phy mock paper



for question 2:

Peter brought a rice cooker that operates at 110 V from Japan to HK. He connects the rice cooker to the mains through a transfomer that steps down the voltage from 220 V to 110 V. he coonects an ammeter to the circuit to study the cuurent drawn by the rice cooker.

1 Answer

  • 天同
    Lv 7
    7 years ago
    Favorite Answer

    1.(a) Magnetic force on a single wire = B.l.J

    Hence, magentic force on N wires = NBlJ

    (b) Torque = (NBlJ).l = NBl^2J

    (c) Torque produced by the object at the axle of the motor

    = mg x (6/100) = 0.06mg = 0.6m

    where g is the acceleration due to gravity, taken to be 10 m/s^2

    The motor stops rotating when the magnetic torque equals the torque produced by the object.

    Hence, NBl^2J = 0.6m

    i.e. 4 x 0.2 x (0.1)^2 x 10 = 0.6m

    m = 0.133 kg

    2. (a) Yes, it is correct.

    (b) How is Peter's circuit? It is not described in the question.

    3. In general, not all momentum of the wind is transferred to drive the wind turbine, and there is energy loss due to friction.

    4. The cooling capacity per unit input power for condition X = 1600/800 = 2 watts per watt

    The cooling capacity per unit input power for condition Y = 2400/1200 = 2 watts per watt

    Therefore, both conditioners X and Y are having the same cooling capacity.

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