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Electromagnetism

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David Burrows

on 28 October 2014

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Transcript of Electromagnetism

Electromagnetism
Electromagnets
Permanent magnets are, in general, not as useful as electromagnets because their magnetism cannot be turned on and off at will.
Main points
The main characteristics of the magnetic field in this situation are:

the field lines are circular and concentric around the wire

the strength of the field decreases away from the wire, that is, it decreases with radius, r, in metres

the direction of the field reverses if we reverse the direction of the current

the strength of the field is proportional to the magnitude of the current, I, in amps.
Equations
The magnetic field strength 'B' is calculated using current 'I', radius 'r' and the magnetic constant 'k' which is equal to 2.0 × 10^–7 N A^–2.

Don't confuse this with the electrostatic constant 'k' used in Coulomb's law.
An
electromagnetic field
exists wherever there is a
current flowing.
The effect was noted by
Hans
Christian Oersted
who observed compass needles influenced in the presence of an electric current. The needles formed tha same pattern found when using a permanent magnet.
The direction that the current flows makes a difference to the direction of the magnetic field and must be shown when drawing diagrams. An 'x' for current 'into the page' and a dot for current 'out of the page'
Right-hand Rule
The magnetic field direction can be easily remembered by making use of the "
Right-hand Rule
". This rule makes use of the right hand and
conventional flow of current
.

It should be noted at this point that all rules associated in physics with electromagnetism normally make use of your right hand. Always keep this in mind or else, if you use your left hand, you’ll be predicting the exact opposite of what occurs in nature.
(sorry, lefties)

Point your thumb along the direction of the current and then curl your fingers around the wire. The direction in which your fingers are pointing represents the direction of rotation of the magnetic field lines.
Multi-turn coils
This diagram shows a wire bent into a single circular loop. This loop can be considered as made up of many small, straight segments each adding its individual magnetic field together at the centre of the loop where the field will be the strongest and will be directed through
the loop as shown.

The direction is once again determined by the right-hand rule. The magnetic field strength at the loop’s centre is given by:

Example

In the figure, two separate wires are in close proximity. If wire A carries a current of 1.5 A and wire B carries a current of 2.5 A, calculate the value of the magnetic field strength at a point x between the two wires.
Note that when more than one wire exists, the magnetic fields have to be vectorially
added to find a resultant, such as is illustrated in this example.
example
Solenoids
Using a cylinder made of cardboard or plastic and winding many hundreds of turns of wire side by side, as shown, produces a device called a
solenoid
. The word solenoid comes from the Greek solen, meaning ‘tube’.

This concentrates the magnetic field lines into a region of space that produces an almost perfectly uniform magnetic field within the hollow body of the solenoid.
The magnetic field at the centre of a very long solenoid is constant and is found to depend only on the current flowing in the coil as well as the number of turns per unit length of the solenoid. This type of field is illustrated below and the formula for the magnitude of the field strength in the solenoid’s centre is:
where N is the number of turns; L is the coil length in metres.
The polarity of the solenoid’s magnetic field is often predicted with the
right-hand grip
rule
, which states that if you grip the solenoid in the right hand so that your fingers naturally
curl around the solenoid in the direction of conventional current flow then the thumb
extended will point to the effective north pole of the solenoid magnetic field.

The field lines are then drawn in such a way that they flow externally from the north pole toward the south pole at the coil’s opposite end.
The solenoid can be made into an electromagnet if the hollow core contains a magnetically
soft material. The core concentrates the lines of force and increases the magnetic
strength through the induction principle. Iron–nickel alloys are the most commonly used
material in the physical construction of electromagnet cores, where they can increase magnetic
field strengths several hundred times above that produced by the solenoid itself. The
greatest advantage of electromagnet assemblies is that the magnetic field can easily be
switched on or off simply by breaking the flow of current through the coil turns. They have
many practical applications.
Questions
Determine the magnetic field strength at a distance of 15 cm to the right of a wire if it carries an electric current of 5.5A north.
A solenoid has a length of 20 cm and contains 8000 turns. If it carries a current of 15 A, what is the magnetic field strength at the centre of the coil?
In the diagram, the direction of the magnetic field around a current-carrying wire is shown. If the magnetic field at point p is 1.5 × 10^–3 T and it is 1.0 cm from the wire, what is the magnitude and direction of the current in the wire AB?
Determine the direction and magnitude of the magnetic field at points P1 and P2 in the diagram shown in the diagram
You are now able to do Q2 1-4 of the Electromagnetism worksheet on Moodle.
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