Q4 Physics

• Facts about Magnetism


  • Magnets have 2 poles (north and south)
  • Like poles repel
  • Unlike poles attract
  • Magnets create a MAGNETIC FIELD around them
• Magnetic Field


  A bar magnet has a magnetic field around it. This field is 3D in nature and often represented by lines LEAVING north and ENTERING south
• To define a magnetic field

  You need to understand the MAGNITUDE and DIRECTION
• 1. Field


  The symbol for a magnetic field, with the TESLA (T) as the unit
• Magnetic Force on a moving charge

  If a MOVING CHARGE moves into a magnetic field it will experience a MAGNETIC FORCE. This deflection is 3D in nature.
• The conditions for the magnetic force are: Must have a magnetic field present, Charge must be moving, Charge must be positive or negative, Charge must be moving PERPENDICULAR to the field.
• Example 1


  • A proton moves with a speed of 1.0x10^5 m/s through the Earth's magnetic field, which has a value of 55 μT at a particular location. When the proton moves eastward, the magnetic force is a maximum, and when it moves northward, no magnetic force acts upon it. The magnitude of the magnetic force acting on the proton is 8.8x10^-19 N. The direction cannot be determined precisely by the given information.
• Example 2


  • A proton moves east with a speed of 4.0 x 10^6 m/s in a magnetic field of 2.0 x 10^-4 T directed into the page. The magnitude of the magnetic force acting on the proton is 1.28 x 10^-14 N.
• Right Hand Rule


  To determine the DIRECTION of the force on a POSITIVE charge, hold your right hand flat with your thumb perpendicular to the rest of your fingers. The Fingers = Direction B-Field, The Thumb = Direction of velocity, The Palm = Direction of the Force. For NEGATIVE charges use left hand!
• Magnetic Force and Circular Motion

  Suppose we have an electron traveling at a velocity, v, entering a magnetic field, B, directed into the page. The magnetic force is equal to the centripetal force and thus can be used to solve for the circular path. Or, if the radius is known, could be used to solve for the MASS of the ion. This could be used to determine the material of the object.
• Example 3


  • A singly charged positive ion has a mass of 2.5 x 10^-26 kg. After being accelerated through a potential difference of 250 V, the ion enters a magnetic field of 0.5 T, in a direction perpendicular to the field. The radius of the path of the ion in the field is 0.0177 m.
• Velocity Selector


  When you inject the sample you want it to go STRAIGHT through the plates. Since you have an electric field you also need a magnetic field to apply a force in such a way as to CANCEL out the electric force caused by the electric field.
• Charges moving in a wire

  If the wire had a CURRENT (moving charges), it too will experience a force when placed in a magnetic field. You simply used the RIGHT HAND ONLY and the thumb will represent the direction of the CURRENT instead of the velocity.
• The MAGNETIC FIELD is being produced by some EXTERNAL AGENT
• Example 1


  • A 36-m length wire carries a current of 22A running from right to left. The magnitude of the magnetic force acting on the wire is 0.0396 N, and the direction is +y (up the page).
• Example 2


  • A current of 20 A flows east through a 50 cm-long wire. A magnetic field of 4.0T is directed into the page. The magnitude of the magnetic force acting on the wire is 4.0 N.
• The wire itself must be magnetic, as the wire has its own INTERNAL MAGNETIC FIELD that is attracted or repulsed by the EXTERNAL FIELD.
• A current carrying wire's INTERNAL magnetic field


  To figure out the DIRECTION of this INTERNAL field you use the right hand rule. You point your thumb in the direction of the current then CURL your fingers. Your fingers will point in the direction of the magnetic field.
• The MAGNITUDE of the internal field


  The magnetic field, B, is directly proportional to the current, I, and inversely proportional to the circumference.
• Example 2


  • A vertical wire carries a current of 45A due south. The magnitude of the magnetic field 2cm to the right of the wire is 2.8 x 10^-5 T, and the direction is counterclockwise.
• Example 3


  • A wire carries a current of 10A. The magnetic field of 8.0x10^-4 T will be produced at a distance of 12.5 mm from the wire.