Practical 15 - Detrmination of h using LED’s

Question 1

What are we determining in this practical?

Answer 1

h (Planck’s constant)

Question 2

What are we using to determine h?

Answer 2

Light emitting diodes (LED’s)

Question 3

How do we use LED’s to determine a value for h?

Answer 3

Measure the minimum voltage, Vmin at which light is just emitted by the diode

Question 4

What’s the equation we use to determine h?

Answer 4

Vmin = hc/eλ

Question 5

What do we plot a graph of?

Answer 5

Vmin against 1/λ

Question 6

What will our graph of Vmin against 1/λ give us?

Answer 6

Straight line, gradient e/hc

Question 7

Draw and label the circuit required

Answer 7

(See notes)

Question 8

What’s the method for this practical?

Answer 8

The voltage should be varied until light is just emitted by the LED
Record the voltage that corresponds to Vmin
Replace the LED and repeat the procedure for LED’s with different wavelengths of light
Plot a graph of Vmin (x-axis) against 1/λ (y-axis) and use it to determine a value for h

Question 9

What are the calculations for determining a value for h?

Answer 9

Gradient = 1/λ/V
Vmin = hc/eλ
h = Vmin x eλ/c
V/1/λ x e/c
1/gradient x e/c
1/gradient x 5.33x10^-28
h = 5.33x10^-28/gradient

Question 10

Where does the equation used in this practical originate from and what is it?

Answer 10

Vmin = hc
—
e λ

Energy of photons emitted by the LED:
E = Vmin x e

Another equation for calculating photon energy:
E = hc/λ

Combining them gives us the equation