1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Flunked this test correct answers needed

  1. Feb 14, 2007 #1
    Flunked this test.... correct answers needed


    1) Classify each of the terms in the left hand column by placing X in the relevant box.

    Base Unit Derived Unit Base Quantity Derived Quantity

    2) The table gives four word equations. Complete the table with the quantity defined by each word equation.

    Word Equation Quantity Defined
    Voltage / Current
    Voltage x Current
    Charge / Time
    Work Done / Charge

    3a) The resistance of an ammeter is assumed to be?

    b) The resistance of a voltmeter is assumed to be?

    c) Calculate the total resistance of four 5.0 ohm; resistors connected in parallel

    4) Three resistors or resistances R1, R2, and R3 are connected in parallel with each other. They could be replaced by a single resistor.
    Show that the resistance R of the equivalent single resisitor can be calculated from:

    1/R = 1/R1 + 1/R2 + 1/R3

    5).... this one includes chart.... nevermind

    6a) A copper wire has a CSA of 0.20 x 10^6 m^2. Copper has 1.0 x10^29 free electrons per cubic metre. Calculate the current through the wire when the drift speed of the electrons is 0.94mm s^ -1

    6b) The wire is 4.0m long. Copper has a resistivity if 1.7 x 10^ -8 ohm; per m. Calculate the resistance of the wire.

    6c) Calculate the potential difference across the wire.

    6d) A second wire with the same diameter is made from a material that has a density greater than that of 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.

    6e) 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.

    7a)A lightning stroke passes between a cloud and a lightning conductor 50m high attached to a tall building. A very large current of 20,000A passes for 4.0 x 10^ -4 s.

    Calculate the charge flowing to the ground in this time.

    7b) The lightning conductor is 50m high and has a CSA of 1.0 x 10^ -3 m^2. It is made from copper which has a resistivity of 1.7 x 10^ -8 ohms per m. Calculate the resistance of the lightning conductor.

    7c) Hence calculate the potential difference between the top and bottom of the current-carrying lightning conductor.

    7d) If lightning strikes a tree such that there is the same current through it as there was through the lightning conductor, then a much larger potential difference exists between top and bottom of the tree. Explain why this is so.

    8b) Two resistors of resistance 2.0 Mohm; and 4.0ohm; are connected in series across a supply voltage of 6.0 V together they form a simple potential divider circuit.
    State the potential difference between each resistor.

    8b) A second potential divider circuit uses a resistor and a diode connected in series with the same supply.
    Calculate the potential difference across each component when the reisitance of the resistor and diode are 45ohm; and 5.0ohm; respectively.

    does anyone know how to work out the right answers?

    thank you
    Last edited: Feb 14, 2007
  2. jcsd
  3. Feb 14, 2007 #2


    User Avatar

    Staff: Mentor

    Thread moved from Advanced Physics to Intro Physics.

    missy, the PF rules require you to show your work before we can help you. Please fill in your best answers to each question, and we'll check your work.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook