Pre-AP Physics Chapter 20 Magnetism Pre-AP Physics Chapter 20

Magnets and Magnetic Fields A magnet will attract objects containing iron. A magnet has two poles, North and South. The end that points north on a compass is truly the north-seeking pole. Magnets exert forces. Like poles repel and opposite poles attract. Magnetic poles are not like electric charges in the fact that electric charges can be isolated; magnetic poles cannot.

Only iron, cobalt, nickel, and gadolinium show strong magnetic effects Only iron, cobalt, nickel, and gadolinium show strong magnetic effects. They are said to be ferromagnetic. There is a magnetic field, B, surrounding the pole of a magnet. This field can be represented by field lines just as electric fields were represented by electric field lines. The direction of a magnetic field is the direction that the north pole of a compass needle would point if placed in it. Thus field lines will point away from north and toward south.

The Earth is a huge magnet The Earth is a huge magnet. Its geographic north pole is a south pole magnetically and vice versa. It is called the north magnetic pole. The Earth’s poles do not coincide with the magnetic poles. There is a magnetic declination, or angle, between the magnetic north and the true north directions. It varies from 0o to 25o depending on the point of reference.

Electric Currents Produce Magnetism In 1820, Oersted found that when a current flows through a wire, a compass needle will be deflected when placed near the wire. Electric current produces a magnetic field. A right-hand rule will help you remember the direction of field lines when considering a current-carrying wire. Grasp the wire in your right hand with the thumb pointing in the direction of conventional current flow. The fingers then point in the direction of magnetic field.

I, current This method also works for a circular current-carrying wire.

Another right hand rule helps determine the directions of magnetic force, conventional current and magnetic field.

The right hand rule tells you the direction a magnetic field (B) will move an already moving charge. Your right hand needs to be open, with your thumb pointing at 90 degrees to your fingers. The fingers point in the direction of the magnetic field (which always points from the north pole of a magnet to the south pole). The thumb points in the direction a positive charge is moving. These charges could be a single charge, a bunch of charges (known as current, I), or a wire being pushed (a wire has charges in it, so moving a wire moves the charges in the wire). All of these examples are all v (thumb).

The palm points in the direction the magnetic field moves the charge The palm points in the direction the magnetic field moves the charge. This is the Fmag, or magnetic force. Fmag MUST BE the direction B moves the charge (or wire) NOT an external force. A current flowing in a wire, due to an external voltage supply is not Fmag; it is the direction of v (moving charges). However, the direction the current carrying wire moves due to B IS Fmag (your palm). Also, when a wire is moved into B, the charges in the wire (q) move due to B, causing an induced current in the wire (Fmag). Two notes: 1) If the charge isn't moving, there is no Fmag. 2) The Right Hand Rule works only for positive charges (protons and conventional current, which flows from positive to negative). If you are asked to find Fmag for an electron or negative current, use your left hand.

Force on an Electric Current in a Magnetic Field Since an electric current exerts a force on a magnet field, then by Newton’s third law, a magnet should exert a force on a current-carrying wire. The electric force will be perpendicular to the magnetic field direction and the current direction. F = I l B sinθ where I is current, l is the length of wire in the field, B is the magnitude of the field and θ is the angle between the current and the magnetic field. Note: Maximum force is obtained when θ equals 90o and zero when θ equals 0o.

Units of magnetism The SI unit for magnetic field is tesla (T) which is also known as a weber per meter squared. Another unit is the gauss (G) which is defined as 10-4 T = 1 G

Force on a moving electric charge F = qvB sin θ where q is the charge, v is the velocity of the charge and B is the magnetic field. Θ is the angle between v and B. The force is greatest at θ = 90o. If the particle moves in a circular path, r = mv/qB.