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Magnetic Fields and Magnetic Forces
Properties of magnets:
- A magnet has polarity - it has a north and a south pole; you cannot isolate the north or the south pole (there is no magnetic monopole)
- Like poles repel; unlike poles attract
- A compass is a suspended magnet (its north pole is attracted to a magnetic south pole); the earth’s magnetic south pole is within 200 miles of the earth’s geographic north pole (that is
why a compass points "north")
- Some metals can be turned in to temporary magnets by bringing them close to a magnet; magnetism is induced by aligning areas called domains within a magnetic field
- Permanent magnets are formed of metallic alloys or metals such as iron, nickel, or cobat
Oersted (1820) found that an electric current in a wire produces a magnetic field around it; a stationary charge does not create a magnetic field
- Magnetic field (symbol is B and SI unit is the Tesla or T
- the environment around a magnet in which
the magnetic forces act
- Magnetic field lines
- they represent the area around a
magnet; magnetic field lines flow from north to south
- Atoms of ferromagnetic materials act in groups called
domains; atomic magnets in each domain are aligned so that each domain is a microscopic bar magnet;
the domains align themselves with an external magnetic field
right-hand rules predict the direction of magnetic fields produced by a current. They are used for
conventional current flow. Use your left hand to predict the direction an electron or negative
charge would follow.
RHR #1 - Straight Wire Conductor
Curl the fingers of the right hand into the shape of a circle. Point the thumb in the direction of the current and the tips of the fingers will point in the direction of the magnetic field.
RHR #2 - Solenoid
Curl the fingers of the right hand in the direction of the current. Your thumb is the north
pole of the electromagnet.
RHR #3 - Magnetic Force
Extend the right hand so that the fingers point in the direction of the magnetic field and
the thumb points in the direction of the current. The palm of the hand then pushes in the direction
of the magnetic force.
Forces Due to Magnetic Fields
Ampere found that a force is exerted on a current-carrying wire in a magnetic field
F = B I L sin f
where B is the magnetic field in Teslas (T), I is the curent, L is the
length of wire in meters, and f is the angle. Only the perpendicular
component of B exerts a force on the wire.
We know how to measure force, current, and length. Thus B can be calculated by using
The force produced by a magnetic field on a single charge depends upon the speed of the charge,
the strength of the field, and the magnitude of the charge.
F = q v B sin f
where q is the charge in Coulombs and v is the velocity of the charge
The magnetic field at any point a distance R away from a straight-wire conductor can be calculated using,
Faraday found that a current could be induced in a wire by moving it in a magnetic field. An electric current is generated in a wire when the wire cuts across magnetic field lines.
- Electromagnetic induction
- process of generating a current by using a magnetic field
- emf = B L v sin f
- where emf is the potential difference measured in volts, v is the velocity with which the wire is moved through the magnetic field B, p is the angle at which the wire is moved in the magnetic field, and L is the length of the wire
- electromotive force (emf)
- a potential difference, measured in volts, that can cause an induced current to flow in a wire. It is not a force, but is a historical term coined before electricity was understood.
Electric Motors and Generators
- Electromagnetic Induction
- the process where current is produced when either a wire or a magnetic field move relative to one another; as long as the wire cuts across magnetic field lines during the motion, a current is produced.
- Electric motor
- uses electrical energy to produce
mechanical energy. In a motor, there must be a source of a magnetic field; brushes serve
as a connection to the split-ring commutator, allowing current flow from the motor to an
outside source. In order to continue rotating, current direction must be reversed. This is achieved
by the use of the split-ring commutator and the brushes. The force on a current-carrying wire in a
magnetic field causes an electric motor to rotate
- Electric generator
- uses mechanical energy to create
electrical energy; rotation of wire loop in a magnetic field causes current to be induced.
This current changes direction every 180 degrees, producing alternating current (AC current).
- Lenz's Law
- The direction of the induced current is
such that the magnetic field resulting from the induced current opposed the change in the flow (or
flux) that caused the induced current. It is the change in the flow or flux that causes the induced
current, not the flux itself.
- induced emf produced in a
coil by a changing current
- Mutual inductance
- a changing current in one coil induces
an emf in another coil
- an electrical device that increases or
decreases AC voltage; a step-up transformer has more turns in the secondary than in the primary;
a step-down transformer has more turns in the primary than in the secondary.
where N is number of turns, V is the voltage, and I is the current. s and
p stand for secondary and primary, respectively.
Magnetism Sample Problems