P1.4 Flashcards
What is Ultrasound
why is it useful?
sound with frequencies higher than 20,000hz
boats and submarines use sonar to detect things in water. they emit waves of ultrasound which reflect off things like other boats and detect them as they arrive back. computes use the time delay between the waves being sent and returning to work out how far away other objects are
animals such as bats and dolphins use it to sense their way around their environment
animals also use it to communicate with each other
What happens to ultrasound at a boundary between medium?
How is this useful?
wave passes from one medium to another , some reflected off boundary between and some is transmitted and refracted (partial reflection)
you can point a pulse of ultrasound at an object and wherever there is a boundary between substances, some is reflected back
the time the reflection takes to reach the detector can be used to measure how far way the boundary is
What is infrasound
why/how is it used?
sound with frequencies below 20hz
long wavelengths so travels long distances and can diffract around objects easily
elephants use to communicate
tigers use in growls and roars so they can be heard by rials/mates far away
it can be used to monitor animal movements in remote locations since some microphones are sensitive enough to detect it
it is produced when there is meteor strikes or volcanic eruptions so can be used to detect them
How do you calculate speed?
speed=distanced divided by time
FACTOR OF 2 - there and back so divide by 2
can be used to find the distance to and fro a reflecting surface if you know the speed of the wave and how long it takes to travel thelre and back
How is the earth structured?
very thin crust - bit we live on, about 20km
below the crust is mantle - has all properties of a solid but can flow slowly. where radioactive decay happens- produces heat and so mantle flows in convection currents
core at the centre - made of iron and nickel. inner core solid and outer core liquid
What are tectonic plates?
large piece of cracked crust and upper mantle
float on mantle
move due to convection currents in the mantle causing them to drift
most are moving a few cm per year
How do earthquakes happen?
tectonic plates may slide past each other at plate boundaries, can cause earthquakes due to friction
What practical shows the unpredictability of earthquakes?
fix sandpaper to surface of a lab bench so it can’t be moved
put a brick at one end on top of sandpaper
(represents the tectonic plates)
attach an elastic cord to the brick and join it to some string
attach a mas holder to the end of the string and hang it over a pulley wheel clamped to the lab bench
gradually add masses to the holder, the force of them pulling the brick represents the forces of convection currents in the mantle
as you continue to add masses to the holder, the brick will eventually slide to the right. this represents the earthquake
you can’t predict how many mass holders it will take to make the brick slip- like you can’t predict when/where an earthquake will happen. if you repeat the experiment, may be different each time
How can you try and predict an earthquake?
Why is this method unreliable?
probabilities based on previous occurrences
e.g if a city had earthquakes at regular intervals over the last century, the pattern may continue
isn’t 100% accurate
e.g you may not get an earthquake at the exact location/time predicted
gives you time to prepare just in case
What are seismic waves
How are they detected and used?
two types - p and s waves
produced when there’s an earthquake
detected over the surface of a planet using a seismometer
seismologists work out the time it takes for shock waves to reach the seismometer and note which parts of the Earth don’t have them
What are the differences between p and s waves?
p longitudinal and s transverse
p travel through solids and liquids, s only soids
p travel faster than s
What happens to p and s waves at boundaries?
How is this useful?
when they reach a boundary, some is reflected
as properties of mantle and core change, they change speed - causes them to change direction (refraction)
mostly change speed gradually so path curved. if suden, path has a kink
by observing reflection and refraction of these waves, scientists can work out where properties of the Earth change dramatically - gives them a better understanding of the Earth’s internal structure
How do seismometers work?
readings called seismograms can be used to work out the distance to an earthquake’s epicentre
p and s waves travel at different speeds so there is two tremors on a seismogram - p first then s
using the time difference between the two arriving at the seismometer, you can work out how far away the earthquake was
How can you work out where an earthquake’s epicentre is
you can draw a circle on a map centred on the location of the seismometer. the radius of this circle will be the distance calculated previously. called a distance arc
three or more distance arcs will cross at one point - the epicentre of the earthquake. called triangulation
