Physics 106 - Chapter 20 Magnetism
How do we draw magnetic field lines? (2(3))
Draw them, so that: 1. the direction of the magnetic field is tangent to a field line at any point, and 2. the number of lines per unit area is proportional to the strength of the magnetic field (3. magnetic field lines continue inside a magnet, they always form closed loops, hence no monopoles can exist.)
20.4 What is the formula for the magnitude of a force on an electric charge travelling through a magnetic field?
F= q*v*B*sin[theta] or F(max)= qvB, if it moves perpendicularly through the field.
What is the formula for the magnitude of a force on a current-flowing wire in a magnetic field and why?
F=IlBsint[theta] or the force on the wire is proportional to the magnitude of the current in the wire, the length of wire exposed to magnetic field, the magnetic field B, and the sine of the angle the wire makes with the field lines. F= i * L * B * sin[theta] (assuming B is uniform, else B can be the average field over length L)
What is the direction of any magnetic field? And what colors are assigned to magnetic north and south poles?
From the north pole to the south pole. The north pole is the red end of the bar magnet and the south pole the blue end.
Describe the first right-hand rule for a current in a wire?
Grasp the wire with your right hand so that your thumb points in the direction of the conventional (positive) current; then your fingers will encircle the wire in the direction of the magnetic field.
20.2 Hans Christian Oersted Where from, when lived, what famous for?
Hans Christian Oersted was a Danish physicist and chemist (1777-1851), who discovered that an electric current produces a magnetic field (he deflected a compass with a wire by accident in class).
A straight wire carries a current directly toward you. In what direction are the magnetic field lines surrounding the wire?
In circles, counterclockwise.
When was first seen that magnetism and electricity were closely related?
In the 19th century
How many times have Earth's magnetic poles reversed direction in the past? And how do we know that?
In the past 330 million years Earth's poles have reversed direction 400 times. Magnetism in rocks can solidify (radioactive dating).
How does the Earth have a magnetic field?
It is thought to be produced by electric currents in the Earth's molten iron outer core. The pattern of field lines is almost like there were an imaginary bar magnet inside.
What is the new definition of 1 Ampere according to the proportionality constant mu0?
One Ampere is defined as that current flowing in each of two long parallel wires, 1 m apart, which results in a force of exactly 2*10^-7 N per meter of length of each wire. (an operational definition)
Formula for waste heat/ power?
P=I²R
Define a uniform magnetic field.
The field is the same everywhere in magnitude and direction.
What is the magnitude of the force when the particle moves parallel to the field lines?
The force is zero
Explain the situation with the poles of the Earth.
The geographic north pole of the earth is actually a magnetic south pole (S). The field lines go from Antarctica to North Pole.
Describe the magnetic field lines produced by a current through a wire.
They surround the wire at their center with circles. (Use first right hand-rule to determine direction.)
20.1 Where and when were the first rocks found that could attract each other?
Thousands of years ago in the area of Magnesia, Asia Minor (now Greece)
20.10 What is required for a useful galvanometer?
We need the angle phi only to be proportional to current, independent of B. Thus we build in a round iron core which causes a magnetic field B that will always point parallel to the face of the coil. formula -> phi =NIABsin[theta]/k
When do parallel magnetic field lines exist?
When two poles (N and S) with a large area compared to the distance between them are held together. Only the more central magnetic field lines will be resembling parallelism.
What is the Hall effect used for?
With a Hall probe we can measure the drift velocity of charge carriers.
Are there other materials besides Fe, Co, Ni, and Gd that exert magnetic behavior?
Yes, but only slightly and difficult to detect.
Give three formulas for an electron in a circular motion (acceleration, Newton's second law) in a uniform magnetic field...
a=v²/r and Sum of F= ma, hence qvB= mv²/qB and r= mv/qB
A wire carrying current I is perpendicular to a magnetic field of strength B. Assuming a fixed length of wire, which of the following changes will result in decreasing the force on the wire by a factor of 2? a. decrease the angle from 90° to 45° b. decrease the angle from 90° to 30° c. decrease the current in the wire to I/2 d. decrease the magnetic field to B/2 e. none of these will do
b, c, d
Which of the following can experience a force when placed in the magnetic field? a. an electric charge at rest b. an electric charge moving c. an electric current in a wire d. another magnet
b. , c. and d.
Name a few examples of devices that depend on magnetism.
compasses, motors, loudspeakers, computer memory, and electric generators.
What do we call the angular difference between the direction of a compass needle and True North?
magnetic declination (in the US it varies from 0° to 20°)
Define paramagnetic vs. diamagnetic
materials that have a magnetic dipole moment vs. no magnetic dipole moment
What is hysteresis?
the fact that a magnetic curve in a B0 B diagram does not retrace itself on the same path.
Define True North.
the geographic North pole is called True North, it is in fact 1000 km away from the magnetic geographic north pole, which is in the Canadian Arctic.
What is mu0?
the permeability of free space
How much does Earth's magnetic pole move per year?
About 10 km per year.
What kind of current is necessary for a continuously turning motor?
Alternating current or a dc motor with commutators and brushes that change connection.
How does a magnet lose its magnetism?
By hammering (distortion of domains alignment) or heating (increasing the random thermal motion of atoms and thus domains).
How can the magnetic field of a solenoid be increased? What do we call the resulting arrangement?
By sticking a piece of iron inside, which becomes a magnet. Consequently, both magnetic fields add. Such an iron-core solenoid is an electromagnet.
Which other elements besides iron (FE) show strong magnetic effects? What is the expression for all of these materials?
Cobalt (Co), Nickel (Ni), Gadolinium (Gd), and some of their oxides and alloys. They are said to be ferromagnetic (lat. ferrum = iron).
Describe the 2. Right-hand rule (describe dangers with charge application)
A wire goes through a magnetic field. We expect a force on the wire. My outstretched fingers point into the direction of the current flow, I bend my fingers into the direction of the magnetic field lines and then my outstretched thumb will indicate the direction of the force on the wire. (only applicable for direction v of positive charges, negative charges will be opposite
How do we use a compass and a map?
On a good map is an angle of declination that shows the disparity between True North and geomagnetic North. Hold the needle North pole towards the magnetic North pole and align the map with True North.
What describes the angle of dip?
The angle of dip, magnetic dip or magnetic inclination describes the angle the Earth's magnetic field makes with the horizontal at any point of Earth's surface, or the angle of the tip of a compass towards the geomagnetic North Pole. It's max. is +90° in the Northern hemisphere and -90° in the Southern hemisphere.
20.7 What is a solenoid?
A long coil of wire consisting of many loops (or turns) of wire is called a solenoid.
20.3 What are Newton's three laws?
1. Law of Inertia A body will remain at rest, or moving at a constant velocity, unless it is acted upon by an unbalanced force. 2. The force experienced by an object is proportional to its mass times the acceleration it experiences F=ma 3. If two bodies exert a force on each other, the forces are equal in magnitude, but opposite in direction F12= -F21
A straight power line carries 30 A and is perpendicular to the Earth's magnetic field of 0.5*10^(-4) T. What magnitude force is exerted on 100m of this power line?
0.15 N
How is 1 Coulomb defined?
1 C= 1A*s
Does the Earth's magnetic field have a greater magnitude near the poles or near the equator?
Near the poles, where the field lines are closer together.
A negative charge is placed at rest near a magnet. Will the charge begin to move? Will it feel a force? What if the charge were positive?
No to all questions. A magnetic field exerts a force only on moving electric charges.
20.5 TRUE or FALSE The magnetic field B due to the current in a long straight wire is directly proportional to the current I in the wire and inversely proportional to the distance r from the wire.
TRUE, B is proportional to I/r
TRUE or FALSE The direction of the force is always perpendicular to the direction of the current.
TRUE, but only half-TRUE, since the direction of the force is always perpendicular to the direction of the current and also perpendicular to the direction of the magnetic field.
TRUE or FALSE Every magnet has two poles.
TRUE, physicists have searched for isolated single magnetic poles (magnetic monopoles), but have never as yet been successful.
What is the SI unit for magnetic field B?
Tesla (T)
Who used the favours of magnetism as an aid to navigation first?
The Chinese in the 11th century (maybe earlier)
