Category Archives: Physics

1. A rotor-resistance starter for a three-phase wound-rotor induction motor

(a) adds external resistance to the wound rotor via sliprings

(b) is connected in series with the machine stator

(c) can only be used in conjunction with a double-cage rotor

(d) applies only to DC motors

2. A large synchronous motor is often started by

(a) connecting it directly to the three-phase supply

(b) pushing the rotor in the required direction before connecting the supply

(c) running it up to speed using a ‘pony’ motor before energising the rotor

(d) first switching on the rotor supply then applying the stator voltage

3.A single-phase induction motor

(a) can only be used for loads rated at less than 0.5 kW

(b) will run in whichever direction the rotor is….

1.The instrument of Exercise 7 is to have a multiplier connected in series with it

to convert it for use as a voltmeter to read up to 50 V. What will be the multiplier resistance? 2. A permanent-magnet moving-coil instrument has a resistance of 3 _, but has a 27 _ swamping resistor connected in series with it. The movement requires a current of 15 mA for full-scale deflection. Calculate the resistance of a shunt required to convert it for use as a 0–10 A ammeter. What will be the resistance of a multiplier to convert the unit for use as a 0–300 V voltmeter if the swamping resistor remains in circuit?

1.To find the resistance of a resistor, various currents were passed through the resistor and the corresponding voltage drops across it were measured. The results obtained were

(a) Draw to a suitable scale the graph connecting these quantities, with current values horizontal, and from the graph estimate the resistance value. (b) One serious reading error has been made; estimate what the voltage reading should be.

2.A resistor R is measured by the ammeter-and-voltmeter method, as in Figure 11.51. The ammeter and voltmeter readings are shown on the figure and the resistance of the voltmeter is 400_. Find the apparent and the true values ofR.

Portable mass spectrometers are now available for monitoring gaseous emissions from volcanoes.

Analyses after the eruption of the Turrialba Volcano in Costa Rica in January 2010 showed the presence of [M]+ ions (i.e. atomic ions and nonfragmented molecular ions) at m/z 64, 44, 40, 34, 32, 28 and 18. Mass spectra recorded before the eruption confirmed that the peak at m/z 64 was missing, and the peak at m/z 34 was substantially diminished.

(a) To what do you assign the peaks at m/z 64 and 34?

(b) Bearing in mind the composition of the Earth’s atmosphere, what molecular ion interferes with the detection of the volcanic emission observed at m/z 34?

(c) Volcanic gases mix with the surrounding air. Suggest what atmospheric gases give rise to the ions at m/z 40, 32 and 28.

(d) Sampling of the volcanic plume….

Q. 2. (a) Describe briefly three factors which justify the

need for modulation of audio frequency signals

over long distances in communication. (b) Draw

the waveforms of (i) carrier wave, (ii) a modulating

signal and (iii) amplitude modulated wave.

Q. 3. (a) Give three reasons why modulation of a

message signal is necessary for long distance

transmission. (b) Show graphically an audio

signal, a carrier wave and an amplitude modulated

wave.

You have a new summer job with NASA and are working on astronomical observations using electromagnetic radiation that is not in the visible range. Your supervisor has explained 21­cm radiation to you and that it is used for a number of observations of interstellar hydrogen. She explains that 21-cm radiation is in the microwave region of the electromagnetic spectrum and comes from a hyperfine splitting of the electron ground state of hydrogen. It is similar to the Zeeman effect, except that it is the spin states that are split, and the magnetic field is internal to the atom: it comes from the magnetic field due to the nucleus. When the atom makes a transition from the higher state to the lower state, a 21-cm photon is emitted. Based….

You are hired as an expert witness by the attorney representing a doctor. The doctor is being sued by a patient who claimed radiation damage from the doctor’s x-ray machine. The plaintiff argues that the machine must not have been properly shielded, exposing him to dangerous radiation. Your visit to the doctor’s office shows the following results. You do indeed measure x-radiation in the doctor’s office, with a minimum wavelength of 30.0 pm. Consultation with the doctor and inspection of his x-ray machine shows that it accelerates electrons through a voltage of 35.0 kV before they strike the target, producing bremsstrahlung. What advice do you give the attorney?

The Dulong–Petit law states that the molar specific heat of solids is 3R at higher temperatures, where R is the gas constant. For metals, this law is obeyed at room temperature, 300 K. The absorption of energy appears as internal energy in the metal in two primary ways: (1) vibration of metal lattice ions locked into crystalline positions and (2) translational kinetic energy of free electrons. The number of free electrons in a metal is approximately proportional to kBT/EF ,  since only those electrons near the Fermi energy can be thermally excited into available states. From this information, work with your group to determine the percentage of the total molar specific heat that is attributed to free electrons in gold.

Your group is a radiology department in a hospital. Two patients in your waiting room are arguing about who “got more radiation” in their cancer treatments. Patient A received 2.0 Gy of radiation, while Patient B received 1.0 Gy. Patient A is claiming that he had twice as much energy delivered to his body based on these numbers. Upon further investigation, it is determined that Patient A received radiation from fast neutrons, RBE 10, affecting 22 g of tissue. Patient B received alpha particles, RBE 18, affecting 30 g of tissue. (a) Who “got more radiation” in terms of biological effectiveness for radiation damage and (b) by what factor?

This activity simulates the statistical decay of radioactive nuclei. Packages of 100 dice can be purchased online. (a) First, think about the following procedure, but don’t do it yet: Put 100 dice in a bag and shake for a few seconds. Roll out the dice on a tabletop. Each such roll of the dice will represent one time interval Dt. Remove all the dice showing a one on the upper face, and set them aside. Record the remaining number N of dice. Put the remaining dice back in the bag, shake, and roll out again. Repeat this procedure, always removing the dice showing a number one from those on the table, until only a few dice remain. Second, after thinking about this procedure, predict the half-life of the….