Redefining the Kilogram Flashcards
Q7: What was the goal of the Avogadro Project?
A7: The goal was to use a silicon sphere to determine Avogadro’s number (the number of atoms in a mole) to redefine the kilogram.
Q8: What challenge did the Avogadro Project face?
A8: The challenge was achieving sufficient accuracy in counting atoms and making a perfect spherical shape.
Q9: What is Planck’s constant, and how does it relate to redefining the kilogram?
A9: Planck’s constant relates the energy of a photon to its frequency. It is used to redefine the kilogram by providing a universal constant not dependent on a physical object.
Q10: How was the kilogram redefined in 2011?
A10: The kilogram was redefined based on a fixed value of Planck’s constant.
Q11: What complementary methods were used to achieve the kilogram’s new definition?
A11: The silicon sphere approach (determining Avogadro’s number) and the watt balance approach (measuring Planck’s constant) were used.
Q12: Why is the new definition of the kilogram considered an improvement?
A12: It is based on universal constants rather than a physical object, providing greater stability and precision.
Q13: How is the new definition of the kilogram similar to the redefinition of the meter?
A13: Both definitions are based on fixing a fundamental physical constant: the speed of light for the meter and Planck’s constant for the kilogram.
Q14: What is the value of Planck’s constant used for the kilogram definition?
A14: The value of Planck’s constant is
h = 6.62607015 × 10−34 kg m2 s
Q: What is the primary purpose of the Kibble balance?
A: The Kibble balance is used to define the kilogram based on fundamental physical constants, specifically Planck’s constant, rather than a physical artifact.
Q: How does the Kibble balance differ from traditional balances?
A: Traditional balances measure mass by comparing the gravitational force on an object to known weights, whereas the Kibble balance measures mass by balancing electromagnetic forces against gravitational forces.
Q: Describe the two operational modes of the Kibble balance.
A:
Weighing Mode: Measures the mass by equating the gravitational force on a kilogram mass to the electromagnetic force generated by a current passing through a coil in a magnetic field.
Velocity Mode: Measures the voltage induced by moving the coil in the magnetic field, which allows for precise calculation of the magnetic field strength and length of the wire in the coil.
Q: What equation is used to balance the gravitational and electromagnetic forces in the Kibble balance?
A: The equation is
𝑚⋅𝑔=𝐵⋅𝐿⋅𝐼
where 𝑚 is mass,
𝑔 is gravitational acceleration,
𝐵 is magnetic field strength,
𝐿 is length of wire,
and 𝐼 is current.
Q: How does the Kibble balance address the challenge of measuring magnetic field strength and coil length?
A: By using the velocity mode to induce voltage, the Kibble balance creates two equations which can be solved to eliminate the variables for magnetic field strength and coil length.
Q: What is the role of Josephson junctions in the Kibble balance?
A: Josephson junctions measure voltage very accurately and are used to balance the induced voltage in the coil by providing a precise voltage reference based on microwave radiation.
Q: How is current indirectly measured in the Kibble balance system?
A: Current is measured indirectly by measuring the voltage across a known resistor and using the relationship
𝐼=V/R
where 𝑅 is the resistance.
Q: What technique is used to measure resistance in the Kibble balance system?
A: The quantum Hall effect is used to measure resistance with high precision.
Q: How did the definition of the kilogram change in 2019?
A: The kilogram is now defined by a fixed value of Planck’s constant, rather than a physical artifact, based on precise measurements using the Kibble balance.
Q: What is the significance of the equation
h= 4⋅π ^2 ⋅m⋅g⋅v /f ^2 n
the context of the Kibble balance?
A: This equation relates Planck’s constant (h) to mass (m), gravitational acceleration (g), velocity (v), and frequency (f), and is used to calculate the kilogram based on these fundamental measurements.
Q: How is the local gravitational acceleration measured for use in the Kibble balance?
A: A gravimeter is used to measure the local gravitational acceleration by dropping a corner reflector down a vacuum tube and measuring its acceleration via interferometry.
Q: Why is precision in measuring velocity critical for the Kibble balance?
A: Precise measurement of velocity is crucial because it directly affects the accuracy of the induced voltage measurement, which is used to determine magnetic field strength and length of the wire.
Q: What is the function of the silicon sphere in the context of measuring fundamental constants?
A: The silicon sphere is used for counting atoms with high precision, helping to define constants such as Avogadro’s number, but it does not directly define the kilogram.
Q: How does the definition of the mole differ from other SI units?
A: The mole is defined as the amount of substance containing exactly
6.022×10 ^23
elementary entities, which can be atoms, molecules, or particles, and is not based on a physical object but a number.
Q: How does the redefinition of the kilogram impact the SI system of units?
A: The redefinition ensures that the kilogram is based on a fundamental physical constant, Planck’s constant, making the unit more stable and reproducible, independent of physical objects.
