SAT物理练习题2
Question 1:
Five particles charged with the same charge +q move with the same speed v in a magnetic field B, as shown above. Which particle experiences the highest magnetic force?
A
B
C
D
E
Question 2:
Two wires of circular cross-sectional areas have resistances R1 and R2.
If ρ1 = 2ρ2, l1 = 2l2 and r1 = 2r2, what is the ratio R1/R2?
ρ1, ρ2 are the resistivities of the two wires,
l1, l2 are the lengths of the two wires,
r1, r2 are the radii of the two wires.
1
1/2
1/4
2
4
Question 3:
Two wires carry identical currents I, as shown above. At which point is the magnetic field the strongest?
A
B
C
D
E
Question 4:
Two metallic bars slide along metallic rails at speeds v1 = .1m/s and v2 = .2m/s as shown above. The bar and the rails are situated in a magnetic field of 4T. What is the induced voltage in the bar and rails?
20mV
30mV
40mV
120mV
150mV
Question 5:
A police car with its siren on travels at a speed v1 toward a person. The frequency of the siren is f1 and the person hears the sound at a frequency f2. If vs is the speed of sound, v1 =
vsf2/f1
vs(1 - f2/f1)
vs(f1/f2 - 1)
vs(1 - f1/f2)
vs(f2/f1 - 1)
Question 6:
Violet light has a wavelength of 400nm. What is its frequency, if the speed of light is 3·108?
5·1014
7.5·1014
1015
2·1015
5·1015
Question 7:
The activity of a radioactive sample at time t = 0s is A0 and the activity of the same radioactive sample at time t1 is A1. What is the decay constant of the sample?
[ln(A1/A0)]/t1
[ln(A0/A1)]/t1
[ln(A1 - A2)]/t1
[ln(A1/A0)]t1
e(A1/A0)t1
SAT2物理练习题参考答案与解析参见下一页
Question 1:
Five particles charged with the same charge +q move with the same speed v in a magnetic field B, as shown above. Which particle experiences the highest magnetic force?
Solution: The magnetic force experienced by a moving particle is given by the formula F = q(v x B). We can maximize F by ensuring that the speed and the magnetic field are perpendicular to one another. Particule A is the correct answer.
Question 2:
Two wires of circular cross-sectional areas have resistances R1 and R2.
If ρ1 = 2ρ2, l1 = 2l2 and r1 = 2r2, what is the ratio R1/R2?
ρ1, ρ2 are the resistivities of the two wires,
l1, l2 are the lengths of the two wires,
r1, r2 are the radii of the of the cross-sectional areas of the two wires.
Solution:
R1 = ρl11/A1 = ρ1l1/(¶r12)
R2 = ρ2l2/(¶r22) = (ρ1/2)(l1/2)/(¶r12/4) = ρ1l1/(¶r12)
R1/R2 = 1.
Question 3:
Two wires carry identical currents I, as shown above. At which point is the magnetic field the strongest?
Solution:
Points D and B are situated at equal distance from the two wires and the magnetic field is 0 at these points. As the magnetic fields created by the two currents have opposite directions at points A, C and E, the strongest magnetic field will be at point A, because the component generated by the "horizontal" current is much lower than the one created by the "vertical" one.
Question 4:
Two metallic bars slide along metallic rails at speeds v1 = .1m/s and v2 = .2m/s as shown above. The bar and the rails are situated in a magnetic field of 4T. What is the induced voltage in the bar and rails?
Solution:
e = -dΦ/dt = -B·dA/dt
dA/dt = l(dx/dt) = l(v1 + v2)
The induced voltage in the bar is B·l·(v1 + v2) = 120mV.
Question 5:
A police car with its siren on travels at a speed v1 toward a person. The frequency of the siren is f1 and the person hears the sound at a frequency f2. If vs is the speed of sound, v1 =
Solution:
The frequency heard by the person is given by the formula:
f2 = [vs/(vs - v1)]f1
f2vs - f2v1 = f1vs
v1 = vs(f1/f2 - 1)
Question 6:
Violet light has a wavelength of 400nm. What is its frequency, if the speed of light is 3·108?
Solution:
f = c/λ = 3·108/400·10-9 = 7.514Hz.
Question 7:
The activity of a radioactive sample at time t = 0s is A0 and the activity of the same radioactive sample at time t1 is A1. What is the decay constant of the sample?
Solution:
A1 = A0eλt1
A1/A0 = eλt1
λ = [ln(A1/A0)]/t1