TOPIC 6.3: Magnetic Fields and Forces

TOPIC 6.3: Magnetic Fields and ForcesThese notes were typed in association with Physics for use with the IB Diploma Programme by Michael Dickinson

6.3 Magnetic Force and Field6.3 Introduction.

We’ve already studied electric fields and seen that they exist in a region of space surrounding an electric chargeThis idea can be applied to magnetism.If iron filings are sprinkled on top of a bar magnet, they will show a pattern which traces the lines of magnetic force around the magnet.

6.3 Magnetic Force and Field6.3 Introduction.

The earth behaves like a massive magnet. The south pole of the magnet at the geographic north pole and visa versa. When a compass is positioned anywhere within the Earth’s magnetic field, the needle will orientate itself along the Earth’s magnetic field with it’s magnetic north pole directed towards the Earth’s geographic north pole.

6.3 Magnetic Force and Field6.3.1 State that moving charges give rise to magnetic fields. 6.3.2 Draw magnetic field patterns due to currents.

When an electrical current flows in a piece of wire then a magnetic field is produced around the wire. We can correctly predict the direction of the magnetic field using the “right hand grip rule”

6.3 Magnetic Force and FieldThe thumb points to the currentThe fingers show the direction of circular magnetic field.The space between the field lines increase with distance from the wire. Meaning a weaker field the further away.As the current increases the strength of the magnetic field increases.

6.3 Magnetic Force and Field6.3.3 Determine the direction of the force on a current-carrying conductor in a magnetic field.

When a current-carrying wire is placed in a magnetic field a magnetic force is produced. This usually causes either the magnet or conductor to move.The force will be perpendicular to the current and the magnetic field. We use “Fleming’s left hand rule”


This acronym might help: TFCHAND WIRE/MAGNETThumb Thrust (force)Fore finger Field (magnetic)Center finger Current


Look at the diagram and identify the direction of the Fmag

Answer: Down


Consider a dart or arrow moving away from you. You would see its tail end. If the dart was moving toward you , you would see its tip.


That means the picture from earlier could be drawn like this.

6.3 Magnetic Force and FieldPractice

6.3 Magnetic Force and Field6.3.6 Solve problems involving magnetic forces, fields and currents.

The size of the magnetic force, Fmag, is proportional to the strength of the magnetic field, B, the size of the current, I, and the length of the wire (that is in the magnetic field), L.

Formula

F = BIL

Magnetic field strength, B, is measured in Tesla, T. Magnetic force, F, is measured in Newtons, NCurrent, I, is measured in Amps, ALength of wire, L, measured in Meters, m

6.3 Magnetic Force and Field6.3.6 Solve problems involving magnetic forces, fields and currents.I SHOW YOU

Sample Problem BA wire 36m long carries a current of 22A from east to west. If the magnetic force on the wire due to Earth’s magnetic field is downward (toward Earth) and has a magnitude of 4.0 x 10-2N, find the magnitude of the magnetic field at this location.Answer: 5 x 10-5 T to the west

6.3 Magnetic Force and Field6.3.6 Solve problems involving magnetic forces, fields and currents.WE TRY TOGETHER

Practice 13A piece of copper wire is held perpendicular to a magnetic field of strength 0.25 Teslas. The length of the conductor within the field is 10cm. If a current of 8 Amps is allowed to flow in the wire, what is the force on the wire?Answer: 0.2 N

6.3 Magnetic Force and Field6.3.6 Solve problems involving magnetic forces, fields and currents.YOU DO

Practice B #2-4 pg 692 in book

Answers:1) 1.7 x 10-7 T in the +z direction2) 0.050 T3) 1.5 T4) 0.59 m

TOPIC 6.3: Magnetic Fields and Forces

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