All 40 questions from the Joint Admissions and Matriculation Board (JAMB) Physics 2019 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
Advertisement
A particular metal has a work function of 2.0eV. If light of wave-length 151nm falls on it, the maximum energy of the photo electron emitted is
W₀=2.0eV=1.6×10⁻¹⁹×2=3.2×10⁻¹⁹J. Energy of light E=hc/λ=(6.63×10⁻³⁴×3×10⁸)/(1.51×10⁻⁷)=1.3172×10⁻¹⁸J. Kinetic energy = E−W₀ = 1.3172×10⁻¹⁸−3.2×10⁻¹⁹ = 9.972×10⁻¹⁹J.
The efficiency of a machine is 75%. If the machine can overcome a force of 6000N with another force of 2000N, what is the velocity ratio of the machine?
Mechanical Advantage (M.A) = Load/Effort = 6000/2000 = 3. Efficiency = M.A/V.R × 100% ⇒ 75% = 3/V.R ×100% ⇒ V.R = 300/75 = 4.
Calculate the angle of minimum deviation when light is refracted through an equilateral triangular prism of refractive index 1.5
Angle of prism A=60°. n=sin[(A+Dm)/2]/sin(A/2) ⇒ 1.5=sin[(60+Dm)/2]/sin30° ⇒ sin[(60+Dm)/2]=0.75 ⇒ (60+Dm)/2=48.59° ⇒ Dm=(2×48.59)−60≈ 37.2°.
A force of 14N is applied to a spring of stiffness 220N/m. Calculate the energy stored in the spring
Extension e=F/K=14/220=0.06364m. Energy stored = ½Fe = ½×14×0.06364 = 0.4455J.
Two forces X and Y act on an object. The angle between the forces is 30°. Given that the magnitude of X is 15N while that of the resultant is 18N, what is Y?
Using the cosine rule with the supplementary angle (150°) between X and Y in the force triangle: 18²=15²+Y²−2(15)(Y)cos150° ⇒ Y²+25.98Y−99=0. Solving the quadratic gives Y=3.375N (taking the positive root).
For a body executing simple harmonic motion, it is true that
Using a=−ω²x and v=ω√(A²−x²): at the equilibrium position (x=0), acceleration is zero while velocity is maximum. At the extreme ends, acceleration is maximum while velocity is zero.
A little sag is left in overhead telephone wires because
A fall in temperature causes contraction of the wire (while a rise causes expansion); the sag allows room for this contraction so the wire does not snap.
Which of the following is true about electromagnetic waves?
Electromagnetic waves are transverse (not longitudinal), most cannot be seen, and they all travel at the same speed in vacuum, though they have different frequencies and wavelengths (radio, infrared, visible light, UV, X-ray, gamma ray — increasing frequency, decreasing wavelength).
The angle of incidence in the denser medium when the angle of refraction in the less dense medium is 90° is called
This defines the critical angle — the angle of incidence in the denser medium beyond which total internal reflection occurs.
A point on a stationary wave where there is no movement of the medium is called
A node is a point on a stationary wave with no movement of the medium; the antinode is the point of maximum movement.
Which of the following properties is not common to all waves?
Polarization is an exclusive property of transverse waves; longitudinal waves cannot be polarized. Reflection, refraction, and diffraction are common to all waves.
A plane progressive wave is represented by y=2sin(100πt−0.2πx) where all the symbols have their usual meanings. What is the velocity of the wave?
From 2πft=100πt: f=50Hz. From 2πx/λ=0.2πx: λ=10m. Velocity V=fλ=50×10=500m/s.
The angle between the earth's magnetic field and the horizontal is known as the angle of
The angle between the earth's magnetic field and the horizontal is the angle of dip (or inclination). (The angle between the magnetic meridian and geographic meridian is the angle of declination/variation.)
An organ pipe closed at one end is 110cm. What is the frequency of the fundamental note, assuming that the speed of sound in air is 330m/s?
For a closed pipe, fundamental frequency f₀=V/4L. L=110cm=1.10m. f₀=330/(4×1.10)=75Hz.
A tuning fork, a violin string and a loudspeaker cone are producing sounds. This is because they are all in a state of
Sound is produced when there is vibration of a material medium; all three sources are vibrating.
Two cars of equal mass moving with a velocity of 50m/s collide head-on. Calculate the velocity of each after the collision if they stick together
For a head-on collision, the cars travel in opposite directions. With equal mass x: (x×50)−(x×50)=(2x)V ⇒ 0=(2x)V ⇒ V=0.
An object weighing 200kg hangs at the end of a rope. If g=9.8m/s², what is the acceleration, when the tension in the rope is 2000N?
Resultant force = T−mg = 2000−200(9.8) = 2000−1960 = 40N. F=ma ⇒ a=40/200=0.2m/s².
An object was launched with velocity of 50m/s at an angle of 30° to the horizontal, the maximum height reached is:
Vertical component Uᵧ=50sin30°=25m/s. At maximum height V=0: 0²=25²+2(−10)Hₘₐₓ ⇒ Hₘₐₓ=625/20=31.25m.
A dynamo converts
A dynamo is a generator that converts mechanical energy into electrical energy.
The quality of sound depends on
The pitch of a sound depends on frequency; the quality/timbre depends on overtone (harmonics); the loudness depends on amplitude.
Which of the following is correct?
The resistance of metals increases with temperature (as conductivity decreases), while for semiconductors, resistance decreases with temperature (conductivity increases) as more charge carriers become available.
Which of the following is most strongly deflected by a magnetic field?
Gamma rays and X-rays are not charged and are not deflected by electric or magnetic fields. Alpha and beta particles are charged and are deflected, but beta particles are more strongly deflected since they are far lighter than alpha particles.
Three capacitors 2μF, 3μF and 6μF are connected in series. If the p.d across the system is 12v, the p.d across the 6μF capacitor is
1/C_T = 1/2+1/3+1/6 = 1 ⇒ C_T=1μF. Total charge Q_T=V×C_T=12×1×10⁻⁶=1.2×10⁻⁵C. In series, all capacitors carry the same charge, so V(6μF)=Q_T/6μF=1.2×10⁻⁵/6×10⁻⁶=2V.
The purpose of a dielectric material in a parallel plate capacitor is to
From C=Akε₀/d, introducing a dielectric (with dielectric constant k>1) between the plates increases the capacitance.
A boat travels due east with a speed of 40m/s across a river flowing due south at 30m/s. What is the resultant speed of the boat?
By Pythagoras' theorem: V_R²=40²+30²=1600+900=2500 ⇒ V_R=√2500=50m/s.
When a plane mirror with a ray of light incident on it is rotated through angle θ, the reflected ray will be rotated through
When a plane mirror is rotated through an angle (with the incident ray fixed), the reflected ray rotates through twice that angle.
In an AC circuit that contains only a capacitor, the voltage
In a purely capacitive AC circuit, the current leads the voltage by 90° — equivalently, the voltage lags the current by 90°.
If the distance between two point charges is increased by a factor of four, the magnitude of the electrostatic force between them will be
From F=Kq₁q₂/r², F∝1/r². If distance increases by 4× (r₂=4r₁): F₂=F₁r₁²/(4r₁)²=F₁/16 — reduced to one-sixteenth.
Which of the following is a set of vectors?
Vector quantities require both magnitude and direction: force, weight, velocity, acceleration and momentum. Mass, volume and density are scalar quantities.
A transformer has a primary coil with 500 turns and a secondary coil with 2500 turns. When the voltage input to the primary is 120v, the output is
Nₛ/Nₚ = Vₛ/Vₚ ⇒ Vₛ = Vₚ×(Nₛ/Nₚ) = 120×(2500/500) = 600V.
If two parallel wires carry currents flowing in the same direction, the wires will
Current-carrying wires create magnetic fields around themselves; when both currents flow in the same direction, the force between the wires is attractive (opposite directions produce a repulsive force).
In what range of temperature is the expansion of water anomalous?
The anomalous expansion of water occurs between 0°C and 4°C: heating water in this range causes it to contract (instead of expand), and cooling it causes it to expand (instead of contract).
A certain radioactive substance has a half life of 20 years. How long will it take the activity to become 1/4 of its original?
Decay constant λ=0.693/t½=0.693/20=0.034651/yr. When activity is ¼ of original: ¼=e^(−λt) ⇒ −λt=ln(¼)=−1.3863 ⇒ t=1.3863/0.034651≈40 years.
Two balls of masses 5kg and 10kg were dropped from a height at the same time. If the ball with the smaller mass reached the ground after 4s, the time taken by the bigger ball to reach the ground is
Neglecting air resistance, all objects released from the same height at the same time reach the ground at the same time, since they experience the same acceleration due to gravity, regardless of mass.
The dimensions of pressure and density are respectively
Pressure = Force/Area = (M×LT⁻²)/L² = ML⁻¹T⁻². Density = Mass/Volume = M/L³ = ML⁻³.
A conducting sphere has a net charge of -4.8×10⁻¹⁷C. What is the approximate number of excess electrons on the sphere if the fundamental natural charge is -1.6×10⁻¹⁹C?
Number of electrons = Total charge/Charge per electron = (−4.8×10⁻¹⁷)/(−1.6×10⁻¹⁹) = 300.
The process of adding impurity to a semi-conductor to increase its conductivity is called
This process is called doping; the resulting semiconductor is known as an extrinsic semiconductor.
The electromotive force induced is in such a direction as to oppose the motion or change producing it. This is a statement of
This is Lenz's law. (Faraday's law gives the magnitude of the induced e.m.f.; Lenz's law gives its direction.)
The transfer of energy from the sun to the Earth's surface is known as
Heat transfer from the sun to the earth is by radiation — the only method of heat transfer that does not require a material medium.
The instrument used for measuring the relative humidity is known as
A hygrometer measures relative humidity. (A hydrometer measures relative density; a wind vane shows wind direction; an anemometer measures wind speed.)
Advertisement
…and/or go further with premium services and No Ads.