Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• draw on children's own experience of action at the distance, probably through experiences with magnets
• draw on children's experiences, some of which will be vicarious, to establish the reality of gravity in space
• explore something of the mystery of action at a distance

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• relate electric, magnetic and gravity forces, without conflating them
• separate the mass of an object from the force of gravity acting on an object, without being dogmatic
• treat freefall as a natural motion

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Magnets

Wrong Track: Magnets attract metals

Right Lines: Magnets can attract or repel other magnets. Some metals and some non-metals can become magnetised.

Explaining how magnets work

Thinking about the learning

Pupils' everyday experiences of magnets are likely to focus on situations where the force of attraction is important: For example, in magnetic catches on cupboard doors. However, not all objects and not all metals are attracted to magnets.

Thinking about the teaching

The process of attraction can be explained in three simple stages:

1. An object needs to be within the field of influence of the magnet for the effect to be noticed.
2. The magnetic field influences the internal structure of the object. Atoms realign themselves. The object becomes a weak magnet by a process called induction. We now have two magnets.
3. There is a force of attraction between the two magnets.

Electric and magnetic forces

Wrong Track: The positive pole of this magnet attracts the negative pole of the other one.

Right Lines: Both magnetic and electric forces can attract and repel, but the the mechanisms to account for these interactions are different; in one case involving magnetic poles and in the other case involving electrically charged objects.

Explaining the difference

Thinking about the learning

The problem here is that pupils confuse the charge story of electric forces with the pole story of magnetic forces. This is hardly surprising since opposites attract and likes repel is a common mantra of the science classroom.

The same pattern of attraction and repulsion is found both for magnets and for electrically charged objects. Two like poles (two norths or two souths) repel each other, whilst opposite poles, a north near a south, attract. However, you should take care not to mix up attraction and repulsion between poles with attraction and repulsion between opposite or similar electrically charged objects.

Here are some children working in this area:

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Thinking about the teaching

The field concept can be used to describe the action-at-a-distance effect in both electric and magnetic situations. Repulsive and attractive forces are also evident in both cases.

However, we interpret electric forces to be the result of the separation of positively and negatively charged particles. Magnets, on the other hand, are not charged. The force between magnets can be accounted for in terms of the effect of an alignment of the atoms within the magnet. Each atom might be considered to be a mini magnet. When these atoms line up together their combined effect is strong enough to reach out beyond their immediate location – the magnetic field, a force field, exists around the magnet.

Electromagnetism

One of the great achievements of 19th century century physics is attributed to James Clerk Maxwell, who explained the link between electricity and magnetism. He showed that electric charge, once moving, creates a magnetic field. The physics of an electric motor makes use of this electromagnetic effect. Radio waves can be thought of as travelling magnetic and electric fields. It has been said that magnetism is just electricity on the move. This is an oversimplification, but Maxwell did pave the way for a unification in our understanding of magnetic and electric forces.

Teacher Tip: Tell stories about the people that invented physics.

Gravity and magnetism

Wrong Track: Gravity is a magnetic force that attracts things to the Earth.

Right Lines: The Earth's gravity and the Earth's magnetic field are independent of each other.

Distinguishing between the two forces

Thinking about the learning

Mixing up gravity and magnetism is a common confusion. Both magnetic and gravitational forces act at a distance and both are in evidence at the Earth's surface.

Thinking about the teaching

One simple distinction to make between gravity and magnetism is that gravitational forces always attract and never repel. The force due to gravity follows from the interaction between the Earth's mass and any other body that happens to be nearby (the effect extends far beyond the Earth to the Moon and beyond).

Unlike gravity, magnetism doesn't work for anthign with mass. Magnetic forces only occur between specific materials (mainly iron and certain iron alloys). This alone is a strong argument that magnetism is not responsible for gravitational forces.

Mass vs. weight

Wrong Track: It weighs 10 kilogram. That's its mass.

Right Lines: The force of gravity acting on an object is a measure of the gravitational pull acting on that object. It is a force. Mass is a measure of how difficult it is to accelerate the object. The force of gravity acting on an object can change with location; the mass always stays the same.

Making the distinction

Thinking about the learning

Once again, here is a distinction made in the sciences that everyday conversations don't bother with. Statements such as: The bag of potatoes weighs 5 kilogram are common parlance. We all know what this means. I weigh 65 kilogram is a similar statement that only physicists might object to.

The distinction between mass and weight can often seem pernickety, unnecessary and not particularly helpful. However, in physics, the mass of an object and the force acting on an object are very different measures.

Thinking about the teaching

Unfortunately, the distinction between mass and weight is not helped by the everyday use of the verb, to weigh. Weighing has become synonymous with finding either the weight or the mass.

We'd suggest avoiding weight as a technical term, and instead associating it with the experience of being supported by the floor, or the bathroom scales. Instead use the force of gravity as a label for the gravitational force acting on an object.

There isn't really a common expression in science equivalent to massing something. If there was such a mass machine, it would work on the principle of accelerating the mass. The force required to produce a certain acceleration would depend on the mass.

Hence a read-out in kilograms would be possible. Such a machine would be making use of the inertia of the mass and would perhaps be called an accelerometer or an inertiameter. Using such a machine, a more massive object would require a bigger force to change its speed by the same amount.

There is gravity in space

Wrong Track: There is no gravity in space or on the Moon because there is no air there.

Right Lines: The gravitational force does not depend upon the presence of an atmosphere.

Atmosphere and gravity

Thinking about the learning

Orbiting astronauts are seen to float about in their spacecraft. This is often attributed, incorrectly, to there being no gravity up there in space. This is one of the most common misconceptions about space. A reason often given for this assertion is that the space craft is above the Earth's atmosphere. Therefore, because there is no atmosphere there is no gravity. The same logic is used to explain the fact that lunar astronauts must wear large heavy boots because there is no gravity on the Moon, because there is no atmosphere.

These clips are worth watching and listening to carefully:

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Thinking about the teaching

This misunderstanding, which involves making a direct link between gravity and atmosphere, is a regular visitor to science classrooms.

A video clip of Apollo 15 astronaut Dave Scott dropping a feather and hammer on the surface of the Moon in 1971 is evidence that things do fall on the Moon – there is plenty of gravity there, but no atmosphere.

Falling freely – at the same rate

Wrong Track: It's obvious. If I drop a cannonball and a cricket ball, the cannonball will fall faster.

Right Lines: All objects fall freely at the same rate irrespective of mass (provided the effects of air resistance can be ignored).

Forces, mass and acceleration

Thinking about the learning

This is held as being so obvious that most people wouldn't even bother to check it out. A heavy block of wood, mass 2 kilogram, is clearly being pulled down with a greater force of gravity (about 20 newton) than a lighter piece of wood, mass 1 kilogram (about 10 newton). It seems clear to most that this larger force will make the heavy object fall faster.

Thinking about the teaching

The fact that a larger block of wood is subject to a greater force from gravity is indeed true. However, the greater mass of this wood requires a greater force to maintain its accelerated motion. Overall, the effect of a small force on a small mass is the same as that of a large force on a large mass. The net effect is the same – they fall together. They have the same force to mass ratio. (There is more on this argument in the Gravity and Space episode in the SPT: Earth in space topic.) The most important strategy for teaching is to set up a simple and effective dropping objects practical activity. Pupils should drop the objects themselves and also watch objects dropped by others. It is important to use objects that will not damage the floor or feet.

Making predictions

Before setting pupils up to drop objects, invite them to articulate their predictions and their reasons. Return to these predictions and reasons after the dropping activity.

The lighter object ought not to be so light as to be influenced significantly by air resistance. Two blocks of softwood, about 200 gram and 800 gram, will do the trick. If you can find two objects with the same volume but different masses then this will be even more convincing. A cricket ball and a tennis ball, perhaps.

Action without contact

Wrong Track: How can the Earth pull on a ball to make it fall? It's not even touching it!

Right Lines: Some forces can act on objects without touching them. Gravity is one example of these non-contact forces.

Explaining action-at-a-distance

Thinking about the learning

If a pupil made this kind of statement, you might think that they were starting to go along the Right Lines with their thinking by questioning the very essence of the gravitational force. Gravitational, magnetic and electric effects happen at a distance.

Thinking about the teaching

Emphasise that non-contact forces are special. If pupils think that the action-at-a-distance effects of gravity are obvious, they are missing the point. Action at a distance is a very strange phenomenon indeed!

As a teacher you are not expected to offer any explanations about the origins of gravity. We can say, however, that we have sufficient knowledge about how gravity works to get people to the Moon and back. A fantastic achievement.

Evolving ideas of gravity

Here is an opportunity to demonstrate to pupils that scientific concepts are the products of the scientific community and therefore may evolve with time.

Aristotle suggested things fell to Earth because it was their natural place.

For Newton there were invisible threads linking all objects together.

A totally different approach was suggested by Einstein. He linked gravity to a distortion in the shape of space itself.

A current speculative idea is that invisible exchange particles called gravitons pass between all objects and that these somehow are responsible for transmitting the gravitational force effect.

Physics never stands still.

Forces on objects with small and large charge

Wrong Track: The big charge will push hard on the little charge, sharing some it's charge with the little one.

Right Lines: The are two forces, one acting on each charged object. The forces are equal in size, but opposite in direction. Both forces are examples of action at a distance, so there does not need to be any transfer or contact to enable the forces.

Working with electrical forces

Thinking about the learning

The idea of action at a distance is hard, and sufficiently counter-intuitive that physicists arguably invented the idea of the field to bridge the gap. But this bridging does not involve the transfer of any properties, such as charge or mass, from one object to another. It's simply a way of re-describing the interaction, different from the approach of isolating objects from their environment and figuring out the forces acting on them as a result of interactions with the environment. No charged particles (electrons or otherwise) move between interacting charged objects. The field demarcates a volume of influence, within which you can expect other charged objects to have an electrical force acting on them. Emphasising the essential nature of the electrical force as action without contact helps to make this clear.

Forces acting on a pair of objects, one with a large charge, and the other with only a small charge, are often seen to be unequal in magnitude. As they're representations of the same interaction, they are necessarily the same (later you might want to connect this principle with Newton's third law – you can find out more in the SPT: Force and motion topic).

Thinking about the teaching

This is another place where the idea of compensation is often ignored: pupils tend to focus on either the charge of, or the distance to, the object in deciding whether to expect the object to exert a large or small electrical force. It's good to be aware of this widespread tendency, and to explicitly model considering both, even if a precise analytic discussion is beyond expectations at this age. The same errors in reasoning turn up elsewhere, for example in the SPT: Machines topic, and later in the SPT: Electricity and energy topic. It's a pervasive mistake.

Pupils are likely to seek to have some physical contact-like interaction between charged objects, whether to reduce their discomfort with action at a distance or for other reasons. This often takes the form of suggesting that charge is transferred from one charged object to the other, when one exerts a force on another.

Charging and charged objects

Wrong Track: Positive charge flows onto the sphere, then it's charged.

Right Lines: The material is made of atoms, which have negative electrons and positive protons. Atoms are neutral, because there are equal numbers of protons and electrons. However the electrons are easy to remove from the atoms, so its these that are transferred to charge an object. Adding electrons makes the object negatively charged, and subtracting electrons makes the object positively charged. The electrons that one object loses, another object gains.

Working on charging

Thinking about the learning

It's likely that pupils don't have a secure description of the structure of atoms. So if you want to discuss the process of charging or discharging, you may well have to re-activate, or perhaps even develop, this knowledge. Fortunately the model of an atom we need is really rather simple: pupils need to know that atoms are neutral, as a result of an equal number of positively charged particles (protons) and negatively charged particles (electrons). Electrons can be removed from atoms rather easily, but protons are very hard to remove.

To charge a material you can remove some of the electrons, or add some electrons. As you cannot lose electrons, making one thing positively charged necessarily makes another negatively charged. To make something neutral again, you restore the balance between the electrons and the protons, a process known as discharging.

In this way any material can be charged or uncharged, as a result of shifting electrons to or from it. This is very different from persuading charged particles to flow through it, which is what you do in electrical circuits. Materials which can be used in electrical circuits are conductors: those which do not permit charged particles to flow are called insulators. This categorisation is to do with charged particles moving through the material, and not with shifting charged particles to or from an object.

Thinking about the teaching

Many pupils will firm encouragement to distinguish between the properties of a material (whether it's an electrical insulator or electrical conductor) and its charged state (whether is positively charged, negatively charged, or neutral). In particular there is a tendency to conflate the state of being charged with the property of being a conductor.

You'll need to emphasise that to charge something you need to shift electrons to or from it, so you'll need to contact it. This makes it easier to understand that isolated means that it cannot become charged as other charged objects come close to it. Contact is needed.

Pupils also tend to think of charge as a something that exists quite separately from electrons or protons, and you'll need to be careful not to reinforce this by talking as if this were the case.

Teacher Tip: Add electrons to something to make it negatively charged, don't add negative charge to an object.

There's a good chance you could improve your teaching if you were to:

Try these

• Giving real experiences of forces acting at a distance
• Exploiting the tangible effects of magnets in regions of space around the magnet
• Keeping magnetic, electric and gravity forces and their effects separate
• Exploiting similarities of behaviour between the forces
• Sharing some of the struggles that clever people had with action-at-a-distance in the past
• Dealing with mass and the force of gravity acting on an object by sharing the reasons for caring about the difference
• Discussing the assumed universality of these forces, and sharing some of the evidence for that

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• Treating action-at-a-distance as obviously acceptable
• Acting as if the similarities between the three non-contact forces always have been obvious
• Over-emphasising the similarities
• Conflating the terminology and representations for the three different forces

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.