SPH4U – Grade 12 Physics Unit 1 Electromagnetism 2 SPH4U – Grade 12 Physics Unit 1

Tribal Challenge Determine the direction of the electric current in (a) and the force on the wire in (b) a) b)

Tribal Challenge Determine the direction of the electric current in (a) and the force on the wire in (b) a) b) N

Review – Mini Quiz from last day Moving electric charges will create a magnetic field (T / F) Moving magnetic fields will create an electric current (T / F) The magnetic field around a solenoid resembles a bar magnet (T / F) A current carrying wire in a magnetic field will experience a force if you place it in an external magnetic field. (T / F)

Review – Mini Quiz from last day Moving electric charges will create a magnetic field (T / F) Moving magnetic fields will create an electric current (T / F) The magnetic field around a solenoid resembles a bar magnet (T / F) A current carrying wire in a magnetic field will experience a force if you place it in an external magnetic field. (T / F)

Calculating Magnetic Force From last day, we learned that a current carrying wire would have it’s own magnetic field, and that it would experience a force in the presence of an external magnetic field. Today we will discuss how to calculate that force.

Measuring Magnetic Field Strength  

Measuring Magnetic Field Strength Some common magnetic fields: common refrigerator magnet: 0.001 Tesla magnetic field near the Earth’s surface: 5 x 10-5 Tesla

The force on a moving charge As we discussed last day, a current carrying wire will experience a force in the presence of a magnetic field. This is because there are moving electrons that make up the current that cause another magnetic field to be created. We said that this is because electricity and magnetism are both fundamentally caused by electric charges.

The force on a moving charge In truth, magnetic forces act on all types of moving charges: electrons, protons, and ions. When one of these moves in the presence of a magnetic field, there will be a magnetic force exerted on it, as long as the charge is in motion.

The force on a moving charge  

The force on a moving charge  

The force on a moving charge Right hand rule: If you point your right thumb in the direction of the velocity of the charge and your straight fingers in the direction of the magnetic field then your palm will point in the direction of the resulting magnetic force. This gives the direction when the charge is positive. If the charge is negative, the particle will go in the opposite direction.

The force on a moving charge The magnetic force will always be directed at a 90º angle to the velocity of the particle and the magnetic field. That is independent of θ, the angle between the magnetic field and the velocity of the particle. The angle θ only helps determine the magnitude of the force.

Example 1 An electron moving at a velocity of 6.7x106 m/s [E] enters a magnetic field with a magnitude of 2.3T directed at an angle of 47º to the direction of motion and upward in the vertical plane. Find the magnitude and direction of the magnetic force on the electron.

Example 1 An electron moving at a velocity of 6.7x106 m/s [E] enters a magnetic field with a magnitude of 2.3T directed at an angle of 47º to the direction of motion and upward in the vertical plane. Find the magnitude and direction of the magnetic force on the electron.

Magnetic force on Current Carrying Conductor Since a current carrying wire is a collection of moving charges, some of the same principals we have just been discussing for a single charge apply to the situation where we have a current running through a wire.

Magnetic force on Current Carrying Conductor  

Magnetic force on Current Carrying Conductor  

Magnetic force on Current Carrying Conductor Much like with charges, this formula only tells us the magnitude of the force. Also, the angle θ only determines the magnitude of the force, not the direction. The direction of the force is determined by the right hand rule which we discussed last day: If the fingers of your open right hand point in the direction of the external magnetic field, and your thumb points in the direction of the conventional current, then your palm faces in the direction of the force on the conductor.

Magnetic force on Current Carrying Conductor Some implications: A magnetic field does not exert a force on a current moving parallel to the direction of the magnetic field. (This would make sinθ = 0 in our formula, cancelling the force out). The magnetic force is greatest when the current moves perpendicular to the magnetic field. (sinθ = 1).

Example 2 Earth’s magnetic field exerts a force of 1.4x10-5 N on a 0.045 m segment of wire in a truck motor. The motor wire is positioned at an 18° angle to the Earth’s magnetic field, which has a magnitude of 5.3x10-5 T at the truck’s location. Calculate the current in the wire.

Example 2 Earth’s magnetic field exerts a force of 1.4x10-5 N on a 0.045 m segment of wire in a truck motor. The motor wire is positioned at an 18° angle to the Earth’s magnetic field, which has a magnitude of 5.3x10-5 T at the truck’s location. Calculate the current in the wire.

Video Another recommended video by Derek Owens: “Magnetic Force on Moving Charges” - http://www.youtube.com/watch?v=XWkhUwX4D5s “The Motor Effect” - http://www.youtube.com/watch?v=3BYXuYybaNg

Cool Stuff Video of ferromagnetic liquid (ferro fluid) http://www.youtube.com/watch?v=PvtUt02zVAs

Homework Read Sections 8.2, 8.3 (again) Make additional notes to supplement the lesson notes. Complete the following questions: Pg. 391 # 3, 4, 5, 7 Pg. 396 # 1, 2, 4