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You have used north and south poles to predict pushes and pulls. Mia now wants to map the field around one bar magnet.
Which tool can show field direction at each place?
به انگلیسی
Magnetic Fields
Describe magnetic poles (north and south), explain attraction and repulsion between poles, describe magnetic field lines plotted using a compass, and explain the Earth's magnetic field and its practical uses
What a learner can do afterwards
- States the rule for attraction and repulsion of magnetic poles
- Draws the magnetic field pattern around a bar magnet from memory or compass readings
- Explains why a compass needle points north
- Explains that the Earth has a magnetic field and describes how it is used for navigation
1 · Read
You have met north and south magnetic poles. Like poles repel, or push apart. Opposite poles attract, or pull together. A magnetic field describes magnetic effects in the space around a magnet. It has a strength and a direction. Magnets can also attract unmagnetised iron.
A compass needle is a tiny magnet free to turn. Its north tip follows the combined field; its south tip points the other way. A strong nearby magnet can dominate Earth's field. Just beyond a bar magnet's north end, in line with the bar, the north tip points away. Beyond its south end, it points toward the magnet. To map direction, place a compass, mark its north-tip direction, move it and repeat.
Field lines are drawing tools, not real threads. Outside a bar magnet, draw curved paths from north to south, bowing around both sides. Inside, the paths return south to north, making loops. Lines cannot cross: that would give two field directions at one point. Iron filings show the pattern but not arrow direction. View a sealed filings display with an adult.
On a map drawn to show strength, closer field lines mean a stronger field. A bar magnet's field is strongest near its poles. A field sensor measures strength; compass swing speed is not a reliable measure. With the magnet fixed and other fields negligible, moving directly outward from a pole lowers the reading. Halving distance does not generally double field strength.
Try it together
Mia measures 12 units at A near a pole and 5 units at B farther out. Subtract to find the drop: 12 minus 5 is 7 units. Double a reading only if told it doubles: twice 6 is 12. To count direction readings, add each group once. Two near each pole and three along the sides make 2 plus 2 plus 3, or 7 readings.
Earth has a magnetic field. Its broad pattern resembles a bar magnet's field, but no giant solid bar magnet is buried inside Earth. Away from magnetic objects, a compass's north tip usually points roughly toward geographic north. It need not point exactly north, and near Earth's magnetic poles it can be unreliable. Without Earth's field or any other magnetic field, a compass would have no magnetic reason to choose north.
A compass helps you navigate by showing direction. Clockwise, its directions are north, east, south and west. Each is a quarter-turn from the next. Facing north, east is on your right. Magnets, speakers and steel objects can disturb a reading. Move away, let the needle settle, then read the compass before choosing your route.
Imagine a bar magnet divided across its middle. Each piece still has a north pole and a south pole, with its own field. Dividing the bar does not separate the two types of pole. This is a thought experiment, not an instruction to snap a magnet.
Field lines form loops. Outside a bar magnet they point north to south. Compasses show field direction, sensors measure strength, and Earth's field helps us navigate.
The lesson, questions and flashcards were written for this dataset. OpenStax College Physics was consulted for facts about magnetic fields, poles and compasses.
The readings, questions and flashcards were written for this dataset. OpenStax College Physics was consulted for factual checks.
More to read
Building a field map
Put a bar magnet on a table and use a small compass to investigate its field. Let the needle settle before recording its north-tip direction. Move the compass and repeat. The directions help you draw smooth paths around the magnet. Outside the bar, arrows point from north to south. The paths bow around both sides. Inside the magnet, they continue from south to north, completing loops. Field lines are part of the drawing, not threads in the air. They cannot cross because the field has one direction at a point. Iron filings can reveal a pattern, but their shapes alone do not show arrow direction. View a sealed filings display with an adult.
Earth guides a compass
Earth's magnetic field reaches a compass even when there is no classroom magnet nearby. The compass needle is a small magnet mounted so it can turn. Its north tip follows the field at the needle. Away from magnetic objects, it usually points roughly toward geographic north. A small difference from geographic north does not mean the compass is broken. Near Earth's magnetic poles, a compass can be unreliable. Earth's broad field pattern resembles a bar magnet's pattern, but there is no giant solid bar magnet buried inside Earth. The comparison is a model of the pattern.
Measuring field strength
A compass gives direction information. A field sensor measures strength. How quickly a compass needle swings is not a reliable strength measurement. To compare sensor positions, keep the magnet fixed. When other fields are negligible, moving directly outward from a pole gives a smaller reading. Use the recorded readings to calculate a change. For a drop, subtract the final reading from the first. If a reading changes from 11 units to 6 units, the drop is 5 units. Halving distance does not generally double the field strength. Multiply by two only when the question states that the measured reading doubles.
Using compass directions
A compass helps you choose a direction, but it does not know your destination. Read your route or map to find the direction you need. Clockwise around a compass, the directions are north, east, south and west. Moving between neighbouring directions is a quarter-turn. If you face north, east is to your right and west is to your left. Before using a reading, move away from magnets, speakers and steel objects. They can change the magnetic field at the compass. Let the needle settle, then read it again. If it changes beside an object, check the reading farther away before choosing your route.
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