15.7 Resonance Flashcards
What process is resonance?
one that occurs when an oscillating body is forced to oscillate by an outside source at a frequency that is equal to its own natural frequency.
What is the driving force then equal to?
Driving force = natural frequency
How can we show Resonance?
By using an experiment involving Barton’s Pendulums.
How do we use Barton’s Pendulums to show resonance?
We have a punch of pendulums of different lenghs. We make one oscillate. This makes all of them oscillate to different amplitudes. The one with the same length gives the same amplitude as the one that was shaken.
Why is one of the same length able to abosrb the most energy ?
As it has the same natural frequency.
When we approach the natural frequency what happens to the amplitudes.
They increase every half an oscillation greatly
When the energy transferred is at maximum (to the bob of the same natural frequency) what happens? as long as system isnt damped
The system is said to be resonating. The frequency is now called resonating frequency.
What set up can we use to investigate resonance?
An arrangement of a spring with masses attatched by a string to a variable frequency signal generator.
How can we use this setup to determine the resonance?
1 - we determine the natural frequency (free oscillations over 10 seconds)
2 - We set the signal generator to about half the natural frequency and then switch it on.
3 - The frequency is gradually increased until the mass vibrates with maximum amplitude - which should happen when set frequency is equal to natural frequency.
4 - The experiment should be repeated but with signal generator set to twice the natural frequency which should be decreased over time.
5 - We find the average value for resonant frequency.
— This experiment can be repeated with different masses
If the system is damped is the resonating frequency when driving frequency is equal natural frequency.
The driving frequency would be lower, this becomes lower and lower as damping increases
If from the practical above we were to plot a graph of amplitude over driving frequency, what shape graph would we get and where would resonating frequency be on it?
A mountain like graph with resonating frequency at peak amplitude
If there is damping would amplitude of the oscillation decrease?
YES
If damping is involved in the system what would happen to the mountain like graph ?
The peak would decrease and the graph broaden.
Where is resonance often used?
musical instruments
e. g. recorder to change note
e. g.tune radios
Example of system where damping takes place and we can see a frequency decrese?
A pendulum in water, it is definitely going to travel slower and hence longer to swing
Do heavier damped systems have a flatter response to oscillations?
Yes, amplitude increases at a lower rate near natural frequency and in general are not as sensitive as less damped systems
Near natural frequncy what happens to little damped systems?
Their amplitude of oscillations increases greatly.
Why is damping important in machine and building designs?
Unwanted vibrations could build up to large amplitudes an hence damages
What type of materials are often used to absorb a lot of energy from vibrations making them die off.
Ductile materials, ones which can deformed plastically without fracture. These work by hysterisis
How do we use resonance tuning the radio?
We alter the capacitance of a variable capacitor in the tuning circuit. When the natural oscillations in the tuning circuit matches the frequnecy of incoming radio signals, resonance occurs absorbing energy strongly from the EM waves.
Is resonance used in microwaves?
Yes, the oscillating electric field must have a frequency close to the natrual frequency of the vibrations of the water molecules. This would mean energy is absorbed strongly from the electric field.
What process does the microwave essentially use to heat up food?
Frictional heating
What does MRI stand for?
magnetic resonance imaging
How does an MRI scanner work?
A strong magnetic field is used to to exert a force on the protons within the hydrogen atoms of patient’s body. Normally the protons move in random directions, but now they’re aligned with the magnetic field. Now radio waves are sent and energy from these waves are absorbed by the protons (by resonance) and are consequently knocked out of alignment. When the emission of radio waves is stopped the protons return to original alignment by magnetic field and release radio signals, which can be detected outside the body.`
How can we make really strong magnetic fields?
superconductivity
