Answer:
Q = 4 Q₀
Explanation:
This is an exercise on capacitors, where the capacitance is
C =
if we apply the given conditions
C = \epsilon_{o} \ \frac{2A}{0.5d}
C = 4 \epsilon_{o} \ \frac{A}{d}
let's call the capacitance Co with the initial values
C₀ = \epsilon_{o} \ \frac{A}{d}
C = 4 C₀
The charge on each plate of a capacitor is
Q = C ΔV
If the potential difference is maintained, the new charge is
Q = 4 C₀ ΔV
let's call
Q₀ = C₀ ΔV
we substitute
Q = 4 Q₀
A radar receiver indicates that a pulse return as an echo in 20 μs after it was sent. The reflecting object would be 3000 m away .
Phenomenon of hearing back our own sound is called an echo. It is due to successive reflection of sound waves from the surfaces or obstacles of large size. To hear an echo, there must be a time gap of 0.1 second in original sound and the reflected sound.
Given
time = 20 μs = 20 * s
let distance to the reflecting surface be = x
total distance travelled by pulse will be = 2x
speed = 3.0 × m/s
distance = speed * time
2x = 3.0 × * 20 *
x = 3000 m
The reflecting object would be 3000 m away
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a) Zinc (work function: 4.3 eV)
The equation for the photoelectric effect is:
(1)
where
is the energy of the incident photon, with
h = Planck constant
c = speed of light
= wavelength
= work function of the metal
K = maximum kinetic energy of the photoelectrons emitted
The stopping potential (V) is the potential needed to stop the photoelectrons with maximum kinetic energy: so, the corresponding electric potential energy must be equal to the maximum kinetic energy,
So we can rewrite (1) as
where we have:
V = 1.93 V
e is the electron charge
First of all, let's find the energy of the incident photon:
Converting into electronvolts,
And now we can solve eq.(1) to find the work function of the metal:
so, the metal is most likely zinc, which has a work function of 4.3 eV.
b) The stopping potential is still 1.93 V
Explanation:
The intensity of the incident light is proportional to the number of photons hitting the surface of the metal. However, the energy of the photons depends only on their frequency, so it does not depend on the intensity of the light. This means that the term E in eq.(1) does not change.
Moreover, the work function of the metal is also constant, since it depends only on the properties of the material: so is also constant in the equation. As a result, the term (eV) must also be constant, and therefore V, the stopping potential, is constant as well.