Final Exam Flashcards
What are the two types of magnets?
The two types of magnets are permanent magnets and electromagnets (currents in wires).
What is the general direction and shape of magnetic fields?
Magnets always have a north and a south pole and the magnetic field points from the North Pole to the South Pole. Magnetic field lines always form closed loops.
What contributions to magnetism were made by (a) Oersted, and (b) Ampere?
(a) Oersted first observed that a compass needle was deflected by a current in a wire. (b) Ampere discovered the mathematical formula that relates the current in a wire to the magnetic field that it produces.
What type of materials are permanent magnets made from?
Permanent magnets are formed from a class of materials known as ferromagnetic.
(a) What is the shape of the earth’s magnetic field? (b) Why does a compass point north?
(a) The earth’s magnetic field points in the direction from the south geographic pole to the north geographic pole. Thus in the northern hemisphere the earth’s magnetic field points downward and at an angle to the ground. (b) The north pole of the compass points to the earths geographic north because the geographic north pole of the earth is the south magnetic pole.
What are the van Allen radiation belts?
two regions surrounding the earth containing a large accumulation of charged particles. These particles come to the earth from the sun and get trapped by the earth’s magnetic field.
What is the effect of a magnetic field on a charged particle?
A magnetic field produces a deflection on a moving charged particle. Charged particles revolve around magnetic field lines in spiral orbits.
What role do magnetic forces play in an electric motor?
An electric motor consists of a coil of wire located inside a permanent magnet. When current flows in the coil the magnetic forces on the current produce a net torque that cause the coil to rotate – this rotation is the mechanical action of a motor.
What is the shape of the magnetic field produced by a current in a long straight wire?
The magnetic field lines of the current in a long straight wire are a series of concentric circles surrounding the wire. The magnetic field is stronger near the wire.
Why do magnets “stick” to certain materials which are not magnets?
Permanent magnets can stick to materials that have properties that allow them to be magnetized when a permanent magnet is brought close to them. These materials have internal microscopic magnets that are usually not aligned to form a permanent magnetic field. However, when a magnetic field is present, the microscopic magnets are temporarily aligned and thus exhibit overall magnetic properties. Thus magnets stick to objects made of steel (like refrigerators). Some materials like copper, aluminum and non‐conductors are not magnetized when a permanent magnet is brought close to them
What is the ultimate source of magnetism in electromagnets and permanent magnets?
The ultimate source of the magnetic field is current. When these currents flow in wires and coils they are called electromagnets. The origin of permanent magnetism is atomic currents due to electron spin and orbital motion in certain materials.
Are neutral particles affected by magnetic fields?
Only moving charged particles can experience magnetic forces. Neutral particles are not affected by magnetic fields whether they are moving or at rest.
Due to electron’s spin-unmagenetized- cancel out e-, random magnetized-lined up e-
Permanent magnetism
What is electromagnetic induction? Who discovered it?
Electromagnetic induction in a phenomena in which current in induced in a coil if the magnetic field that penetrates through the coil changes in any manner.
Why do transformers not work with DC?
Transformers are an application of the principle of electromagnetic induction, and therefore only work if the current in the primary changes in time.
What was Maxwell’s contribution to the laws of electricity and magnetism?
Maxwell theorized that a changing electric field would generate a magnetic field.
What were Maxwell’s and Hertz’s contributions to electromagnetic waves?
Maxwell predicted theoretically on the basis of the laws of electricity and magnetism that electromagnetic waves should exist. Hertz first demonstrated experimentally the existence of electromagnetic waves.
What is an electromagnetic wave?
An electromagnetic wave is a particular combination of time‐varying electric and magnetic fields that propagate through space at the speed of light.
What are the relative orientations of the electric and magnetic fields of an electromagnetic wave?
In an electromagnetic wave, the electric and magnetic fields are perpendicular to each other and to the direction of propagation of the wave.
What is the relation between the wavelength and frequency of an electromagnetic wave?
(lambda) f =c.
What is a typical wavelength and frequency of a microwave?
Microwaves have typical wavelengths of approximately centimeters. The corresponding
frequency of a 1 cm microwave = (3x108 m/s) / 0.01 m = 3x1010 Hz.
The frequency of an AM radio station is 850 kHz. Why is it possible to pick up this station anywhere between 845 and 855 kHz?
Broadcast transmitters produce electromagnetic waves over a certain bandwidth or range of frequencies.
Why are microwavable means not packaged in aluminum containers?
Microwaves do not penetrate through conductors but are reflected from them. The aluminum container will prevent the microwaves from penetrating through and cooking the food.
WORK Most cellular phones transmit and receive at 1.8 GHz (1 GHz = 10^9 Hz). What is the wavelength of this wave?
(lambda) c/f =3x108 /1.8x10^9 =0.17m=17cm.
WORK The green laser pointer used in class has a frequency of 5.645 x 1014 Hz. What is the wavelength?
(lambda) c/f =3x108 /5.64x1014 =5.32x107 m=532nm.
WORK What is the frequency of an x‐ray that has a wavelength of 1 nm (nm = 109 m)?
f =c/(lambda) =3x108 /1x109 =3x1017 Hz.
What is alternating of AC electrical power?
With alternating current AC power the current reverses direction 60 times per sec.
How is AC power produced?
AC power is produced in an electrical generator which consists of a coil of wire that is rotated in a magnetic field.
What are the advantages of AC vs. DC power?
Voltages in an AC power system can be changed using transformers, and AC plants can be located far away from the cities they supply.
What energy sources are used to produce AC power?
Steam produced in a fossil fuel burning plant or nuclear power plant, hydroelectric power, and wind power.
What is the role of transformers in the electric power grid?
Transformers are used to step‐up the voltages coming from the power plant for transmission and then stepped‐down for distribution to homes.
What is a Ground Fault Circuit Interrupter (GFCI)?
A GFCI is an electrical safety device that detects small amounts of current flowing in the ground circuit. Its purpose is to prevent electric shocks.
Why is AC power transmitted at high voltages?
The power transmitted P is the product of current and voltage. If large currents are transmitted this requires that the transmission lines use large diameter conductors to minimize losses. Power is more efficiently transmitted at high voltage and low currents.
When you pay your electricity bill, do you pay for power used or energy?
Your electricity bill is the cost for the total amount of electrical energy used over a certain period.
Why is it advantageous to replace conventional incandescent lights with compact fluorescent lights?
Incandescent bulbs produce a lot of heat compared to the light energy. Compact fluorescent lights produce less heat and are more efficient and produce the same light output fopr considerably less electrical power.
WORK A light bulb operates at 2 A on a 100 V power line. What would it cost to operate this light bulb for an entire week in South Dakota, where electricity costs are 6 cents/KWH, and in Hawaii, where electricity costs are 17 cents/ KWH?
The number of kWh used = the power in kW x time in hours.
The power P = IV = 2 A x 100V = 200 W = 0.2 kW.
So # kWh = 0.2 x (7 days x 24 hr/day) = 33.6 kWh
Cost = # kWh x $/kWh
(a) SD: cost = 33.6 kWh x $0.06/kWh = $2.02
(b) HI: cost = 33.6 kWh x $0.17/kWh = $5.71
Current flows in same direction-> batteries
DC
Reverse direction periodically-> Power Company
AC
How does a battery charge
convert AC-> DC
Mainly, why use AC?
Can step up or down voltage/easier to generate
Electric Outlet formulas; P=IV; W=AV; 1 Watt=x Joules/sec; How much is 1 Kilowatt?
Power=Current (Voltage) OR P=IV; Watts=Amps x Volts W=AV; 1 watt=1J/s; Kilowatt=1000W
Due to electron’s spin-Unmagnetized-cancel out e-, random magnetized-lined up e-
Permanent Magnetism
WORK Astronomers express very long distances in light years (LY), which is the distance that light travels in one year. The nearest star system to our solar system is Alpha Centauri which is 4.37 LY from our Sun. How far in meters is Alpha Centauri to the Sun.
In 1 LY, light travels a distance in meters = ct, where t is the number of seconds in one year:
1 LY = ct (3X10^8 m/ s) X (365days X 24hr/ day X 60min/ hr X 60s min)
= (3X10^8 m s) X (3.15 X 10^7 s)= 9.46 X 10^15 m
so 4.7 LY = 4.7 X 9.46 X 10^15 m= 4.45 X 10^16 m
What is the index of refraction?
The index of refraction n is the ratio of the speed of light in vacuum to the speed of light in a medium: n = c/v.
(a) What is the range of wavelengths of visible light? (b) Where do UV wavelengths lie relative to visible light? (c) Where do the IR wavelengths lie relative to visible light?
(a) the wavelengths of visible light range from approximately 400 nm to 700 nm. (b) UV wavelengths smaller than 400 nm, and (c) IR wavelengths are longer than 700 nm.
What is the phenomenon of refraction and how is it characterized by the index of refraction?
Refraction is the bending of a light ray as it enters the boundary between two materials. The amount of bending depends on the indices of refraction of the two media.
(a) How is a ray of light bent when it goes from air into glass? (b) How is a ray of light bent when it goes from water into air?
(a) A ray propagating from air into glass is bent toward the normal. (b) A ray foing from water into air is bent away from the normal. The normal is the imaginary line that is perpendicular to the boundary surface.
What is total internal reflection? What is a practical application of this phenomenon?
Total internal reflection is an optical phenomenon that occurs when a light ray starting in a medium of index of refraction n1 is reflected at the boundary of another medium with index of refraction n2 < n1. This only occurs if the angle of incidence is greater than some critical value that depends on the materials. Fiber optics communications is based on this phenomenon.
What should have been the distance between Galileo and his assistant Massimo so that he would have measured a time delay of 20 seconds between when he sent his light signal and received Massimo’s light signal back? Is this feasible?
Suppose Galileo and Massimo are separated by a distance D. The total distance that the lightbeamtravelsis2D.So2D=ct=3x108 m/sx20s=6x109 mD=3x109 m.(Thisis larger than the diameter of the earth!)
WORK Calculate the speed of light in: (a) glass with index of refraction 1.5 (b) beer with index of refraction 1.345 (c) diamond with index of refraction 2.417
a) v=c/n =(3x10^8 m/s)/1.5=2x10^8 m/s
(b) v = c/n = ( 3x10^8 m/s) / 1.345 = 2.23x10^8 m/s
(c) v = c/n = ( 3x10^8 m/s) / 2.417 = 1.24x10^8 m/s
Calculate the frequencies for the following colors of visible light:
(a) red,=660nm (b) green,=550nm (c) violet,=420nm (1 nm = 1 nanometer = 10–9 m)
(a) f = c/lambda = ( 3x10^8 m/s )/ 6.60x10^‐7 m = 4.55x10^14 Hz
(b) f = c/lambda = ( 3x10^8 m/s )/ 5.50x10^‐7 m = 5.45x10^14 Hz
(c) f = c/lambda = ( 3x10^8 m/s )/ 4.20x10^‐7 m = 7.14x10^14 Hz
Light rays enter two slabs of glass at the same angle of incidence. One piece of glass has an index of refraction n1 = 1.4, while the other has an index of refraction n2 = 1.6. The diagrams below show the refracted (bent) rays for these slabs of glass. Which diagram corresponds to which piece of glass?
The light ray is bent more in material (a) than in material (b), so (a) is the material with the higher index of refraction.
What is the phenomenon of dispersion?
Dispersion is the name for the fact that the index of refraction of a material depends on the wavelength of light. Dispersion is observed when a beam of white light enters a prism and emerges as multiple beams of different colors. It is also the phenomenon that causes the formation of rainbows due to the dispersion of sunlight from water droplets in the atmosphere.
What phenomenon is the cause of blue skies and red sunsets?
Blue skies and red sunsets are the result of the scattering of sunlight from air molecules. The scattering depends strongly on wavelength.
