Electricity Demystified, 2nd edition
Stan Gibilisco
Explanations for Quiz Answers in Chapter 10
1. Let's recall the formula for flux density Bt (in teslas) near a long, straight, thin wire in free space that carries a direct current I (in amperes). At any point at a distance r (in meters) away from the wire, we'll observe a flux density of

Bt = 2 x 10-7 I / r

The coefficient of 2 is exact to as many significant figures as we need. We're told that the wire carries 10 A, and a certain point P lies 100 mm (which equals 0.100 m) from the wire. Therefore, I = 10 and r = 0.100. Before we do any calculations, let's take note of the fact that all the choices in this question express flux density values in gauss, not teslas. Remembering that 1 T = 104 G, we can modify the above formula to

Bg = 104 x 2 x 10-7 I / r
= 2 x 10-3 I / r

where Bg represents the flux density in gauss. Now, for point P, we can calculate the flux density BP (in gauss) as

BP = 2.0 x 10-3 x 10 / 0.100
= 2.0 x 10-3 x 100
= 200 x 10-3
= 0.20 G

The correct choice is B.

2. For point Q, we have r = 50 mm = 0.050 m. We can calculate the flux density BQ (in gauss) as

BQ = 2.0 x 10-3 x 10 / 0.050
= 2.0 x 10-3 x 200
= 400 x 10-3
= 0.40 G

The correct choice is C.

3. If we double the DC in the wire from 10 A to 20 A, we get I = 20. Therefore, the flux density BQ (in gauss) is

BQ = 2.0 x 10-3 x 20 / 0.050
= 2.0 x 10-3 x 400
= 800 x 10-3
= 0.80 G

The correct choice is D.

4. We'll observe repulsive magnetic force in only two circumstances: When we bring two magnetic north poles near each other, or when we bring two magnetic south poles near each other. None of the choices given here satisfy either of these two requirements, so the correct answer is D, "None of the above."
5. The magnetomotive force near a current-carrying wire coil depends on the number of turns and the current, and on no other factors whatsoever. The only choice that gives us a "hit" here is A. Remember, magnetomotive force fundamentally differs from flux density or magnetic-field quantity!
6. If the electrons in a current-carrying wire move directly toward us, the conventional current goes straight away from us. Using the right-hand rule (and twisting our wrists in an awkward fashion), we can determine that the magnetic flux flows in concentric circles or rings going clockwise around the wire axis from our point of view. The correct choice is D.
7. Scientists define the geomagnetic inclination (or simply the inclination) as the vertical slant of the geomagnetic flux lines at a specific location on the earth's surface. The correct choice is A.
8. Near a magnetic dipole such as we would observe in a bar magnet, the magnetic lines of flux form closed curves. The correct choice is C. You might suppose that choice D could be right as well; after all, circles are always closed curves! But choice D is too specific. It would hold true only if we were talking about a current-carrying, straight wire rather than a magnetic dipole in general.
9. If we have a coil with 100 turns and we send 100 mA (which equals 0.100 A) of current through it, we get a magnetomotive force Fm (in ampere-turns) of

Fm = 0.100 x 100
= 10.0 At

This problem asks for the result in gilberts, not ampere turns. To get gilberts from ampere-turns, we must multiply by 1.257. In this case, we end up with 10.0 x 1.257 = 12.57 Gb, which we should round off to 12.6 Gb because our input data is accurate to only three significant figures. The correct choice is A.

10. The south geomagnetic pole attracts the south poles of ordinary magnets, so it's actually a north magnetic pole. The answer is B. Choice A won't work because the geographic poles do not coincide with the geomagnetic poles. Choices C and D are completely wrong!