Explain forces that act at a distance with the field model

Learning goal: Use the field model to explain how an object can experience a force from a magnet, a charged object or a massive body without touching it; compare magnetic, electrostatic and gravitational fields; and map a magnetic field safely with a compass.

Forces without contact. Most forces you have studied so far involve touching: a hand pushes a cart, a rope pulls a sled, a floor holds up a chair. Three familiar forces are different. A magnet pulls a steel paper clip before the two touch. A balloon rubbed on hair lifts small bits of paper from a centimeter away. A dropped ball falls toward Earth with nothing pulling on it that you can see. Magnetic, electrical and gravitational forces can act at a distance, and the question is how an object "knows" that another object is nearby when nothing connects them.

The field model. Scientists answer that question with a model: an object is thought to have a region of influence around it, called a field. When a second object with the right property is placed anywhere in that region, the field exerts a force on it, and that force can change the second object's motion. A field is not a substance you can scoop up; it is a way of describing what would happen to a suitable object placed at each point around the source. At this grade, the details of the field model are not needed. What matters is the idea: an external field explains a force at a distance, and, for the same test object placed outside the source and moved straight away from it, the force is stronger close to the source and weaker farther away. No mathematics is used to describe fields at this level; the field is described by its direction and by comparing stronger and weaker regions.

Gravitational fields. Every object with mass exerts a gravitational force on every other object with mass. Two books on a shelf attract each other, but the force is far too small to notice. Gravitational forces are hard to detect unless at least one of the objects is very massive, such as a planet, a moon or the sun. The gravitational force increases with the masses of the objects, decreases rapidly as the distance between them increases, and, for round bodies such as Earth and the moon, points toward the center of each object. Earth's field is why a released ball moves toward the ground and why the ground is "down" on every side of a round planet. Weight is the force an object's mass experiences in a gravitational field. Weight is often confused with mass. Mass is the amount of matter; it is the same anywhere. Weight is proportional to mass but depends on the gravitational field at the object's location, so a rock has the same mass on the moon as on Earth but a much smaller weight there, because the moon's field at its surface is weaker than Earth's.

Electrostatic fields. An electrostatic field exists around an object with a net charge, meaning it has more positive than negative charge or the reverse. Place a second charged object in that field and the two objects experience electric forces. Whether they attract or repel depends on the signs of the charges: like charges repel, opposite charges attract. Rubbing a balloon on hair transfers charge, so the balloon and the hair end up with opposite net charges and attract. A charged balloon can also attract uncharged paper, because the charges inside the paper shift slightly, bringing the opposite charge a little closer than the like charge. That is an attraction caused by the field; it does not break the rule about signs.

Magnetic fields. A magnetic field exists around a magnetic object. A second magnetic object placed in the field experiences a magnetic force that attracts or repels depending on the orientation of the two objects: a north pole faces the other magnet's south pole and they attract; two north poles or two south poles face each other and they repel. Unmagnetized iron and steel are pulled toward either pole, which is why a magnet sticks to most steel cabinets with whichever end you offer. Magnetic field lines can be seen when iron filings are sprinkled on a sheet of paper resting over a magnet: the filings line up in curves that leave one pole and enter the other. A compass shows the same pattern one point at a time, because its needle turns to lie along the field.

Poles are not charges. Magnetic poles are often confused with electric charges, and the difference matters. A single positive or negative charge can exist on its own, but poles always come in north-south pairs; cutting a bar magnet in half produces two shorter magnets, each with both poles. A magnet exerts a magnetic force only on magnetic material or on moving charges, and a charged object at rest exerts an electric force only on charge, including the charge it shifts inside a neutral object. So a charged balloon and a magnet are not doing the same thing to each other: any slight pull between them is the ordinary attraction of a charged object for any nearby neutral material, not a magnetic effect of the charge or an electric effect of the poles.

Electricity and magnetism are related. An electric current produces a magnetic field around the wire that carries it. Wrapping a wire into a coil around an iron nail and connecting it to a battery makes an electromagnet, a temporary magnet that loses almost all its magnetism as soon as the current is switched off. Under some conditions a changing magnetic field can also produce an electric current in a conductor. You can test the first idea with an electromagnet kit, or a circuit, that your teacher supplies and checks. A bare coil of wire connected straight across a cell is close to a short circuit: the current is large, and the wire and the cell can quickly get hot enough to burn, so never build one that way. The teacher's circuit includes a switch and a resistor or small bulb in series that limits the current; switch it on only for the short trials the teacher sets. Count paper clips picked up with the current on, then off.

A worked mapping model. Suppose a bar magnet lies on a sheet of paper and a small compass is moved around it. At a point beyond the north pole the needle's north end points away from the magnet; at a point beyond the south pole it points toward the magnet; above the middle of the magnet it points from the north side across to the south side. Joining the directions gives curved lines that leave the north pole, loop around and enter the south pole. Moving the compass farther away, the magnet's hold on the needle weakens and Earth's own field increasingly takes over, so the needle swings back toward north; that is the evidence that the magnet's field weakens with distance. The diagram shows this pattern with four compasses. It shows directions only: it cannot show how strong the force is at each point, and the straight-sided loops are a drawing convenience, not the true curved shape.

Investigation: map a field and test distance. Materials: one bar magnet, a small compass, paper, a ruler, a stack of index cards and ten steel paper clips. Steps: lay the magnet on the paper, place the compass at eight positions around it and draw a short arrow for the needle's north end at each; then count how many paper clips the magnet can hold in a hanging chain with no card between the magnet and the first clip, and repeat with one, two, three and four cards between them, recording the count for each stack. Record the arrow map and the clip counts. A result that supports the claim: arrows form loops from one pole to the other, and the number of clips held falls as the number of cards rises. Safety: keep magnets away from phones, cards with magnetic strips and pacemakers; small magnets are a swallowing hazard; if iron filings are used, keep them in a sealed bag or under a covered tray and never blow on them.

Worked example 1 — identify the objects and the direction of the force. A steel cabinet door swings shut against a magnet mounted on the frame. The interaction is between the magnet and the steel door. The magnet's field exerts a force on the door toward the magnet, and the door exerts an equal pull on the magnet toward the door; the door moves because it is free to swing while the frame is fixed. Neither object touched the other when the force began, so this is a force at a distance explained by a magnetic field.

Worked example 2 — weight and mass on the moon. A rover component has a mass of 20 kg on Earth. On the moon its mass is still 20 kg, because mass measures the amount of matter and does not depend on location. Its weight on the moon is smaller than on Earth, because the moon's gravitational field at its surface is weaker; the moon is less massive than Earth. Weight changed; mass did not. If a scale reading is used to compare the two locations, it is the weight that the scale measures.

Worked example 3 — attract or repel? Two charged balloons are hung side by side. Both were rubbed on the same wool cloth, so both carry the same sign of net charge. Prediction: like charges repel, so the balloons swing apart and hang at an angle. If one balloon is discharged, for example by wiping it all over with a damp cloth, it no longer has a net charge, and the remaining charged balloon can now attract it weakly by shifting the charges inside it. Compare with two bar magnets placed end to end: north facing south attracts; turning one magnet around so that north faces north makes them repel. The rule for magnets is about orientation, the rule for charges is about sign, and neither rule applies to the other kind of object.

Worked example 4 — distance. In a fictional trial, a magnet holds 9 paper clips through zero cards, 6 through one card, 3 through two cards and 1 through three cards. The counts fall as the gap grows, which supports the claim that the field's pull weakens with distance. The counts do not give a formula: the cards also change the gap unevenly and the clips themselves are magnetized by the field, so the pattern is evidence of the direction of the trend, not a measurement of field strength.

Representation task: a three-column comparison. Make a table with rows for gravitational, electrostatic and magnetic fields and columns for: what property an object needs to feel the force (mass; net charge, or, more weakly, any material whose charges can shift; magnetic material), whether the force can attract only or attract and repel, what decides attraction or repulsion (always attracts; sign of charge; orientation of poles), and one everyday example of each. Under the table, write two sentences explaining what all three have in common: each is a force at a distance explained by a field, and, acting on the same test object, each weakens with distance.

Independent performance: map and report. Carry out the compass mapping and the paper-clip distance test from the lesson, or use a teacher-provided recorded version, and write a one-page report. Success criteria: the field map has at least eight arrows that form loops from one pole to the other; the distance record has at least four gaps with a clip count for each; the conclusion states what the pattern of arrows shows and what the falling counts show; the report names one thing the compass map cannot tell you (field strength at each point) and one safety precaution you followed. Explain in one paragraph why a magnet and a charged balloon are not doing "the same thing", using the words pole, charge and orientation correctly.