What Is the Magnetic Constant and Why Does It Matter?

This intends these 3 values can’t beryllium independent; if you cognize 2 of them, you tin deduce nan third. How do physicists woody pinch this? We specify nan velocity of ray arsenic precisely 299,792,458 meters per second. (How do we cognize it’s exact? Because we specify a metre arsenic nan region ray travels successful 1/299,792,458 of a second.) Then we measurement nan magnetic changeless (μ0) and usage that worth on pinch nan velocity of ray to cipher nan electrical changeless (ε0).

Maybe that seems for illustration cheating, but to moreover commencement doing existent science, astatine immoderate constituent we person to dress up arbitrary units and specify immoderate parameters. In fact, erstwhile you travel down to it, each systems of measurement are made up, conscionable for illustration each words are made up.

Permeability of Free Space

Magnetic fields (represented by nan awesome B) tin beryllium created by magnets, arsenic shown successful nan photograph up top. But because of that interdependence we talked about, they tin besides beryllium made by moving electrical charges. (I’m utilizing nan shorthand word “charges” for charged particles, for illustration electrons.) This is described by nan Biot-Savart law:

You tin spot nan magnetic changeless (μ0) successful there. We besides person nan worth of nan electrical complaint (q) moving pinch a definite velocity (v). So this says nan magnetic section increases pinch nan electrical complaint and decreases pinch nan region (r) from nan moving charge—and nan magnetic changeless tells america precisely really overmuch it varies.

Of course, we don’t woody pinch individual moving electrons very often. But we woody pinch streams of moving electrons each nan time: That’s electrical current, which we tin measure. If we cognize nan complaint connected nan particles successful coulombs, past nan number of coulombs flowing per 2nd gives america nan existent (I) successful amperes. And we tin constitute nan equation supra successful position of current: B = μ0I/(2πr).

It’s Everywhere

What this tells america is that electric existent generates a magnetic field. This is utilized successful each kinds of machines. For instance, it gives america electromagnets, wherever nan magnetic unit tin beryllium turned connected and disconnected to move metallic objects successful factories and scrapyards. It’s besides really audio speakers create sound: An electrical awesome vibrates a magnetic driver, which generates unit waves successful nan air.

Also magnetic fields power electrical currents. This is really motors work. There's a existent moving done a coil of ligament successful nan beingness of a magnetic section that's usually created pinch immoderate imperishable magnets. The unit connected nan coil of ligament causes it to turn, and there's your motor. It could beryllium a instrumentality motor, portion of your AC compressor, aliases nan main thrust for an electrical car.

Wait! There's more. Just arsenic a changing electrical section creates a magnetic field, a changing magnetic section creates an electrical field—and that produces an electrical current. This is really astir of our powerfulness is generated. Some power source—steam, wind, moving water, whatever—spins a turbine that rotates a coil wrong a magnetic field. The changing magnetic flux induces a voltage successful nan coil, converting mechanical power into electrical power that tin beryllium transmitted to your home.

Measuring nan Magnetic Constant

How tin we measurement μ0? One method uses what’s called a existent balance. A elemental type of this has 2 parallel wires carrying electrical existent (I) successful other directions, arsenic shown successful nan sketch below. Then you suspend nan 2 wires pinch strings truthful that they tin move apart, for illustration this:

The existent successful each ligament creates a magnetic section astatine nan location of nan different wire, and this pushes them apart. As they move away, nan magnetic unit decreases and nan horizontal constituent of nan hostility successful nan support drawstring increases (because of nan alteration successful angle). Once these 2 forces are equal, nan wires will beryllium “balanced.”

If you cognize nan worth of nan electrical existent and nan region betwixt nan wires (r), you tin find nan magnetic constant, μ0. Then, arsenic we showed above, you tin usage this worth on pinch nan defined velocity of ray to cipher nan electrical constant, ε0.

So yeah, each successful all, you could opportunity nan magnetic changeless is beautiful important. Oh, and what is that changeless value? According to nan International Committee for Weights and Measures, μ0 = 1.256637061272 × 10–6 N/A2. No more, nary less.