Magnetic Fields and Forces

  • EM6.1 Bouncing Spring over Cup Hg

    A DC power supply is connected to a spring at one end and grounded in a cup of Hg. The spring is suspended above the cup and will begin to oscillate when the battery is turned on as contact is made and broken. SAFETY IS OF THE UTMOST CONCERN WHEN HANDLING MERCURY.

  • EM6.2 Cathode Ray Tube/Evacuate Tubes

    This is also done in lab. The electrons of a cathode ray tube are deflected by electric and magnetic fields. Turn the CRT on and isolate a spot on the screen. Bringing a magnet close to the CRT will move the spot.

  • EM6.3 Current Balance

    This is two wires parallel to each other carrying current in the opposite directions. By the law of Biot-Savart, the magnetic field produced by each wire is equal in magnitude and in the same direction so that the wires repel each other. Placing a mirror on the edge of the top wire and pointing a laser at the mirror will show the repulsion on the lecture hall's side wall. Extensions of this could be discussions of the definition of ampere and the calibration of an ammeter. After plugging the apparatus in, allow it to warm the gallium swivel so that it can rotate easily.

  • EM6.4 Current Carrying Wire above a Compass

    This is better known as Oersted's discovery. A wire is connected to a battery. Placing the compass on a vu graph the class can see that placing the wire parallel to the compass needle does not affect it, but placing in perpendicular to the direction of the needle will rotate it 90 degrees.

  • EM6.5 Field Mapping with Iron Filings and Compass

    Classic, nonetheless visually stunning. Place magnets in various orientations underneath a sheet of paper, and sprinkle iron filings on top of the paper. Tap the paper to force the filings to align with the field. Make sure not to allow filings to come into contact with the magnets.

  • EM6.6 Magnetic Domains

    This is an array of magnetized arrows. A magnet can be brought near to change the direction of the arrows. They align themselves according to the magnetic field. However, they reform into domains when the field is removed. Once arranged, the arrows will only realign when another magnet is brought near. This can be used with a Helmhotz coil on the vu graph to create a hysteresis representation.

  • EM6.7 Magnetic Forces

    This is a general magnetic interaction. A magnet is hanging in the air. See how the magnet responds when another magnet and a piece of iron are brought nearby . Place a solenoid below the magnet and attach it to a power supply. See how the magnet reacts when the power supply is turned on. See how the magnet reacts when the power supply is turned on. The same could be done with a suspended solenoid connected to a big galvanometer. See how the current changes when a magnet is brought nearby and inserted, when another solenoid is turned on below it, and how the iron's presence affects it.

  • EM6.8 Measurement of B

    The strength of the magnetic field of an electromagnet can be measured using an ammeter, a meter stick, and a spring balance via the equation B=F/(Il).

  • EM6.9 Model of Magnetic Fields

    These are small models of magnetic fields. Mainly they are compasses arranged to align themselves with magnetic fields.

  • EM6.10 Current Carrying Wire in a Magnetic Field

    Using a power supply (Shelf C1-1), apply a current through a wire suspended between two poles of a magnet. Magnet poles are labeled, so students should predict which way the wire will deflect (up or down) with the current is increased from zero. Switch the polarity to watch the wire deflect the other way.

  • EM6.11 Varying B Field

    This is a rod of magnetic material spinning about its center. Iron filings on top will keep rearranging themselves. The pattern is supposed to be reminiscent of a nebula. Its success is questionable, though it does create rather interesting patterns.

  • EM6.12 Currents Producing Magnetic Fields

    Use with an overhead transparency projector, iron filings and power supply (Shelf C1-1). By driving a current through a wire, the magnetic field can be observed using iron filings. Different wire configurations are available - perpendicular to the plane (resulting B field is circumferential), suspended above and parallel to the plane (field points either left or right depending on direction of current), solenoidal (field is vertical inside the coil), toroidal (field is circular inside the coils). An alternative to this demo can be used well with the document camera - involves a perpendicular wire, and several strategically placed compasses to show the field circling the wire.

  • EM6.13 Magnetic Forces on Wires (PASCO)     Magnetic forces

    Magnets are mounted on an iron yoke and placed on a balance (resolution of at least 0.01 g - use Ohaus centigram balance on shelf H1-2). One of the conducting paths is suspended between the magnets. The balance is used to measure the mass of the magnets and yoke prior to any current passing through the conducting path. Current is then passed through the conducting path, producing a force. The change in reading on the balance can be converted to find the magnetic force between the conductor and magnetic field. Conductors of different length are included to measure the effect of length on magnetic force. Magnetic field can be varied by changing the number of magnets in the yoke. The power source is used to change the current supplied to the conductor.