Download Forces and Fields Concept Check 15 Solutions

Forces and Fields Concept Check 15 Solutions Concept:
describe magnetic interactions in terms of forces and fields
1.
How is it possible for a compass to point north?
The magnetic domains in the compass needle (permanent magnet) will align
themselves with the Earth’s external magnetic field.
2.
What direction are the field lines pointing at the equator and at our location?
At the equator the magnetic field lines are parallel to the Earth’s surface and pointing
towards geographic North. At our location the magnetic field lines are directed at an
angle of about 25° towards the surface and pointing towards geographic North.
3.
How is it possible for the north end of a magnet to attract a non-magnetized paper
clip?
The magnetic field lines produced by the magnet align the domains inside the paper
clip, so that the paper clip becomes a magnet itself, and therefore can be attracted to
the magnet
Concept:
4.
compare gravitational, electric and magnetic fields (caused by permanent
magnets and moving charges) in terms of their sources and directions
What shape do magnetic field lines make? How does this shape compare to that of
electric and gravitational fields?
Magnetic field lines are circular in shape, forming loops that externally extend from
the North pole to the South pole. Internally the field lines are from the South pole to
the North pole of the magnet, Electric field lines around a single charge extend
radially, either inward or outward. For a single mass gravitational field lines extend
radially only towards the centre of mass.
5.
How do you determine the direction of a magnetic field? The direction of a
gravitational field? The direction of an electric field?
To determine the direction of an electric field, a charged object could be used and
the direction of electrical force acting on it could be used to determine field
direction. A positive charge will be forced in the direction of electric field.
To determine a gravitational field, a mass could be placed in the field. The direction
that this mass would move, would indicate the direction of the gravitational field.
To determine the magnetic field direction, the simplest test, would be to place a
suspended magnet in the field. The permanent magnet will align itself with the
magnetic field. The force acting on charged particles or current carrying conductors
when placed in the magnetic field can also be used to determine the field direction,
using hand rules.
6.
What is the range of each of the three fields (magnetic, electric, gravitational)?
Infinity
7.
What produces a gravitational field? What produces an electric field? What
produces a magnetic field?
Mass (m) produces gravitational fields, charge (q) produceds electrical fields and
moving charges produce magnetic fields
Concept:
8.
describe how the discoveries of Oersted and Faraday form the foundation
of the theory relating electricity to magnetism
How did Oersted’s and Faraday’s experiments shape our understanding of
magnetism?
Oersted observed that a current carrying conductor produces a magnetic field.
(motor effect). Faraday showed that a magnet can induce a current in a conductor
when there is relative motion. These two effects show that the two effects are the
same.
Concept:
9.
describe, qualitatively, a moving charge as the source of a magnetic field
and predict the orientation of the magnetic field from the direction of
motion
What field is produced around a stationary electrical charge?
An electrical field only
10. What field is produced around a moving electrical charge?
An electrical field, and a magnetic field.
11. Describe the direction of the magnetic field of electron moving out of the page
towards you?
Clockwise
12. A conductor has a current moving out of the page. Describe the direction of the
magnetic field around this conductor.
Clockwise. This answer is the same as question 10, because the flow of electrons
(default current) is out of the paper, the same as question 10
13.
Is the magnetic field shown below correct for this cross section of a solenoid?
Ignore the abcd loop
Use hand rule for solenoids
Yes or No. No, if we use a default current of electron flow, but yes if we use
conventional current.
14. Explain how an electromagnet could be made more powerful.
More loops of wire – adds more magnetic field lines.
Increase the current – increases the magnitude of the field already present
Metal core – increases the number of domains that can be aligned in the direction of
the magnetic field
Concept:
explain, qualitatively and quantitatively, how a uniform magnetic field
affects a moving electric charge, using the relationships among charge,
motion, field direction and strength, when motion and field directions are
mutually perpendicular
15. Describe the path that a charged particle travels when moving parallel to magnetic
field lines.
If the particles travel
parallel to the magnetic field lines, this indicates that the angle


between v and B is either 0° or 180°, so the amount of force acting the particle must


be 0. Fm = IlBsin θ , where θ is the angle between v and B .
16. Describe the path that a charged particle travels when moving perpendicular to
magnetic field lines.

If particles travel perpendicular to the magnetic field, then the angle between v and

B will be 90° and the magnetic force will be a maximum. Using the hand rule for
particles moving in a magnetic field, we see that the force is always perpendicular to
the velocity of the particle. This scenario describes circular motion, so the particle
will follow a circular path.
17. Which direction is the magnetic force that acts on a charged particle moving in a
magnetic field directed? Describe how to determine the direction of the magnetic
force.
Using the hand rule for particles moving in a magnetic field, we see that the force is
always perpendicular to the velocity of the particle. This scenario describes circular
motion, so the particle will follow a circular path.
Negative particles – Left hand
Positive particle – Right had
The hand rule used states that the thumb represents the direction of particle motion
(velocity), the extended fingers point in the direction of the magnetic field and the
direction that the open palm faces would be the direction that the particle will be
pushed.
18. A proton is accelerated through a potential difference of 3000 V. It enters a
magnetic field of 0.030 T. What is the radius of curvature of the proton?
The potential difference of 3000 V gives us our energy, which we can use to
determine the speed of the particle
ΔE p = ΔEk
Fnet = Fm ; Fnet = Fc
1 2
mv
2
2qV
v=
m
Fc = Fm
qΔV =
v=
(
)
2 1.6 ×10 −19 C (3000V )
1.67 ×10
v = 7.6 ×10 5
mac = qv⊥ B;
m
s
−27
kg
ac =
v2
r
mv 2
= qv⊥ B
r
mv
r=
qB
m⎞
⎛
1.67 ×10 −27 kg ⎜ 7.6 ×10 5 ⎟
⎝
s⎠
r=
−19
1.6 ×10 C ( 0.030T )
r = 0.26m