Magnetic Fields (1) Elliott

Basics Magnetic fields are different to other force fields because they are formed by dipoles. Electric fields are made using a single positive or negative point charge, and gravity is caused by point masses. You never get a magnetic monopoles. If you break up a magnet, you still get north and south poles:

Basics Unmagnetised materials are attracted to either pole. Like poles repel; unlike poles attract. Magnetic 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.

Earth’s Magnetic Field 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.

Magnetic Materials Only iron, cobalt, and nickel and their alloys are magnetic. these elements are next to each other in the periodic table. They are transition elements.

Attraction Vs Repulsion There is a neutral point where there is zero force.

Domain Theory of Magnetism Magnets 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.

Unmagnetised materials Domains all jumbled up. Currents caused by domains cancel one another out.

Partially Magnetised Materials Some domains line up. Partially magnetised material occurs.

Retaining Magnetism Some materials like soft iron lose their magnetism quickly.  These are used for temporary magnets.   Permanent or hard magnetic materials do not lose their magnetism

Magnetised Materials If the domains are fully lined up. The magnet is saturated, and cannot be magnetised further

Magnetic effect of an electric current Electric currents are always associated with magnetic fields.  The domains in a magnet are caused by the movement of electrons in shells.   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:

Interaction of Magnetic Fields Same direction Different Direction There is a neutral point between the two wires where the magnetic field cancels If the currents are in the opposite direction, there is repulsion. Notice that there is a resultant magnetic field. The two wires attract.

Important Magnetic field strength or flux density is a vector quantity. The direction is important Magnetic fields are three-dimensional, although we show them as two dimensional.

Magnetic Field of a Solenoid

If the current goes clockwise, we get a south pole If the current goes clockwise, we get a south pole. If the current goes anti-clockwise, it's a north pole.

Force on a Current Carrying Wire 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. A carbon rod is placed in the magnetic field of a large permanent magnet.  The brass rails connect the carbon rod to the power supply.  When a current flows through the carbon rod in the directions indicated by the arrows, the rod experiences a force and moves from left to right.  This shows that the current in the carbon rod makes a magnetic field that interacts with the magnetic field from the permanent magnet to produce a force.

Direction of the Force Flemming’s left hand rule.

Basic Rules Force is proportional to: The current; The strength of the magnetic field ; The length of wire within the magnetic field.

Associated Formula The term B is called the magnetic field strength, or the flux density, and is measured in Tesla, T. Flux density is a vector quantity 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.

Quick Question Explain why increasing the number of coils on a solenoid increases the Force felt from it’s magnetic field.

Answer Length is proportional to force. Increasing length increases force.

Check Your Progress The length of a 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?

Answer Use F = BIl 0.01 N = B ´ 2.5 A ´ 0.05 m B = 0.080 T

Fields at Angles If the field is at anything other than 90 degrees, you can use the formula.

Check Your Progress 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 if the wire is at an angle of 35o to the field?

Answer