Static electricity · Model

Electric fields

Nothing crosses the gap. A charged object changes the space around it instead, and anything charged that arrives in that space feels a push or a pull straight away.

Start here

The comb bends the water without touching it.

Run a tap down to the thinnest steady stream it will give and bring a comb you have just pulled through dry hair up to the side of it. The stream bends towards the comb, from a couple of centimetres away, and nothing is touching anything.

What is in the gap, doing the work?

• ANothing at all — the comb changes the space around it, and the water responds to that
• BAir, pushed sideways by the charge on the comb, which then shoves the stream over
• CA thin thread of charge travelling from the comb to the water and pulling on it
• DNothing — the water is already charged, and two opposite charges pull across the gap

Two charged objects push and pull on each other across a gap with nothing in between, and that is genuinely strange. Physics deals with it by splitting the problem in two. First, a charged object fills the space around it with an electric field. Second, anything charged that finds itself in that space feels a force from the field where it is standing.

So the field is a property of space itself, not of the object that made it and not of the object that feels it. It has a value at every point, and that value has a size and a direction. A field arrow is drawn pointing the way a small positive charge would be pushed if you put one there — which means arrows point away from positive charges and towards negative ones. A negative charge put at the same point feels a force the other way.

Two things follow. The field is there whether or not anything is in it, waiting; and the field is what carries the interaction across the gap, so no air, no wire and no contact is needed. In a vacuum it works exactly the same.

At the bench · a field map and one test point

Move the test point around.

Change a control to begin

The small arrows are the field, sampled on a grid: each one shows which way a small positive charge would be pushed there, and how hard. The big arrow is your test point.

Commit first. Two equal positive charges sit a few centimetres apart. What is the field at the point exactly half-way between them?

• AThe strongest field on the whole map, because it is close to both charges
• BZero — the two pushes are equal and exactly opposite, so they cancel
• CTwice the field of one charge on its own at that distance
• DHalf the field of one charge on its own, because the two are shared

The figure

Three forces that reach across a gap

The electric field is not a special case. Every force you meet at this stage that acts without contact is described the same way: the first object fills the space with a field, and the second one responds to the field where it is.

Gravitational field

Anything with mass makes one. Always a pull, never a push, which is why there is no opposite of mass.

Magnetic field

A magnet or a current makes one. Pull or push, depending on which poles face each other.

Electric field

A charge makes one. Pull or push, depending on the two signs — and it is enormously stronger than gravity.

In all three the arrows come in pairs of equal size and opposite direction, and in all three the gap can be a perfect vacuum. What crosses it is not a substance. It is a field.

!

Think again

“The field only exists when there is something in it to feel it.”

The field is there first. That is the whole point of inventing it: instead of saying two objects mysteriously know about each other across a gap, we say the first one changes the space, and the second one only ever responds to the space it is standing in. Take the second object away and the field is unchanged — put anything charged back at that point, at any moment, and it is pushed the same way. Bring in something twice as charged and the force doubles while the field stays exactly as it was.

“The air in between must be carrying it.”

Pump the air out and nothing changes. Two charged objects in an evacuated jar attract and repel exactly as they did, and the same is true of the Sun's gravitational pull on the Earth across 150 million kilometres of almost nothing. Air is not the messenger; it is not even in the way. What is between them is the field, and a field does not need a material to sit in — the same discovery that made light so strange, for exactly the same reason.

Mastery ladder

Not started yet.

Rungs 3 and 4 you mark yourself.

Rung 1 · Read the map

A field map shows arrows pointing outwards in every direction, away from a single object at the centre. What can you say about the object and about a small negative charge released nearby?

• AThe object is negatively charged, and a negative charge would be pushed outwards along the arrows.
• BThe object is positively charged, and a negative charge would be pushed outwards along the arrows.
• CThe object is positively charged, and a negative charge would be pulled inwards, against the arrows.
• DYou cannot tell the sign from the map, only the strength.

Rung 2 · The one that catches people

A student says the field between two equal positive charges must be at its strongest half-way between them, because that point is close to both. What is right?

• AThe student is right — being close to two charges instead of one makes it the strongest point on the map.
• BIt is zero there. The two charges push a positive test charge in exactly opposite directions with equal strength, so the pushes cancel.
• CIt is zero there, because the two charges cancel each other out everywhere between them.
• DIt is half the strength of a single charge at the same distance, because the two contributions are averaged.

Rung 3 · Explain

A charged comb bends a thin stream of water from a couple of centimetres away, with nothing touching. Explain what is happening, using the word field, and say what would change if the whole thing were done in a vacuum.

Your explanationComplete

Rung 4 · Take it somewhere new

People are told they are safe from lightning inside a car, and that it is nothing to do with the rubber tyres. Explain why, using the idea of a field and what you know about conductors.

Your answerComplete

Key note

An electric field is what a charged object does to the space around it: at every point there is a size and a direction. Anything charged placed at a point feels a force from the field there, so nothing has to cross the gap and no air or contact is needed. Field arrows are drawn the way a small positive charge would be pushed, so they point away from positive charges and towards negative ones; a negative charge at the same point is pushed the opposite way. The field is strongest close to a charge and weakens quickly with distance, and between two like charges there is a point where it cancels to nothing. Gravity and magnetism are described in exactly the same way.

Going further

Michael Faraday invented the idea, and he was not taken seriously at first. He had almost no mathematics and described the space around a magnet as filled with lines of force — a picture, not an equation. It took James Clerk Maxwell to write that picture down properly, and when he did, the equations predicted something nobody had asked for: that a field which changes can travel through empty space at a fixed speed. That speed turned out to be the speed of light, and light turned out to be exactly that. Faraday's picture of empty space having a state is now the foundation of every field theory in physics.

Fields also explain why you are safe inside a car in a thunderstorm, and it is not the tyres. A metal shell is a conductor, so its free electrons rearrange themselves until the field inside the shell is very nearly zero, whatever is happening outside — the outside surface takes the whole strike and the inside stays quiet. The same trick shields the cable to a television aerial, protects the electronics inside an aircraft struck by lightning, and is why a mobile phone loses signal in a lift.

It is the metal shell that protects you, so this only works in a car with a metal roof and metal sides. A convertible, a fibreglass boat or an open tractor cab will not do it, and you should not touch metal inside the car while the storm is overhead. If you are caught outside in a storm, get indoors or into a car — never shelter under a tree.

The bench is a teaching model. Charges are treated as points of equal size, and the field is calculated with the standard inverse-square rule and reported as a relative figure with 100 set at the strongest point the grid samples — no field strength in newtons per coulomb is given, because the unit is beyond this stage. Grid arrows have a shortest and a longest length, so very weak and very strong regions are drawn clipped rather than to scale, and points closer than a small distance to a charge are left blank because the model gives no sensible value there. The test point moves along the centre line only, where the field happens to be horizontal; away from that line it is not. Real charged objects are not points, and a real charged sphere's field differs from this near its surface.

Lesson content © MrBadmusAI.