
A microwave source produces pulses of 20.0-GHz
(a)

The wavelength of the microwave.
Answer to Problem 72P
The wavelength is 0.015 m.
Explanation of Solution
Write the expression to calculate the wavelength.
λ=cf
Here, λ is the wavelength, f is the frequency and c is the speed of light.
Conclusion:
Substitute 20.0 GHz for f and 3.00×108 m/s for c in the above equation to calculate λ.
λ=3.00×108 m/s20.0 GHz(109 Hz1 GHz)=0.015 m
Thus, the wavelength is 0.015 m.
(b)

The total energy of each pulse.
Answer to Problem 72P
The total energy is 25.0×10−6 J.
Explanation of Solution
Write the expression to calculate the total energy.
U=Pt
Here, U is the total energy, P is the power and t is the time.
Conclusion:
Substitute 25.0 kW for P and 1.00 ns for t in the above equation to calculate U.
U=(25.0 kW)(103 W1 kW)(1.00 ns)(10−9 s1 ns)=25.0×10−6 J
Thus, the total energy is 25.0×10−6 J.
(c)

The average energy density inside the pulse.
Answer to Problem 72P
The average energy density is 7.37×10−3 J/m3.
Explanation of Solution
Write the expression to calculate the average energy density.
u=Uπr2l
Here, u is the average energy density, r is the radius and l is the length of the pulse.
Write the expression to calculate the length of the pulse.
l=ct
Use the expression l=ct in the equation for u to rewrite.
u=Uπr2(ct)
Conclusion:
Substitute 25.0×10−6 J for U, 1.00 ns for t, 6.00 cm for r and 3.00×108 m/s for c in the above equation to calculate u.
u=25.0×10−6 Jπ(6.00 cm(10−2 m1 cm))2(3.00×108 m/s)(1.00 ns)(10−9 s1 ns)=7.37×10−3 J/m3
Thus, the average energy density is 7.37×10−3 J/m3.
(d)

The amplitude of electric and magnetic field.
Answer to Problem 72P
The amplitude of electric and magnetic field respectively 4.08×104 V/m and 1.36×10−4 T.
Explanation of Solution
Write the expression to calculate the magnitude of electric field.
Emax=√2uε0 (I)
Here, ε0 is the permittivity of free space and Emax is the magnitude of electric field.
Write the expression to calculate the magnitude of magnetic field.
Bmax=Emaxc (II)
Here, Bmax is the magnitude of magnetic field.
Conclusion:
Substitute 7.37×10−3 J/m3 for u and 8.85×10−12 C2/N⋅m2 for ε0 in the equation (I) to calculate Emax.
Emax=√2(7.37×10−3 J/m3)8.85×10−12 C2/N⋅m2=4.08×104 V/m
Substitute 4.08×104 V/m for Emax and 3.00×108 m/s for c in the above equation (II) to calculate Bmax.
Bmax=4.08×104 V/m3.00×108 m/s=1.36×10−4 T
Thus, the amplitude of electric and magnetic field respectively 4.08×104 V/m and 1.36×10−4 T.
(e)

The force exerted on the surface.
Answer to Problem 72P
The force exerted on the surface is 8.33×10−5 N.
Explanation of Solution
Write the expression to calculate the force exerted on the surface.
F=uA
Here, F is the force and A is the area.
Write the expression to calculate the area.
A=πr2
Use the above equation to rewrite the equation for F.
F=u(πr2)
Conclusion:
Substitute 7.37×10−3 J/m3 for u and 6.00 cm for r in the above equation to calculate F.
F=7.37×10−3 J/m3(π(6.00 cm(10−2 m1 cm))2)=8.33×10−5 N
Thus, the force exerted on the surface is 8.33×10−5 N.
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Chapter 24 Solutions
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