TOPIC 6.3: Magnetic Fields and Forces
6.3 Introduction.
The earth behaves like a massive magnet.
The south pole of the magnet at the geographic ______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.
6.3.1 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 ______the wire.
We can correctly predict the direction of the magnetic field using the “______”
The ______points to the current
The 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.
A ______is a long wire wrapped around a metal core which produces a magnetic field when electric current is passed through it. They are important because they create controlled magnetic fields and can be used to convert energy into motion.
The fingers point in the direction of the ______. The thumb points in the direction of the magnetic field lines.
6.3.3 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 ______is produced. This usually causes either the magnet or conductor to move.
The force will be ______to the current and the magnetic field.
We use “Fleming’s ______”
This acronym might help: TFC (or FBI)
HAND / WIRE/MAGNETThumb / ______(force)
Fore finger / ______(magnetic)
Center finger / ______
Look at the diagram and identify the direction of the Fmag
Answer:
Drawing 3-D sketches can get confusing some times, so there is a convention used. 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.
6.3.4 Determine the direction of the force on a charge moving in a magnetic field.
When a charged particle moves in a magnetic field it will feel a force. Current is a ______charge.
Conventional current is in the ______direction to the flow of electric charge.
When a charge enters the magnetic field it will feel a force.
This force causes its direction to change. As the direction of motion changes so does the force and it creates a ______motion.
Let’s look at this diagram starting at the top.
The size of the magnetic force, Fmag, is proportional to the strength of the magnetic field, B, the velocity of the charge, v, and the size of the charge, q.
IB Equation: F = ______
Sample Problem A
A proton moving east experiences a force of 8.8 x 10-19N upward due to the Earth’s magnetic field. At this location, the field has a magnitude of 5.5 x 10-5T to the north. Find the speed of the particle.
Answer: 100,000 m/s
Practice A #1-3, pg 689 in book
6.3.5 Define the magnitude and direction of a magnetic field.
Magnetic field strength, B, is measuered in Tesla, T. It is a vector quantity, so if two magnetic fields interact, vector addition must be used to calculate the resulting magnetic field.
The size of the magnetic force, Fmag, is proportional to the ______of the magnetic field, B, the size of the current, I, and the length of the wire (that is in the magnetic field), L.
“sin θ” takes in account for the cases where the conductor and the magnetic field are not perpendicular. Normally when they are perpendicular, the angle between them is ______and the sinθ disappears.
Equation: F = ______
Sample Problem B
A 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 and direction of the magnetic field at this location.
Answer: 20 T to the west
Practice B #2-4 pg 692 in book
6.3.6 Solve problems involving magnetic forces, fields and currents.
Practice 13
A 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: 0.2N
Practice 14
The same piece of wire is now tilted so that it makes an angle of 30º to the magnetic field. What is now the force on the wire?
Answer: 0.1N