Ask A Physicist Lecture 3 September 28, 2015 Electricity and Magnetism

Electric Fields From Point Charges

Dipole Electric Field

Question: Why doesn’t the girl get electrocuted? Answer: Same reason that a bird doesn’t get electrocuted when it perches on a high voltage power line. There isn’t a complete circuit and no current flows through the body. It’s current through the heart that interferes with its operation.

Examples ofMagnetic Fields of a Current Carrying Wires The direction of the magnetic field of a straight wire is perpendicular to the direction of the current and falls off as 1/r.

Force between two current carrying wires.

The direction of the magnetic field is perpendicular to the direction of the current and falls off as 1/r. Iron filings line up around a current carrying wire

Magnetic field of a coil with iron filings

Bar magnet with iron filings

The earth’s magnetic field looks as if there were a giant bar magnet embedded in the center of the earth.

Q: If you break a magnet length wise, they will not fit back together because the like poles repel. I was wondering why a magnet doesn't just fly apart from all those like poles next to each other? - mike (age 31) napavine wa. A: Great question. The magnetic forces are pushing the parts of the magnet apart, just as you say. It doesn't fly apart only because the short-range bonds between the atoms are strong enough to keep it together. That raises a related question. Why doesn't the magnetism on one side just flip directions, so that it sticks well with the magnetism on the other side? In a plain piece of iron, that's pretty much just what happens. Unless the iron is a very thin needle, the magnetism breaks up into domains pointing different directions, so that they don't repel their neighbors. There's an art to making permanent magnets, finding materials in which the domains are strongly magnetized yet have trouble flipping around. Mike W.

A Bubble Chamber Photograph Direct evidence that a magnetic field will exert a force on a moving charged particle. The force is perpendicular to both the velocity of the particle and the direction of the magnetic field which is into the paper in this photo. The right hand rule applies. The reaction is  +e -  e - +e + +e -. The charged particle will cause bubbles to form along its path in a superheated liquid. The trajectory of the particle will be a circle whose radius is inversely proportional to its momentum

Change in the magnetic flux of the left coil induces a current in the right coil. Faraday’s Law of Induction The amount of the deflection depends on the strength of the magnetic field, it’s area, it’s algebraic sign, and it’s time rate of change.

Electromagnetic waves. A new phenomenon 1. Faraday’s Law says that a changing magnetic field can induce an electric field. 2. Ampere’s Law says that a moving charge can induce a magnetic field. Question: Could these two effects conspire to produce a continuous electro- magnetic field? The answer is yes! In 1861 James Clerk Maxwell showed that indeed they could. These solutions are called Maxwell’s Equations and are of the form of sinusoidal waves travelling at the speed of light c 2 =  o  o. If you put in the experimentally measured values of  o and  o sure enough you get the experimentally value for the speed of light.

These EM waves are sine waves or a mixture of sine waves at various frequencies and wavelengths. There is a relationship between the frequency f, wavelength  and the speed of light c: f = c/2 . These waves were first observed by Hertz around Marconi got the idea of using them for telegraphy and around 1900 succeeded eventually transmitting them across the Atlantic. Today they are ubiquitous in all forms of electronic communication.