Magnetic Fields Tutorial 1 - Magnetic Fields
On A Current Carrying Wire In A Magnetic Field
You will be familiar with the basic notion of a magnetic field, in which magnetic materials experience a magnetic force. However it is worth revising some of the basic ideas that you will have come across in early secondary school.
fields can be shown by field lines, which go from North to South.
The field lines in a strong magnetic field are more closely packed than in a weak field.
materials are attracted to either pole.
poles repel; unlike poles attract.
In the Earth’s magnetic field, the North pole will align itself to point to the North, if the magnet is allowed to swing freely.
The Earth has a magnetic field like a bar magnet. Notice that the S-pole is under the North geographic pole. Be careful not to be confused by this.
never get single magnetic poles; if there is an N-pole, there must also be
an S-pole to go with it.
are thought to result from the action of tiny atomic magnets called domains.
This can be explained by the movement of electrons that represent a tiny
electric current that results in magnetism. In most materials, the
currents cancel out.
iron, cobalt, and nickel and their alloys are magnetic.
a magnetic material is unmagnetised, the
domains are all jumbled up. If some
of the domains are lined up, then the
material is partially magnetised. If
the domains are fully lined up, the magnet
is saturated, and cannot be magnetised
materials like soft iron lose their
magnetism quickly. These are used for temporary
magnets. Permanent or hard
magnetic materials do not lose their magnetism.
Electric currents always produce a magnetic field, even if the wire itself is not made of a magnetic material. The magnetic field of a single current carrying wire is like this:
direction of the current is determined by the Screwdriver
The magnetic field of a solenoid is like a bar magnet.
You will be familiar with the motor effect. If we put a current carrying wire in a magnetic field, we see that there is a force.
can work out the force that is exerted on the wire quite simply.
Experiment shows us that the force is proportional to:
strength of the magnetic field
length of wire within the magnetic field.
is summed up in a simple formula:
F = BIl
[B – magnetic field strength;
– current in A;
– length in m]
is called the
magnetic field strength,
or the flux density, and is measured
in Tesla, T. The magnetic flux
density can be thought of as the concentration of field lines.
We can increase the force by increasing any of the terms within the
equation. If we coil up the wire,
we increase its length within the magnetic field.
|A current of 8.5 A flowing through a magnetic field is found to exert a force of 0.275 N. The length of wire in the magnetic field is 5 cm. What is the value of the magnetic field?|
|Formula first: F = BIl.|
B = F
= ___0.275 N ___ = 0.647
Il 8.5 A ´ 0.05 m
In a demonstration of the above equation, the length of wire in a magnetic field is 0.05 m. When a current of 2.5 A flows, a force of 0.01 N is shown. What is the magnetic field strength?
This relationship holds true as long as the current is at 90o to the magnetic field. If the wire is at an angle to the field, the relationship takes this into account by changing to: