PHY 1020: Solution to 3rd exam

PHYSICS 1020

SOLUTION TO THIRD EXAM
3 March 1997
(Correct solution is indicated by ``(*)'')

Conservation of energy is related to invariance of physical laws under ``translation in time''.

(a)(*) True (b) False
The work done by a force acting on an object depends on the angle between the force and the displacement of the object under the influence of the force.

(a)(*) True (b) False
The effects of friction and air resistance are examples of instances in which total energy is not conserved.

(a) True (b)(*) False
If net work is done on an object, its kinetic energy must increase.

(a) True (b)(*) False
Power and energy have the same units.

(a) True (b)(*) False
The higher the temperature of a substance, the smaller is the average kinetic energy of random motion of the molecules.

(a) True (b)(*) False
Ice (frozen water) at C is less dense than water at C.

(a)(*) True (b) False
Sound waves are electromagnetic waves of very long wavelength.

(a) True (b)(*) False
During the course of this century, engines and motors have become more and more efficient. Further improvements in design methods and better materials will make it possible to build engines which are 100% efficient.

(a) True (b)(*) False
To make a bar magnet, you have to take a piece of iron and add magnetic north poles on one end and magnetic south poles at the other end.

(a) True (b)(*) False
Overall, the total entropy of the universe keeps increasing, but some systems (e.g. living things and beings) decrease their entropy, at the cost of increasing the entropy of the rest of the universe.

(a)(*) True (b) False
The speed of light in vacuum is 300 000 km/s, but in very dense materials, due to the strong electric fields caused by the many protons and electrons, light can travel much faster than this.

(a) True (b)(*) False
The speed of light in vacuum is 300 000 km/s, but very massive objects can go much faster if sufficiently strong forces act on them.

(a) True (b(*) False
The observed frequency of light depends on the relative motion of light source and observer.

(a)(*) True (b) False
Time is measured by counting the number of occurrences of periodic events.

(a)(*) True (b) False
From the point of view of physics, in which of the following situations has work been done?
1. You try to lift a tree blocking your driveway, but it doesn't budge.
2. (*) Your brother throws his dirty clothes into the corner of his room.
3. Your mother holds a sleeping baby.
4. You work hard studying for your physics exam.
5. You spend many hours standing in line for a ticket to the football game.
You push your physics book across the table. The book weighs 10 N, you push with a steady 5 N force, and the force of friction against the book is 3 N. You push the book a distance of 2 m. The amount of work you have done is
1. 0 J
2. 5 J
3. (*) 10 J
4. 15 J
5. 20 J
During the motion of a swinging pendulum, a repetitive conversion of different types of energy into each other takes place. Which of these types of energy is at its maximum at the lowest point of the pendulum's swing?
1. (*) kinetic
2. gravitational potential
3. elastic
4. thermal
5. chemical
One Joule is equal to
4. (*)
An automobile travels at an of 60 km/hr on a straight level road. Which of the following is the best description of the energy transformation taking place?
1. chemical energy gravitational potential energy
2. thermal energy chemical energy
3. kinetic energy chemical energy
4. (*) chemical energy thermal energy
5. chemical energy kinetic energy
You run a 5 kW clothesdrier for 2 hours. How much electrical energy did you consume (i.e. convert into thermal energy)?
1. 10 Joules
2. 10 watt-hours
3. 10 kilowatts
4. 10000 Joules
5. (*) 10 kilowatt-hours
A temperature of 77 degrees Fahrenheit corresponds to
4. (*)
Find the true statement about heat and temperature:
1. The calorie unit used to measure heat and the Calorie unit used on food labels are exactly the same unit.
2. The larger the temperature difference between two objects, the more slowly will heat be transferred from one to the other.
3. The main reason we wear clothes is in order to reduce heat loss by radiation.
4. A gallon of boiling water and a pint of boiling water contain the same amount of thermal energy.
5. (*) A gallon of boiling water and a pint of boiling water are at the same temperature.
What is the efficiency of a heat engine whose thermal energy input is 500 joules and whose exhaust is 400 joules?
1. 11%
2. (*) 20%
3. 35%
4. 80%
5. 120%
What is the maximum efficiency of a heat engine which takes heat in at and exhausts waste heat at ?
1. 16%
2. 37%
3. (*) 56%
4. 89%
5. cannot be determined from the information provided.
How much work do you have to do to move a particle with a positive charge of 5 Coulombs from some point to another one at a potential which is 12 V higher?
1. 3 J
2. 30 J
3. (*) 60 J
4. 300 J
5. 600 J
If the distance between two charged objects is doubled, the electric force between them
1. (*) is 1/4 its previous value;
2. is halved;
3. is doubled;
4. is 4 times its previous value;
5. stays the same;
You are listening to WFSQ which is broadcasting at a carrier wave frequency of 91.5 MHz. What is the wavelength of the carrier wave?
1. 3.3 cm
2. 33 cm
3. (*) 3.3 m
4. 8.9 m
5. 328 m
Two continuous waves with the same wavelength and amplitude pass through the same position in some medium such that they are ``out of phase'', i.e. the crests of one coincide with the troughs of the other. The resulting wave has
1. (*) zero amplitude;
2. twice the wavelength and the same amplitude as each of the individual waves;
3. twice the wavelength and twice the amplitude;
4. the same wavelength and the same amplitude;
5. the same wavelength and twice the amplitude;
Which one of these waves has the longest wavelength:
1. blue light;
2. (*) radio waves;
3. infrared light;
4. X-rays;
5. red light;
Which of the following statements is false:
1. In an electromagnetic wave, the magnetic field is perpendicular to the direction of wave propagation.
2. The speed of propagation of electromagnetic waves can be calculated from Maxwell's equations.
3. Electromagnetic waves can be generated by accelerated charges.
4. (*) Electromagnetic waves are longitudinal waves, i.e. the disturbance in the electric field is in the direction of wave propagation.
5. Electromagnetic waves do not need a material carrier medium to travel in -- they can exist in vacuum.
The unit of electric current is the
1. newton
2. volt
3. ohm
4. coulomb
5. (*) ampere
A 100 W lightbulb is connected to a power source of 100 V; what is the current flowing through the wire of the lightbulb?
1. 0 A
2. 0.5 A
3. (*) 1 A
4. 2 A
5. 4 A
Which of the following statements is false:
1. (*) An electric current flowing in a copper wire consists of protons moving along the wire from the high potential to the low potential end of the wire.
2. Electric motors are based on the facts that electric currents generate magnetic fields, and that magnetic fields exert forces on moving charges.
3. When a wire loop is moved in a magnetic field, an electric field is created which causes a potential drop along the wire. This phenomenon is used in the generation of electric energy.
4. Magnetic fields which vary in time cause the creation of electric fields.
5. The force exerted by a magnetic field on a moving charge is stronger the faster the charge moves.
Identify the false statement:
1. In order to define position, velocities, accelerations,..., one has to define frames of reference with respect to which these quantities are measured.
2. In an inertial frame, the law of inertia is valid.
3. Any frame moving in uniform motion with respect to an inertial frame is also an inertial frame.
4. (*) According to Galilean relativity, all inertial frames of reference are equivalent for all laws of physics.
5. Velocities depend on the frame of reference, but accelerations are the same in all inertial frames.
The Michelson-Morley experiment
1. measured the speed of the ether wind to be equal to the speed of light.
2. (*) measured the speed of light in different directions with respect to the Earth's motion to be the same, thus providing evidence against the existence of the ether as the medium for the propagation of light.
3. found that the speed of light was different in different directions, in contradiction with Maxwell's equations.
4. was not sensitive enough to be able to measure the influence of the ether wind on the speed of light.
5. used very precise clocks to determine differences in traveltime of light rays split by semi-transparent mirrors.
Identify the false statement:
The special theory of relativity
1. solved the inconsistency between mechanics and electrodynamics with respect to Galilei invariance.
2. states that all inertial frames are equivalent for all laws of physics.
3. states that the speed of light in vacuum is the same for all observers, independently of their relative state of motion.
4. does away with Newton's concept of absolute space and time, replacing it by a 4-dimensional space-time continuum.
5. (*) provides a new explanation of gravity in terms of the geometry of space.

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