4 N on each other. Suppose the charge on object #1 is halved. What happens to the force? What if the charge on both objects is halved? If q1 is halved the force is also halved. If both charges are halved the force is reduced to 1/4th its original value.

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