Live readout
Field at the probe6.47 mT
Compared with Earth's field (50 μT)129×
Field directionright
Probe distance from magnet 1's centre10.0 cm
Strength at each end face0.10 T
Magnet 1 angle0°
Second magnetOff
Field vs. distance
Field along the line from magnet 1's centre through the probe.
Probe.
Twice as far.
Far from a magnet, twice the distance gives about 1/8 the field (it falls off as 1/r³).
Within a few magnet lengths the ratio is different: along the magnet's axis it's bigger than 8, and straight out from the middle of its long side it's smaller.
That's because the two poles are at different distances from you. Move the probe farther out and the ratio heads toward 8.
With the second magnet on, the graph adds both fields, so the ratio follows no simple law.
What you're learning
Field. A magnet fills the space around it with a magnetic field. At every point the field has a direction and a strength.
Direction. The field points the way the north end of a compass needle points. Outside a magnet, field lines leave the N pole and curve around into the S pole. Inside, they run from S back to N (the small arrow on each magnet), so every line is a closed loop. Lines never cross, because a compass can only point one way at each spot.
Strength. Lines crowd together near the poles, where the field is strongest. Far away the field falls off as 1/r³: twice as far, about 1/8 as strong. Field strength is measured in tesla (T).
Poles. Like poles push apart (repel). Unlike poles pull together (attract).
Iron filings. Each filing becomes a tiny magnet and turns to line up with the field, so the filings trace out the field lines. Where the field is very weak, friction wins and they stay jumbled.
Real magnets and materials
Earth's field, at the surface25–65 μT
Fridge magnet, at its surface5–10 mT
Strong neodymium magnet, at its surface0.3–0.7 T
Hospital MRI scanner1.5–3 T
Units: 1 T = 1000 mT = 1,000,000 μT. Older books use the gauss (G): 1 T = 10,000 G.
A magnet pulls on
- Iron
- Most kinds of steel
- Nickel
- Cobalt
No noticeable pull on
- Aluminum
- Copper and brass
- A US nickel coin (75% copper)
- Wood, plastic, glass
How the lab works
Drag a magnet to move it. Drag its round handle to turn it, or hold Shift and drag. On a phone, twist a magnet with two fingers.
Drag the yellow probe to measure the field. With the keyboard, press Tab to reach the lab, then use the arrow keys to move the probe (hold Shift for bigger steps).
Double-click to bring magnet 1 back to the middle.
Each magnet is an evenly magnetized bar, 10 cm × 2.5 cm × 2.5 cm. The lab calculates its field exactly and adds up the fields of both magnets.
The picture is a flat slice through the middle of the magnets. The arrows show the push or the pull between two magnets, not the twist a real pair would also feel.
