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I am reattaching my homework, as it seems that for some reason it didn't go through, if it did, then please ignore this homework help, but also please try and give me the answers as soon as you can, I need them in max 4-5 more hours. Answer the first 3 questions (all parts in Q 1,2 and 3) with thorough explanation of the steps you've used ( as you usually do ).
Thanks, and please respond as soon as you can, I can see that your time is currently 11:41AM, unfortunatley though I can only select Friday, so please answer me as soon as you can ( in 4-5 hours max from now prefferebly) .
Thanks again

PHYS1002 Problem Sheet 7.pdf


S2 2010

Problem Sheet 7
Due: 5pm Friday 17th September
• Use the Model, Visualize, Solve and Assess pattern as described in the text.
• Fill in and attach a cover sheet to the front of your solutions
• Submit your individual solutions to the three problems to the tutorial slot in the Physics
Annexe (outside room 6-320).

1. Consider the simple circuit below. Leaving the positive terminal of the battery is the
copper wire A with a diameter of 1.0mm. As shown, wire A is connected to wire B
which is also copper but has a diameter of 3.0mm. Wire B is connected to the resistor
via wire C which has a diameter of 1.0mm and is made of gold. The resistance of the
combination of wires is still negligible.

a. What is the current in the circuit? Is it the same everywhere?
b. Find the current densities for wires A, B and C.
c. Find the drift velocity of the charged particles in wires A, B and C.
d. What is the electric field in each section of wire?


S2 2010

2. A real battery can be modelled as an ideal battery in series with a small ʻinternal
resistanceʼ, as shown in the circuit diagram, where the internal resistance is r = 1Ω.
Using Kirchoffʼs loop law, determine the potential difference across each
component and draw a graph of the potential as a function of distance travelled
(clockwise) through the circuit. By how much does the voltage of the real battery
differ from that of the ideal?
Determine the power ʻwastedʼ in the internal resistance of the battery. If the
battery were hooked up to a load resistance that was significantly smaller than
the 10 Ω and 20 Ω resistors shown here, would the rate of heat generation inside
the battery increase or decrease?

3. A very long wire, carrying current I = 1A, is bent in the middle to make a sharp rightangle bend. If the wire is oriented so that its arms lie along the positive x and y axes,
with the current coming in along the x-axis, what is the magnetic field strength and
direction at the point (x,y) = (1cm, 1cm) ?


S2 2010

Optional Extra Problems
4. The electron beam inside a television picture tube is 0.40 mm in diameter and carries a
current of 50 µA. This electron beam impinges on the inside of the picture tube screen.

How many electrons strike the screen each second?


What is the current density in the electron beam?


The electrons move with a velocity of 4.0 x 107 m/s. What electric field strength is
needed to accelerate electrons from rest to this velocity in a distance of 5.0 mm?


Each electron transfers its kinetic energy to the picture tube screen upon impact.
What is the power delivered to the screen by the electron beam?

5. The biochemistry that takes place inside cells depends on various elements, such as
sodium, potassium and calcium, that are dissolved in water as ions. These ions enter
cells through narrow pores in the cell membrane known as ion channels. Each ion
channel, which is formed from a specialized protein molecule, is selective for one type
of ion. Measurements with microelectrodes have shown that a 0.30-nm-diameter
potassium ion (K+) channel carries a current of 1.8 pA.

How many potassium ions pass through if the ion channel opens for 1.0 ms?


What is the current density in the ion channel?

6. A hollow metal sphere has inner radius a, outer radius b, and conductivity σ. The
current is radially outward from the inner surface to the outer surface.

Find an expression for the electric field strength inside the metal as a function of
the radius r from the centre.
Evaluate the electric field strength at the inner and outer surfaces of a copper
sphere if a = 1.0 cm, b = 2.5 cm, and I = 24 A .

What is the magnetic field at the center of the loop in the figure below?

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