Why the field model is needed

Definition

Non-contact force

A non-contact force is a force that acts between two objects that are not touching, through a gravitational, electrostatic or magnetic field.

Definition

Electric field

An electric field is a region in which an electric charge experiences a force.

Every static effect involves a force between objects that are not touching, so it has to be a non-contact force.
Saying only that charges attract or repel does not explain how one object can affect another across an empty gap.
The electric field fills that gap: the first charge sets up a field in the space around it, and the second charge feels a force because it is sitting in that field.
The size of the force on the second charge depends on its own charge and on the strength of the field where it sits, not on any contact between the objects.
Because the field grows weaker further from its source, the force falls as the objects are separated, which is exactly what is observed.
Both objects set up fields and both sit in the other's field, which is why the two forces always come in a pair of equal size.
The field is a model, so it cannot be seen directly, but every prediction it makes about direction and strength can be tested.

Attraction and repulsion as field effects

Near a positive charge the field points away from it, so a second positive charge placed there is pushed outwards and the two objects repel.
A negative charge placed in that same field feels a force opposite to the arrows, so it is pulled inwards and the two objects attract.
The like-and-unlike rule is therefore not a separate fact to memorise; it follows from the direction of the field and the sign of the charge placed in it.
Moving a charged rod closer to another charged object moves it into a stronger part of the field, so the force it feels grows.
Rubbing a rod for longer puts more charge on it, which makes its field stronger at every point and increases the force on anything nearby.
The field also explains why the effect fades with distance rather than stopping abruptly, because the field lines fan out gradually.

Explaining attraction of uncharged objects

Definition

Induced charge separation

Induced charge separation is the small shift of electrons inside a neutral object when a charged object is brought close to it, making one side of the object negative and the other side positive while it stays neutral overall.

A field acts on the charges inside a neutral object as well, even though that object carries no overall charge.
Electrons in the neutral object are pushed one way by the field while the fixed positive nuclei are pushed the other way, which produces induced charge separation.
The side of the object that ends up with the charge opposite to the nearby charged object is also the side closest to it.
That near side sits in the stronger part of the field, so the force there is larger than the force on the far side.
The attraction on the near side therefore beats the repulsion on the far side, and the neutral object is pulled towards the charged one.
The field model explains this attraction, which the like-and-unlike rule on its own cannot do because the neutral object has no overall charge.
The effect disappears as soon as the charged object is removed, because the field that separated the charges has gone and the electrons move back.

Explaining sparks and shocks

Definition

Ionisation of air

Ionisation of air is the removal of electrons from molecules in the air, leaving charged particles that allow the air to conduct electricity.

As charge collects on an object, the electric field in the air around it becomes stronger.
Air molecules contain charges of their own, so a strong enough field pulls electrons away from them, and this is ionisation of air.
The freed electrons and the charged molecules left behind are both free to move, so the ionised air has become a conductor.
Charge then flows through that narrow conducting channel all at once, which is the spark, and the object loses its charge.
This is why a spark needs either a large build-up of charge or a small gap, because both of those make the field in the air stronger.
Field lines crowd together at sharp points and thin edges, so the field is strongest there and sparks tend to start from corners and points rather than flat surfaces.
The same reasoning covers every scale, from the click felt when touching a door handle to a lightning stroke several kilometres long.

Practical

Investigating attraction and repulsion between charged objects

Aim: to investigate the forces of attraction and repulsion between charged objects, and how the force depends on the signs of the charges and on the separation between them.
Apparatus: two polythene rods, one perspex or acetate rod, woollen duster, paper stirrup or wire cradle hung from nylon thread, retort stand with boss and clamp, or an upturned watch glass, protractor and metre rule taped to the bench, coulombmeter with collecting plate, dry cloth, and a dry, warm room.
Variables, sign of the force: the material of the approaching rod is the independent variable, the direction in which the suspended rod turns is the dependent variable, and the duster, the number of strokes, the starting separation and the room conditions are controlled.
Variables, effect of separation: the separation between the rods is the independent variable, the angle the suspended rod turns through is the dependent variable, and the charge on both rods, the suspension arrangement and the direction of approach are controlled.
Method, set-up:
- Wipe both rods, the duster, the cradle and the thread with a dry cloth, and work in a dry room, because moisture on a surface lets charge leak away within seconds.
- Hang the stirrup from a length of nylon thread clamped to the retort stand and balance one polythene rod horizontally in it, so that a very small sideways force makes it turn. An upturned watch glass with the rod resting in the groove works just as well.
- Tape a protractor to the bench directly beneath the free end of the suspended rod, with its zero line along the rod's rest position, and tape a metre rule alongside so separations can be measured.
- Agree a fixed charging routine, for example ten firm strokes along the last $cm10\ \text{cm}$ of a rod, and use it every single time.
- Charge the suspended rod by rubbing one end with the duster, holding the rod by the end you are not rubbing, then let it settle before taking any reading.
Method, finding the direction of the force:
- Charge the second polythene rod with the same duster and the same number of strokes.
- Bring it slowly towards the free end of the suspended rod, keeping the two rods horizontal and at the same height, and stop about $cm5\ \text{cm}$ away without letting them touch.
- Record whether the suspended rod turns away from the approaching rod or towards it.
- Move the approaching rod away, let the suspension settle, then repeat the test twice more, recharging both rods before each run.
- Replace the second polythene rod with the charged perspex rod and repeat the same three runs, recording the direction of turning each time.
Method, finding the effect of separation:
- Recharge both polythene rods, hold the approaching rod at a measured separation of $cm20\ \text{cm}$ and record the angle the suspended rod turns through once it settles.
- Repeat at separations of $cm15\ \text{cm}$ , $cm10\ \text{cm}$ , $cm7\ \text{cm}$ and $cm5\ \text{cm}$ , recharging both rods before every reading so the charge stays as close to constant as possible.
- Take three readings at each separation and calculate a mean angle, investigating any reading that is far from the other two rather than deleting it.
- Use the coulombmeter between runs to check that each freshly charged rod carries a similar amount of charge.
Results: two charged polythene rods repel, so the suspended rod turns away from the approaching rod. A charged perspex rod attracts the charged polythene rod, so the suspended rod turns towards it. The mean angle increases as the separation is reduced.
Processing: plot the mean angle turned through on the vertical axis against separation on the horizontal axis. A larger angle means a larger force, so the curve rises steeply as the separation gets smaller, which shows that the force grows quickly as the charges are brought closer together.
Watch out: charge leaks away in seconds, so take readings promptly and recharge often. Touching the rubbed end with bare fingers earths the charge through you. Letting the rods touch transfers charge and changes both rods, so the next reading is meaningless. Draughts, open windows and breathing on the apparatus move the suspended rod and look exactly like a force. Rubbing never produces the same charge twice, so angles should only be compared within one set of readings taken quickly.
Improvements: use a longer thread so that a smaller force gives a measurable turn, enclose the suspension in a clear box to keep draughts out, read the protractor from directly above to avoid a parallax error, and use the coulombmeter to reject any run where the charge is clearly different.
Safety: low risk. Keep the charged rods away from faces and eyes, keep them well clear of mains sockets and sensitive electronics, clamp the stand so it cannot topple, and clear the bench of anything the swinging rod could knock over.

Example

Changing the distance between two charged rods

A charged polythene rod hangs in a cradle so that it can turn freely, and a second charged polythene rod is held $cm20\ \text{cm}$ from its free end.
Both rods carry the same sign of charge, so the suspended rod turns away and the force between them is repulsion.
At $cm20\ \text{cm}$ the suspended rod sits in a weak part of the second rod's field, so it turns through only a small angle.
The second rod is moved in to $cm5\ \text{cm}$ , which places the suspended rod in a much stronger part of the same field.
The force on the suspended rod is now larger, so it turns through a much bigger angle.
Neither charge has changed; only the strength of the field at the suspended rod's position has changed.

Exam technique

Using the field in an explanation

Build the answer in three steps: the charged object sets up an electric field, a charge placed in that field experiences a force, and that force produces the effect described.
Say where the field is stronger whenever the question changes a distance or an amount of charge, because that is what changes the force.
For attraction of an uncharged object, state that the field causes induced charge separation and that the near side gains the opposite charge.
For a spark, say that the field becomes strong enough to ionise the air so that the air conducts, then that charge flows through it.
Use the word because to join each step to the next, so the chain of reasoning is easy to follow and easy to award.
Avoid explaining a non-contact effect by contact of any kind, since the whole point of the field is that nothing has to touch.

Common Mistake

Do not say that objects must touch for an electrostatic force to act, because the field lets the force act across a gap.
Do not write that positive electrons move, because electrons are always negatively charged.
Do not say a neutral object gains an overall charge when it is attracted, because its charge has only been separated.
Do not treat the field arrows as the path a negative charge takes, because a negative charge is forced the opposite way.
Do not say that a spark happens because air conducts electricity normally, because air only conducts once the field has ionised it.

Self review

Explain why the electric field model is needed to describe an electrostatic force.
Explain, using the field, why two negatively charged rods repel each other.
Explain why the force on a charged object grows as it is moved closer to another charged object.
Explain how a field causes a neutral object to be attracted to a charged one.
Explain how a strong electric field allows a spark to cross an air gap.
Describe how you would show that two charged polythene rods repel each other.

Why the field model is needed

Definition

Non-contact force

A non-contact force is a force that acts between two objects that are not touching, through a gravitational, electrostatic or magnetic field.

Definition

Electric field

An electric field is a region in which an electric charge experiences a force.

Every static effect involves a force between objects that are not touching, so it has to be a non-contact force.
Saying only that charges attract or repel does not explain how one object can affect another across an empty gap.
The electric field fills that gap: the first charge sets up a field in the space around it, and the second charge feels a force because it is sitting in that field.
The size of the force on the second charge depends on its own charge and on the strength of the field where it sits, not on any contact between the objects.
Because the field grows weaker further from its source, the force falls as the objects are separated, which is exactly what is observed.
Both objects set up fields and both sit in the other's field, which is why the two forces always come in a pair of equal size.
The field is a model, so it cannot be seen directly, but every prediction it makes about direction and strength can be tested.

Attraction and repulsion as field effects

Near a positive charge the field points away from it, so a second positive charge placed there is pushed outwards and the two objects repel.
A negative charge placed in that same field feels a force opposite to the arrows, so it is pulled inwards and the two objects attract.
The like-and-unlike rule is therefore not a separate fact to memorise; it follows from the direction of the field and the sign of the charge placed in it.
Moving a charged rod closer to another charged object moves it into a stronger part of the field, so the force it feels grows.
Rubbing a rod for longer puts more charge on it, which makes its field stronger at every point and increases the force on anything nearby.
The field also explains why the effect fades with distance rather than stopping abruptly, because the field lines fan out gradually.

Explaining attraction of uncharged objects

Definition

Induced charge separation

A field acts on the charges inside a neutral object as well, even though that object carries no overall charge.
Electrons in the neutral object are pushed one way by the field while the fixed positive nuclei are pushed the other way, which produces induced charge separation.
The side of the object that ends up with the charge opposite to the nearby charged object is also the side closest to it.
That near side sits in the stronger part of the field, so the force there is larger than the force on the far side.
The attraction on the near side therefore beats the repulsion on the far side, and the neutral object is pulled towards the charged one.
The field model explains this attraction, which the like-and-unlike rule on its own cannot do because the neutral object has no overall charge.
The effect disappears as soon as the charged object is removed, because the field that separated the charges has gone and the electrons move back.

Explaining sparks and shocks

Definition

Ionisation of air

Ionisation of air is the removal of electrons from molecules in the air, leaving charged particles that allow the air to conduct electricity.

As charge collects on an object, the electric field in the air around it becomes stronger.
Air molecules contain charges of their own, so a strong enough field pulls electrons away from them, and this is ionisation of air.
The freed electrons and the charged molecules left behind are both free to move, so the ionised air has become a conductor.
Charge then flows through that narrow conducting channel all at once, which is the spark, and the object loses its charge.
This is why a spark needs either a large build-up of charge or a small gap, because both of those make the field in the air stronger.
Field lines crowd together at sharp points and thin edges, so the field is strongest there and sparks tend to start from corners and points rather than flat surfaces.
The same reasoning covers every scale, from the click felt when touching a door handle to a lightning stroke several kilometres long.

Practical

Investigating attraction and repulsion between charged objects

Aim: to investigate the forces of attraction and repulsion between charged objects, and how the force depends on the signs of the charges and on the separation between them.
Apparatus: two polythene rods, one perspex or acetate rod, woollen duster, paper stirrup or wire cradle hung from nylon thread, retort stand with boss and clamp, or an upturned watch glass, protractor and metre rule taped to the bench, coulombmeter with collecting plate, dry cloth, and a dry, warm room.
Variables, sign of the force: the material of the approaching rod is the independent variable, the direction in which the suspended rod turns is the dependent variable, and the duster, the number of strokes, the starting separation and the room conditions are controlled.
Variables, effect of separation: the separation between the rods is the independent variable, the angle the suspended rod turns through is the dependent variable, and the charge on both rods, the suspension arrangement and the direction of approach are controlled.
Method, set-up:
- Wipe both rods, the duster, the cradle and the thread with a dry cloth, and work in a dry room, because moisture on a surface lets charge leak away within seconds.
- Hang the stirrup from a length of nylon thread clamped to the retort stand and balance one polythene rod horizontally in it, so that a very small sideways force makes it turn. An upturned watch glass with the rod resting in the groove works just as well.
- Tape a protractor to the bench directly beneath the free end of the suspended rod, with its zero line along the rod's rest position, and tape a metre rule alongside so separations can be measured.
- Agree a fixed charging routine, for example ten firm strokes along the last $cm10\ \text{cm}$ of a rod, and use it every single time.
- Charge the suspended rod by rubbing one end with the duster, holding the rod by the end you are not rubbing, then let it settle before taking any reading.
Method, finding the direction of the force:
- Charge the second polythene rod with the same duster and the same number of strokes.
- Bring it slowly towards the free end of the suspended rod, keeping the two rods horizontal and at the same height, and stop about $cm5\ \text{cm}$ away without letting them touch.
- Record whether the suspended rod turns away from the approaching rod or towards it.
- Move the approaching rod away, let the suspension settle, then repeat the test twice more, recharging both rods before each run.
- Replace the second polythene rod with the charged perspex rod and repeat the same three runs, recording the direction of turning each time.
Method, finding the effect of separation:
- Recharge both polythene rods, hold the approaching rod at a measured separation of $cm20\ \text{cm}$ and record the angle the suspended rod turns through once it settles.
- Repeat at separations of $cm15\ \text{cm}$ , $cm10\ \text{cm}$ , $cm7\ \text{cm}$ and $cm5\ \text{cm}$ , recharging both rods before every reading so the charge stays as close to constant as possible.
- Take three readings at each separation and calculate a mean angle, investigating any reading that is far from the other two rather than deleting it.
- Use the coulombmeter between runs to check that each freshly charged rod carries a similar amount of charge.
Results: two charged polythene rods repel, so the suspended rod turns away from the approaching rod. A charged perspex rod attracts the charged polythene rod, so the suspended rod turns towards it. The mean angle increases as the separation is reduced.
Processing: plot the mean angle turned through on the vertical axis against separation on the horizontal axis. A larger angle means a larger force, so the curve rises steeply as the separation gets smaller, which shows that the force grows quickly as the charges are brought closer together.
Watch out: charge leaks away in seconds, so take readings promptly and recharge often. Touching the rubbed end with bare fingers earths the charge through you. Letting the rods touch transfers charge and changes both rods, so the next reading is meaningless. Draughts, open windows and breathing on the apparatus move the suspended rod and look exactly like a force. Rubbing never produces the same charge twice, so angles should only be compared within one set of readings taken quickly.
Improvements: use a longer thread so that a smaller force gives a measurable turn, enclose the suspension in a clear box to keep draughts out, read the protractor from directly above to avoid a parallax error, and use the coulombmeter to reject any run where the charge is clearly different.
Safety: low risk. Keep the charged rods away from faces and eyes, keep them well clear of mains sockets and sensitive electronics, clamp the stand so it cannot topple, and clear the bench of anything the swinging rod could knock over.

Example

Changing the distance between two charged rods

A charged polythene rod hangs in a cradle so that it can turn freely, and a second charged polythene rod is held $cm20\ \text{cm}$ from its free end.
Both rods carry the same sign of charge, so the suspended rod turns away and the force between them is repulsion.
At $cm20\ \text{cm}$ the suspended rod sits in a weak part of the second rod's field, so it turns through only a small angle.
The second rod is moved in to $cm5\ \text{cm}$ , which places the suspended rod in a much stronger part of the same field.
The force on the suspended rod is now larger, so it turns through a much bigger angle.
Neither charge has changed; only the strength of the field at the suspended rod's position has changed.

Exam technique

Using the field in an explanation

Build the answer in three steps: the charged object sets up an electric field, a charge placed in that field experiences a force, and that force produces the effect described.
Say where the field is stronger whenever the question changes a distance or an amount of charge, because that is what changes the force.
For attraction of an uncharged object, state that the field causes induced charge separation and that the near side gains the opposite charge.
For a spark, say that the field becomes strong enough to ionise the air so that the air conducts, then that charge flows through it.
Use the word because to join each step to the next, so the chain of reasoning is easy to follow and easy to award.
Avoid explaining a non-contact effect by contact of any kind, since the whole point of the field is that nothing has to touch.

Common Mistake

Do not say that objects must touch for an electrostatic force to act, because the field lets the force act across a gap.
Do not write that positive electrons move, because electrons are always negatively charged.
Do not say a neutral object gains an overall charge when it is attracted, because its charge has only been separated.
Do not treat the field arrows as the path a negative charge takes, because a negative charge is forced the opposite way.
Do not say that a spark happens because air conducts electricity normally, because air only conducts once the field has ionised it.

Self review

Explain why the electric field model is needed to describe an electrostatic force.
Explain, using the field, why two negatively charged rods repel each other.
Explain why the force on a charged object grows as it is moved closer to another charged object.
Explain how a field causes a neutral object to be attracted to a charged one.
Explain how a strong electric field allows a spark to cross an air gap.
Describe how you would show that two charged polythene rods repel each other.

11.2.4 Electric fields and static electricity

Why the field model is needed

Attraction and repulsion as field effects

Explaining attraction of uncharged objects

Explaining sparks and shocks

Investigating attraction and repulsion between charged objects

Changing the distance between two charged rods

Using the field in an explanation

Review 11.2.4 Electric fields and static electricity by teaching Genie

11.2.4 Electric fields and static electricity

Why the field model is needed

Attraction and repulsion as field effects

Explaining attraction of uncharged objects

Explaining sparks and shocks

Investigating attraction and repulsion between charged objects

Changing the distance between two charged rods

Using the field in an explanation

Review 11.2.4 Electric fields and static electricity by teaching Genie

11.2.4 Electric fields and static electricity

Why the field model is needed

Attraction and repulsion as field effects

Explaining attraction of uncharged objects

Explaining sparks and shocks

Investigating attraction and repulsion between charged objects

Changing the distance between two charged rods

Using the field in an explanation

Review 11.2.4 Electric fields and static electricity by teaching Genie

11.2.4 Electric fields and static electricity Revision Guide

11.2.4 Electric fields and static electricity

Why the field model is needed

Attraction and repulsion as field effects

Explaining attraction of uncharged objects

Explaining sparks and shocks

Investigating attraction and repulsion between charged objects

Changing the distance between two charged rods

Using the field in an explanation

Review 11.2.4 Electric fields and static electricity by teaching Genie

11.2.4 Electric fields and static electricity Revision Guide