ElectroMag

Magnetism

1. Magnetism

Magnetic poles exist in pairs.

breaking a magnet, there is no magnetic monopole, even down on the atomic scale there is no magnetic monopole, field lines heal themselves

Only some materials experience the force of magnetism.

types of magnets

permanent
induced
electromagnet

A material that has become magnetized has a direction of magnetism, this goes from one side (pole) of a magnet to the other.

Opposite poles attract, like poles repel.

Magnetic effect of an electric current.

Demonstration using magnets, coils, and nails.

The Magnetic Field

*A magnetic field is the space around a magnet where certain materials and other magnets will experience a magnetic force.

A magnetic Field line,

joins places of equal magnetic attractive force. These lines always go from North to South. When drawing these lines place an arrow showing this direction. N → S

Magnetics Field lines around a wire wally

2. Magnetic fields

Magnetic field due to

• magnets

• current in

- a long straight wire Magnetic Field lines around a wire wally

- a loop

- a solenoid.

Description without mathematical details.

Vector nature of magnetic field to be stressed.

Demonstrations.

Electro Magnetism

How Electromagnet Works?

Electric current when passes through the soft iron core, produces an energy which is called magnetic flux. If insulated wire is wrapped around an iron or steel object, a powerful magnetic field is produced. When electricity is passed through a coiled wire a stronger magnetic field is generated. The strength of the magnetic field depends upon and is directly proportional to the number of coils, the strength of the current, and the magnetic permeability of the core material. Apart from these factors, the number of turns made by the coil will determine the strength of the field.

magnetic effect of electricity

Battery 9V approx

switch

3m of single core wire (bell wire)

needle and thread

tube with (diameter 50mm - 100mm) (2 litre cyclinder bottle of pop)

2 small (but similiar) pieces of wood to act as a base

To make the solenoid wrap the wire around the bottle a nnumber of times, taking care to leave 100mm at both ends to connect to the battery.

Use tape (or wax?) to keep the solenoid from unravelling.

remove the mould, and using the thread suspend the needle within the loop (prefereably in the middle)

connect the wire via the switch to the battery.

allow the needle to settle (stop swinging about)

close the switch (allow the electricity to go thru the solenoid)

observe

extensions

reverse the polarity of the battery. what do you expect? what happens?

make different diameter solenoids. what do you expect? what happens?

use different voltage size batteries what do you expect? what happens?

use a hall probe to measure the magnetic flux density what happens?

Earth’s magnetic field.

Humans have used compasses since the 11th century to find direction, and since the 12th century for navigation.

Inclination can be measured with a dip circle

3. Current in a magnetic fieldCurrent-carrying conductor experiences a force in a magnetic field.

Lorentz Force by wally

Direction of the force.

Force depends on

• the current

• the length of the wire

• the strength of the magnetic field.

F ∝ I l B

Magnetic flux density B =

Derivation of F = qvB

Appropriate calculations.

Forces between currents (non-mathematical treatment).

Demonstration of the force on a conductor and coil in a magnetic field.

Appropriate calculations.

Alpha particles deflect upward in this field obeying the right hand rule of a positively charged particle. Beta particles deflect the opposite way indicating negative charge. Gamma particles are unaffected by the field and so must carry no charge.

Applications in motors, meters, and loudspeakers.

Definition of the ampere.

4. Electromagnetic induction

Magnetic flux: Φ = BA

Faraday’s law.

Lenz’s law.

http://www.magnet.fsu.edu/education/tutorials/slideshows/eddycurrents/index.html

Interesting and recent, cool understanding about the induced magfield and energy

https://www.youtube.com/watch?v=8a3r-cG8Wic

Change of mechanical energy to electrical energy.

http://science.howstuffworks.com/electricity3.htm

Demonstration of the principle and laws of electromagnetic induction.

Appropriate calculations.

Application in generators

5. Alternating current

Variation of voltage and current with time, i.e. alternating voltages and currents.

Use oscilloscope to show a.c.

Peak and rms values of alternating currents and voltages.

from http://www.pcbheaven.com/userpages/Fluorescent_to_LED_Lamp_Mod/

Where do those 34 volts come form?

If you ask "where do those 34 volts come from", here is the quick answer. When we say that this transformer is rated at 2x12VAC, we mean 2 x 12 Vrms. When this voltage is rectified to a DC voltage and smoothed with a capacitor, it will be higher because we then talk about peak voltage. The formula to convert RMS to Peak voltage is simple:

Vpeak = Vrms/0.707

So, in our case:

Vpeak = Vrms/0.707 = 2x12/0.707 = 24/0.707 => Vrms=33.9 Volts

Compare Peak and rms values of alternating currents and voltages.

National grid and a.c.

6. Concepts of mutual induction and self-induction

Mutual induction (two adjacent coils): when the magnetic field in one coil changes an emf is induced in the other, e.g. transformers.

Demonstration

Self-induction: a changing magnetic field in a coil induces an emf in the coil itself, e.g. inductor.

Demonstration

Structure and principle of operation of a transformer.

Effects of inductors on a.c. (no mathematics or phase relations).

Capicitor and Inductance circuits by wally

Oscillating em waves wally

Demonstration.

Appropriate calculations. (voltage)

Demonstration.

Uses of transformers.

Dimmer switches in stage lighting – uses of inductors.