Motors and Generators

Check Your Learning FOR THESE QUESTIONS ASSUME DIRECTIONS ARE IN A FLAT PLANE. 1.A proton is travelling South in a magnetic field that is directed upward. What is the direction of the force on the proton? a.down b.East c.West d.South The answer is c.

Check Your Learning 2.An electron is travelling upward through a magnetic field that is directed west to east. The force on the electron will be a.East b.North c.South d.Zero The answer is c – remember that it is an electron so if using your right hand your thumb must point downward.

Check Your Learning 3.A proton is travelling west in a magnetic field that is directed east. The force on the proton is a.East b.North c.South d.Zero The answer is d – there is no force since the proton is travelling parallel to the magnetic field.

Check Your Learning 4.A wire is carrying a current across this page in a magnetic field that is directed down the page. The force on the wire is into the page. The current must be a.Left to right b.Right to left c.Zero The answer is a.

Motors A motor is a device that will convert electrical potential energy into kinetic energy Motors work on the principle of torque on a coil (remember the RHT rule 3 example?) –When a coil of wire carrying a current is placed in a magnetic field it will experience force. –If the coil is free to rotate, it will, as the force exerts a torque.

Motors All electric motors contain 2 key components –A coil of wire –Permanent magnets Kinetic energy is possible because when an electric current occurs in a magnetic field, the induced magnetic field in the wire will interact with the magnetic field

How does a motor work? ors.htmlhttp:// ors.html

The Ampere Ampere theorized that the magnetic fields between two charge carrying wires would either result in attractive or repulsive forces Based on this, Ampere was able to develop an equation to describe the magnetic field:

Calculating Motor Force (F M ) Where F M = motor force (N) B ┴ = magnetic field intensity in Tesla (T) (must be perpendicular component) I = current in amperes (amps, A) L = length of conductor in the magnetic field (m)

Motor Force Note 1: For a looped conductor (ie, coil): L = nC Where n = number of loops (an integer) C = circumference of one loop Note 2: If the magnetic field is not perpendicular to the current, you need to modify the equation:

Example 1 A conductor carries a current of 4.8A to the right through a magnetic field pointing into the board. The length of wire inside the field is 25cm and it experiences a force of 0.60N. What is the intensity of the field? What direction is the force in?

Answer Force points up.

Example 2 A 3.2 cm wire is carrying a 34 mA current as shown in the magnetic field below that is directed to the right. The strength of the magnetic field is 0.23 T. Find the force (magnitude and direction) on the wire.

Solution The magnetic field is to the right, and a component of the current is up the page. Using the third right hand rule, Fingers point to the right (with the magnetic field) Thumb points up the page Palm is left facing into the page The force is therefore into the page.

Example 5 (from other sheet) A strong current is suddenly switched on in a wire, but no force acts on the wire. Can you conclude that there is no magnetic field at the location of the wire? No. It is possible that there is a magnetic field but that it is parallel to the wire. There is no force when a magnetic field and a wire carrying a current are parallel.

Practice Problems Page 778, questions 1 to 4 Worksheet (given out on Friday): Questions 7, 8, 9 (current is 1.9A – we no longer cover this section of the textbook)

Calculating Motor Force on Moving Charged Particles This is how devices like particle accelerators work. Any moving charged particle generates a magnetic field (like electrons moving in a wire – current). IzRz_f8https:// IzRz_f8

Motor Force on Moving Charged Particles Where F M = magnetic force (N) q = charge of particle (C) B ┴ = magnetic field intensity (T) v = speed of particle (m/s)

Example 3 An electron moving at 1.0 x 10 5 m/s (initially moving to the left) enters a 0.25T field pointed out of the board. Calculate the force on the electron and draw its path through the field. ** NOTE: If the moving particle is negative, you must point your thumb in the direction opposite the motion of the particle.

Answer UP

Example 4 A particle, moving with a velocity of 8.00×10 4 m/s at an angle of 30’ with respect to a magnetic field of 5.60 × 10 −5 T, experiences a force of 2.00×10 −4 N. Calculate the magnitude of the particle’s charge. (NOTE: use sinθ to get perpendicular angle)

8.93 x C

Practice Problems Worksheet: Questions 11, 12, 13