PHYSICS 20B Lecture Notes - Lecture 7: Electric Field, Cecilia Payne-Gaposchkin, Barcode
Lecture 7
Touching the Stars
● Wavelength
○ An object’s temperature is what determines the wavelength of a light, emitted by
a glowing object.
■ A glowing object can be a star, a star emits light, that light’s wavelength is
determined by the star’s temperature
○ Hotter things glow in shorter wavelength light
■ The hotter the object, the shorter the wavelength
● Electric and Magnetic Fields
○ Electric and magnetic fields are quite similar
○ Electric charges create “fields” around them that attract or repel other charges
■ This can be similar to who magnets function, being that magnets also
create a field in which deflects the path of charged particles
○ A changing electric field creates a magnetic field, and a changing magnetic field
creates an electric field, the fields are transmitted as waves
● Electromagnetic waves
○ By wiggling a charged particle, we set up oscillations (moving back and forth) in
the electric field around it
■ Moving a charged particle will move the electric field around it
● Hot objects radiate
○ Just like with any molecules, temperature affects the molecules movement, a
hotter temperature increases its movement, and cooler temperatures decreases
and locks molecules, like water when being frozen and boiled, this explains why
hot objects would radiate
○ However, the motion of their electrons create electromagnetic waves (i.e. light)
○ If the photons and molecules are in equilibrium with each other (stable), the
result is a blackbody spectrum
● Blackbody spectrum
○ A blackbody spectrum is a spectrum owing to temperature of an object
■ Describing the temperature of objects
○ A black body is an idealized object that gives off radiation
○ A “perfect” absorber: this is when all incoming radiation is absorbed, and nothing
is reflected,
■ Like a Bounty towel soaking up all the spilled water, not leaving anything
behind, except in this case is “reflected”
○ Atomic motions within the objects cause it to emit energy in a way that depends
only on its temperature, not on chemical composition (or anything else)
■ Again, temperature is the leading factor
○ The shape of a black body spectrum depends only on the temperature of the
object
● What radiates like a blackbody?
○ Hot embers from a fire (like charcoal when barbecuing)
○ Hot lava
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Document Summary
Hotter things glow in shorter wavelength light. The hotter the object, the shorter the wavelength. Electric and magnetic fields are quite similar. Electric charges create fields around them that attract or repel other charges. This can be similar to who magnets function, being that magnets also create a field in which deflects the path of charged particles. A changing electric field creates a magnetic field, and a changing magnetic field creates an electric field, the fields are transmitted as waves. By wiggling a charged particle, we set up oscillations (moving back and forth) in the electric field around it. Moving a charged particle will move the electric field around it. An object"s temperature is what determines the wavelength of a light, emitted by a glowing object. A glowing object can be a star, a star emits light, that light"s wavelength is.
