Magnetism
Source of all magnetism = moving charges
domain - a cluster of electrons all spinning in the same direction
*Not magnetized if the domains aren't all facing the same direction.
Why does a paper clip stick to a magnet?
A domain is a cluster of electrons that are spinning in the same direction. The domains in a paper clip are randomly aligned. The magnet has a magnetic field, and when the magnet is close to the paper clip the domains of the paper clip align to match the magnetic field lines of the magnet. The paper clip now has a north and a south pole, and since opposite poles attract the north pole of the paper clip is attracted to the south pole of the magnet, and thus the paper clip sticks to the magnet.
compass - a magnet that is free to move and respond to a magnetic field.
*A compass' needle only moves when it encounters a different magnetic field, and it will align with that magnetic field.
Magnetic fields can exert forces on other things (usually magnets) with magnetic fields.
*symbol for magnetic field = b
Motors
A motor relies on current bearing wires and magnets.
How a motor works = a current carrying wire feels a force in a magnetic field and causes a torque
All moving charges feel a force in a magnetic field if their velocity is moving perpendicular to the magnetic field. If it's moving parallel it won't feel a force, which is how things enter our atmosphere.
*This is why the equator is generally shielded from cosmic rays entering, and the northern countries aren't.
Current is moving perpendicular to the magnetic field (the charges have a force from the magnetic's magnetic field).
Electromagnetic Induction
Electromagnets have a current carrying wire that has a magnetic field. The domains of an unmagnetized object can align with that field and then have a magnetic field of its own.
Electromagnetic induction - when you bring a coil of wire near a magnet, the magnetic field of the coil will change. The induces a voltage (from electromagnetic difference) and that causes a current.
*Used in traffic lights, airport metal detectors, credit card machines, generators, transformers, etc.
*The # of loops is directly proportional to the amount to current (2 loops = 2x current). The greater the number of loops, the greater the induced voltage.
Voltage is caused, or induced, by the relative motion between a wire and a magnetic field. The magnetic field of a magnet moves near a stationary conductor or vise versa - the conductor moves near a stationary magnetic field.
1) move the loop near a magnet
2) move the magnet near a loop
3) change the current in a nearby loop
The amount of current produced by electromagnetic induction depends not only on the induced voltage, but also on the resistance of the coil and the circuit to which it's connected.
Faraday's Law: The induced voltage in a coil is proportional to the product of its number of loops and the rate of which the magnetic field changes within the loops.
How do credit card machines work?
Credit card machines work by electromagnetic induction, which is when you bring a coil of wire near a magnet, the magnetic field of the coil will change. The induces a voltage (from electromagnetic difference) and that causes a current. In the machine are several loops of wire, and when the series of magnets passes through the coil a voltage is induced, which causes a current. This current acts as a signal to approve/decline the card.
Transformers
Transformers use electromagnetic induction to either step-up or step-down the voltage.
The primary (1) is connected to a power source.
*more loops = more V induced
*more length = more resistance
As the I changes direction the magnetic field in the primary constantly changes direction as well, and that changing b of the primary induces a voltage in the secondary and will cause a current in the secondary.
You can't use DC current in a transformer and have to use AC current, because you need a constantly changing magnetic field, to induce the voltage thus to cause a current, so you need an alternating current.
Generators
Generators rely on turning objects.
1) magnets
2) coils of wire
When one end of magnet is repeatedly plunged into and back out of a coil of wire, the direction of the induced voltage alternates (AC current).
Both motors and generators use coils of wire and magnets to transform energy from one form to another, but their techniques are different.
A motor transforms electrical energy into mechanical energy. It uses a current carrying wire that feels a force in a magnetic field and that force causes a torque.
A generator transforms mechanical energy into electrical energy. It relies on electromagnetic induction: a spinning wire over a magnet or vice versa. This creates a charge in magnetic field, induces a voltage and then causes a current.
No comments:
Post a Comment