Intermediate physics / by W. Watson.

  • Watson, W. (William), 1868-1919.
Date:
1912
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Licence: In copyright

Credit: Intermediate physics / by W. Watson. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    3. The terminals of a galvanometer are connected by a wire which has the same resistance as that of the galvanometer, and a lead from a battery is connected to one end of this wire. Find the point on the wire at which the other lead from the battery has to be connected so that exactly one-tenth of the current supplied by the battery will traverse the galvanometer. 4. A battery of which the E.M.F. is 4 volts and the internal resistance is 0*2 ohm has its terminals joined by two wires of resistances 10 and 5 ohms placed in parallel. Find the current in each wire and the amount of heat iu ergs generated per second in each. 5. Show liow by means of an accurately calibrated ammeter and voltmeter we can determine the mechanical equivalent of heat. Give a diagram of apparatus and connections. A wire is immersed in 100 grams of water in a calorimeter. Find the rise of temperature of the water in 5 minutes when a current of 5 amperes passes through the wire whose resistance is 3 ohms. (/=4*2 x 10^ C.G.S. units.) 6. A battery has an E.M.F. of 18 volts and an internal resistance of 3 ohms. The P.D. across the terminals when connected by a wire A is 15 volts, when by a wire B 12 volts. Compare the heat developed in A and B in equal time. 7. Explain what is meant by the term conductance, and show that the conductance of two conductors placed in parallel is equal to the sum of their conductances. 8. If the resistivity of platinum is 8 x 10~^ ohms per cm. per sq. cm., calculate the length of a wire of diameter -2 mm. which Avill have a resistance of 1 ohm. 9. The P.D. of an electric supply is 100 volts ; what must be the resistance of an electric radiator so that when connected to this supply it may give out as much heat as is obtained by burning 10 lbs. of coal j)er hour, the calorific value of the coal being 36 x 10 caloi'ies per lb. ? 10. How many kilowatt-hours of electrical energy will the above radiator require if it is run 6 hours a day for a week ? CHAPTER VI 1. Define the unit of current in electro-magnetic measure, and state what is 1 ampere. A coil of 10 turns and 10 cm. radius is placed in the magnetic meridian with a needle at its centre. Find the current in amperes which will deliect the needle 45° if the horizontal intensity is 0*16. 2. Deduce a simple expression for the work done when a magnetic pole passes along any closed path round a current, and hence deduce the intensity of the magnetic field inside a long solenoid. 3. A current of 5 amperes is passed through a coil containing 20 turns of mean radius 15 cm.; find the strength of the magnetic field in C.G.S. units at the centre of the coil. 4. Describe and explain the use of a tangent galvanometer. If the coil of a tangent galvanometer has 20 turns, and a current of 9 amperes produces a deflection of 45° at a jplace where ^='18 C.G.S. units, what is the mean radius of the coil ? 5. Give Ampere's expression for the magnetic field due to an element of current, and deduce the intensity of the field at any point on the axis of a circular wire carrying a current. 6. Explain what is meant by the terms magnetomotive force and reluctance. 7. Find an expression for the field inside an anchor-ring which is uniformly wound with a layer of insulated wire through which a current is passed, and discuss the question as to whether this field is uniform. 8. A straight wire rolls on two parallel horizontal cylinders 2 metres apart. If a current of 20 amperes is passed from one cylinder through the wire to the other cylinder, find the force which must be applied to the wire to prevent it moving. The vertical component of the earth's field is 4 C.G.S.