PAG 06.1 Determining The Planck Constant Flashcards
Describe the IV characteristic for an LED
Light-emitting diode so current can only pass through it in one direction. Also requires a minimum voltage (threshold voltage) before current can flow
What is a threshold voltage?
The minimum potential difference that needs to be across a diode before it allows a current to flow
How can the threshold voltage be obtained experimentally?
Slowly increase the potential difference across the LED until it just begins to emit light, and current flows through it. The value at which this occurs is the threshold value
How can you determine the point at which the LED begins to glow more precisely?
The LED can be viewed through a matte black paper tube to help prevent external light interfering with your view
How could you vary the potential difference across the LED without altering the power supply?
The LED can be placed in a potential divider circuit with a rheostat. As the resistance of the rheostat decreases, the potential difference across the LED will increase
What device can be used to measure the potential difference across the LED?
A voltmeter connected in parallel to the LED will measure the potential difference across it
How can the energy of a photon be obtained from its frequency?
Energy=Planck’s Constant x frequency
E=hf
How can the energy of a photon be obtained from its wavelength?
Energy=Planck’s Constant x Speed of Light)/ Wavelength
E=hc/(lambda)
What is the minimum energy of an electron moving through a potential difference?
Energy=charge of electron x potential difference
E=eV
What equation can be formed by equating the electron energy and photon energy?
eV=hc/(lambda)
Is the wavelength of a green photon larger or smaller than the wavelength of a red photon?
Red photons have a larger wavelength than green photons. This means the red photons have a smaller energy
How can the Planck constant be determined from a graph of threshold voltage against 1/(lambda)?
The gradient of the graph will be V/(lambda)
V(lambda)=hc/e (rearranged energy balance)
Therefore the Planck constant is given by e/c multiplied by gradient
How can the percentage difference in your experimental value and accepted value by calculated?
(Your value-accepted value)/accepted value x 100%
What safety precautions should be taken when carrying out this experiment?
Ensure that the currents passing through the LEDs don’t exceed their current ratings. Avoid touching any bare metal contacts and handle the components with care since they may become hot
