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NT Exampro Drift Speed Questions - G482 Highcliffe School Physics Department 105 min 79 marks NT Exampro 1 1. A copper wire is 2.0 m long and has a cross-sectional area of 1.00 mm2. It has a p.d. of 0.12 V across it when the current in it is 3.5 A. Draw a circuit diagram to show how you would check these voltage and current values. (3) Calculate the rate at which the power supply does work on the wire. .............................................................................................................................................. .............................................................................................................................................. Rate = ......................................................... (2) Copper has about 1.7 × 1029 electrons per metre cubed. Calculate the drift speed of the charge carriers in the wire. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. Drift speed = ......................................................... (3) The power from the supply connected to the wire is equal to the total force Ft on the electrons multiplied by the drift speed at which the electrons travel. Calculate Ft. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. Ft = ......................................................... (3) (Total 11 marks) NT Exampro 2 2. A semiconducting strip, 6mm wide and 0.5 mm thick, carries a current of 8mA. The carrier density is 7 × 10-23 m-3. Calculate the carrier drift speed. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. Drift speed = ......................................................... (3) An approximate value for the drift speed in a copper wire of the same dimensions and carrying the same current would be about 10-7 ms -1. Compare this figures with your calculated result and account for any difference in terms of the equation I = nAqv. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (3) Explain why the resistance of a semiconducting strip decreases when its temperature rises. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (2) (Total 8 marks) 3. A wire 6.00 m long has a resistivity of 1.72 10–8 m and a cross-sectional area of 0.25 mm2 Calculate the resistance of the wire. ...............................…........................................................................................................… ...............................…........................................................................................................… ...............................…........................................................................................................… Resistance = .............................................… (3) NT Exampro 3 The wire is made from copper. Copper has 1.10 1029 free electrons per metre cubed. Calculate the current through the wire when the drift speed of the electrons is 0.093–1 mm s–1. ...............................…........................................................................................................… ...............................…........................................................................................................… ...............................…........................................................................................................… Current = ..............................................… (3) The wire is cut in two and used to connect a lamp to a power supply. It takes 9 hours for an electron to travel from the power supply to the lamp. Explain why the lamp comes on almost as soon as the power supply is connected. ...............................…........................................................................................................… ...............................…........................................................................................................… ...............................…........................................................................................................… ...............................…........................................................................................................… (3) (Total 9 marks) NT Exampro 4 4. Complete the table by selecting a typical value for each physical quantity from the list below. Each value may be used once, more than once or not at all. 0.05 6.0 2 k 10 M 0.3 mm s–1 30 cm s–1 2.0 × 10–8m 2.0 × 1015 m Physical quantity 300 K 3000 K Typical value Resistance of a voltmeter Internal resistance of a car battery Internal resistance of an e.h.t. supply Resistivity of an insulator Drift velocity of electrons in a metallic conductor Temperature of a working filament bulb (Total 6 marks) 5. The current I flowing through a conductor of cross-sectional area A is given by the formula I = nAQ where Q is the charge on a charge carrier. Give the meanings of n and . n ........................................................................................................................................... ........................................................................................................................................... (2) NT Exampro 5 Show that the equation is homogeneous with respect to units. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (3) With reference to the equation, explain the difference between a metal conductor and a plastic insulator. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. .............................................................................................................................................. (2) (Total 7 marks) 6. A 500 W electric toaster operates from the 230 V mains. Calculate the current in the heating element of the toaster. .......................................................................................................................................... .......................................................................................................................................... .......................................................................................................................................... .......................................................................................................................................... Current = ........................................................ (3) The heating element of the toaster is made of much thinner wire than the wire in its supply cable. In which of these two wires do the electrons have the greater drift velocity? .......................................................................................................................................... (1) NT Exampro 6 Explain your answer. You may be awarded a mark for the clarity of your answer. .......................................................................................................................................... .......................................................................................................................................... .......................................................................................................................................... .......................................................................................................................................... .......................................................................................................................................... .......................................................................................................................................... (4) (Total 8 marks) 7. A thick wire is connected in series with a thin wire of the same material and a battery. In which wire do the electrons have the greater drift velocity? Explain your answer. ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... (3) A battery is connected across a large resistor and a small resistor is connected in parallel. The currents through the resistors are different. Which resistor has the higher dissipation of power? Explain your answer. ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... (3) (Total 6 marks) NT Exampro 7 8. A copper wire has a cross-sectional area of 0.20 ×10–6 m2. Copper has 1.0 × 1029 free electrons per cubic metre. Calculate the current through the wire when the drift speed of the electrons is 0.94 mm s–1. ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... Current = ................................................. (3) The wire is 4.0 m long. Copper has a resistivity of 1.7 × 10–8 m. Calculate the resistance of the wire. ............................................................................................................................................... ............................................................................................................................................... ............................................................................................................................................... Resistance = ............................................ (3) Calculate the potential difference across the wire. ............................................................................................................................................... Potential difference = .............................. (1) A second wire with the same dimensions is made from a material that has a greater resistivity than copper. Explain how, if at all, the current will differ from that in the copper wire when the same p.d. is applied across it. ............................................................................................................................................... ............................................................................................................................................... (2) The number of free electrons per cubic metre in this wire is the same as that in the copper wire. Compare the drift velocities of the free electrons in the two wires. ............................................................................................................................................... ............................................................................................................................................... (1) (Total 10 marks) 9. The current I through a metal wire of cross-sectional area A is given by the formula I = nAe NT Exampro 8 where e is the electronic charge on the electron. Define the symbols n and . .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... .................................................................................................................................... (2) Two pieces of copper wire, X and Y, are joined end-to-end and connected to a battery by wires which are shown as dotted lines in the diagram. The cross-sectional area of X is double that of Y. Y X In the table below, nx and ny denote the values of n in X and Y, and similarly for the other quantities. Write in the table the value of each ratio, and alongside it explain your answer. Ratio Value Explanation nY nX IY IX Y X (6) (Total 8 marks) NT Exampro 9 10. A 100 W tungsten filament lamp operates from the 230 V mains. Calculate its resistance. ...............................…............................................................................................................ ...............................…............................................................................................................ ...............................…............................................................................................................ ...............................…............................................................................................................ ...............................…............................................................................................................ Resistance =........................................................... (2) The drift speed of the electrons in the filament is much higher than the drift speed of electrons in the rest of the circuit. Suggest and explain a reason for this. ...............................…............................................................................................................ ...............................…............................................................................................................ ...............................…............................................................................................................ ...............................…............................................................................................................ ...............................…............................................................................................................ ...............................…............................................................................................................ (4) (Total 6 marks) NT Exampro 10
