3.1.1 Positive and negative charge

Charge in all matter

1. Electric charge is a property of matter, in the same way that mass and volume are properties of matter.
2. Every object around you, from a plastic comb to a sheet of paper to your own body, is made of matter, so every object contains electric charge.
3. There are only two types of electric charge, called positive charge and negative charge.
4. The names positive and negative are labels that tell the two types apart, and they do not mean that one type is larger, better or more important than the other.
5. Charge cannot be seen directly, so its presence is shown by its effects, such as the forces that charged objects exert on each other.
6. Charge is carried by the tiny particles that make up atoms, so explaining charge in matter starts with the structure of the atom.

Charged particles in atoms

1. Every atom has a tiny central nucleus surrounded by electrons.
2. The nucleus contains protons, which carry positive charge, and neutrons, which carry no charge.
3. The electrons occupy the space around the nucleus and carry negative charge.
4. The charge on one proton and the charge on one electron are equal in size but opposite in type.
5. This is shown with relative charges of $+1$ for a proton, $-1$ for an electron and $0$ for a neutron.
6. Because the sizes match exactly, the positive charge of one proton is cancelled completely by the negative charge of one electron.
7. An atom has the same number of electrons as protons, so the positive and negative charges inside it cancel and the atom has no overall charge.
8. For example, a carbon atom has $6$ protons and $6$ electrons, so its total charge is $(+6)+(-6)=0$.
9. The protons are locked inside the nucleus at the centre of the atom, whereas the electrons are found in the outer part of the atom.

Neutral and charged objects

1. In most objects the number of protons equals the number of electrons, so there are equal amounts of positive and negative charge.
2. The positive and negative charge cancel, so the object has a net charge of zero and is described as neutral or uncharged.
3. A neutral object still contains a vast amount of charge, and the word neutral only means that the two types are balanced.
4. A pencil, a desk and a glass of water are all neutral in everyday conditions, even though each contains billions of billions of protons and electrons.
5. An object is charged only when this balance is upset, so that one type of charge outnumbers the other.
6. An object with more positive charge than negative charge is positively charged.
7. An object with more negative charge than positive charge is negatively charged.
8. The net charge depends on the difference between the amounts of positive and negative charge, not on the total amount of charge in the object.
9. This is why a large, heavy object can be neutral while a tiny speck of dust is charged.

3.1.2 Static electricity and forces between charged objects

Charging by rubbing

1. When two different materials are rubbed together, charge is moved from one surface to the other, so both surfaces become charged.
2. The two objects always end up with opposite charges, so one becomes positively charged and the other becomes negatively charged.
3. Charge that builds up on a surface and stays there is called static electricity, because it stays in one place instead of flowing.
4. Rubbing does not make new charge, it only separates charge that was already in the two materials.
5. Familiar examples include a balloon rubbed on dry hair, a plastic comb pulled through hair, a polythene rod rubbed with a duster and a jumper pulled off over your head.
6. Walking across a nylon carpet in rubber-soled trainers charges your body, and sliding across a car seat charges both you and the seat.
7. Static charge only builds up on insulators, such as plastic, rubber, glass, wool, nylon and dry hair.
8. In an insulator, charge cannot move through the material, so it stays on the patch of surface where the rubbing put it.
9. In a conductor, such as a metal, charge moves freely through the material.
10. If you rub a metal rod held in your hand, any charge on it flows through the metal and through your body to the ground, so no charge builds up.
11. A metal object can only hold static charge if it is insulated from the ground, for example by standing it on a plastic block.
12. Static effects are strongest on dry days, because in damp air a thin film of water forms on surfaces and lets the charge leak away.

Forces between charged objects

1. Charged objects push or pull on each other, and this push or pull is called the electrostatic force.
2. Objects with the same type of charge, called like charges, repel each other.
3. Two positively charged objects repel, and two negatively charged objects repel.
4. Objects with different types of charge, called unlike charges, attract each other.
5. A positively charged object and a negatively charged object attract.
6. The electrostatic force is a non-contact force, which means it acts across a gap without the objects touching.
7. The force is larger when the charged objects are closer together and becomes weaker as they move apart.
8. The force is also larger when the objects carry more charge.
9. The evidence that charged objects exert forces without contact comes from seeing them move before they touch.
10. A rubbed balloon held a few centimetres above small pieces of paper makes them jump up to it before the balloon reaches them.
11. Strands of hair lift towards a rubbed balloon held just above your head, even though the balloon does not touch the hair.
12. A thin, smooth stream of water from a tap bends sideways towards a charged comb held close to it.
13. A rubbed rod hanging from a thread swings round when another charged rod is brought near it, even though there is a clear gap between the rods.
14. In each case an object starts to move across a gap, and a change in motion needs a force, so a force must be acting at a distance.

Sparking

1. The longer and harder an insulator is rubbed, the more charge builds up on it.
2. When a large charge has built up, it can suddenly jump across an air gap to a nearby conductor, such as a metal tap, radiator or door handle.
3. This sudden flow of charge through the air is a spark, and it gives out light and a crackling sound.
4. Air is normally an insulator, but a large build-up of charge can make the air conduct for an instant so that the charge crosses the gap.
5. The bigger the charge, the wider the gap that the spark can jump.
6. Sparks usually jump just before contact, which is why you feel a sharp shock when your fingertip is a few millimetres from a metal door handle after walking across a nylon carpet.
7. Pulling off a synthetic jumper in a dark room often produces tiny sparks that you can see and hear crackling.
8. Lightning is the same effect on a huge scale, when charge built up in a thundercloud jumps through the air to the ground or to another cloud.

3.1.3 Electron transfer and static electricity

Charging by friction

1. Static electricity is explained by the movement of electrons, the negatively charged particles in the outer part of every atom.
2. The protons carry the positive charge, but they are held inside the nucleus at the centre of each atom, so rubbing cannot move them.
3. When two different insulators are rubbed together, friction pulls some electrons off the surface atoms of one material and onto the surface of the other.
4. Different materials hold on to their electrons with different strengths, so the material that holds its electrons less tightly loses electrons and the other material gains them.
5. The material that gains electrons now has more electrons than protons, so it becomes negatively charged.
6. The material that loses electrons now has fewer electrons than protons, so it is left positively charged.
7. A positive charge is therefore caused by a shortage of electrons, never by extra protons.
8. Every electron that leaves one surface arrives on the other, so the two objects gain charges that are equal in size and opposite in type.
9. Charge is moved but not made or destroyed, so the total charge of the two objects is still zero after rubbing, just as it was before.
10. The direction of electron transfer depends on the pair of materials, so the same cloth can make a polythene rod negative but an acetate rod positive.

Why only insulators hold static charge

1. In an insulator, electrons cannot move freely through the material, so electrons gained by rubbing stay on the patch of surface where they arrived.
2. In the same way, a patch of an insulator that has lost electrons cannot quickly get them back from the rest of the material.
3. In a metal, some electrons are free to move through the whole object.
4. If a metal rod held in your hand gains electrons, they spread through the metal, into your hand and through your body to the ground, so the rod is left neutral.
5. If the metal rod loses electrons, electrons flow up from the ground through your body to replace them, so again the rod is left neutral.
6. A metal object standing on an insulating support keeps its charge, because the electrons have no path to or from the ground.

Discharging and sparks

1. A charged object loses its charge when electrons flow onto it or away from it until its numbers of protons and electrons are equal again.
2. A negatively charged object is discharged when its extra electrons flow away from it.
3. A positively charged object is discharged when electrons flow onto it to make up the shortage.
4. Connecting a charged conductor to the ground with a metal wire is called earthing.
5. The Earth is so large that it can give or take any number of electrons without its own charge changing noticeably.
6. When a negatively charged object is earthed, electrons flow from the object to the Earth.
7. When a positively charged object is earthed, electrons flow from the Earth to the object.
8. A spark is a burst of electrons crossing an air gap when a large charge has built up on an object.
9. If your body is negatively charged, the extra electrons jump from your fingertip to a metal door handle as you reach for it.
10. If your body is positively charged, electrons jump the other way, from the handle to your fingertip.
11. Either way, the flow of electrons stops as soon as your body is neutral again, which is why a static shock lasts only an instant.

Attracting neutral objects

1. A charged object can also attract a neutral object, such as a rubbed balloon sticking to a wall or a charged comb picking up scraps of paper.
2. When a negatively charged balloon is held near a wall, it repels electrons in the surface of the wall, pushing them a tiny distance away from the balloon.
3. The surface of the wall nearest the balloon is left with a small positive charge, while the displaced electrons sit slightly further away.
4. The balloon attracts the nearby positive charge more strongly than it repels the more distant negative charge, because the electrostatic force is stronger over shorter distances.
5. The overall force is therefore attraction, and the balloon sticks to the wall.
6. A positively charged object attracts a neutral object in the same way, except that it pulls electrons in the surface towards itself.
7. The wall as a whole stays neutral, because no electrons have left it and they have only shifted slightly within it.

3.1.4 Electric fields

What an electric field is

1. Every charged object is surrounded by an electric field, a region of space in which any other charged object feels a force.
2. The field is there whether or not a second charge is present, and a second charged object placed in it shows up the field by being pushed or pulled.
3. The field explains how charged objects can affect each other without touching, because each object is acted on by the field of the other.
4. The field is strongest close to the charged object and gets weaker with distance, so a charge feels a larger force the closer it is.
5. An object carrying more charge produces a stronger field, so it exerts a larger force on a charge at the same distance.
6. A positive charge placed in a field is pushed in the direction of the field.
7. A negative charge, such as an electron, is pushed in the opposite direction to the field.
8. An uncharged particle, such as a neutron, feels no force in an electric field.
9. Electric fields are invisible, so their shape and direction are shown on diagrams using field lines.

Drawing electric field patterns

1. A field line shows the direction of the force on a small positive charge placed at that point, and an arrow on the line shows that direction.
2. Field lines point away from positive charges and towards negative charges.
3. Around an isolated charged sphere, the field lines are straight and radial, spreading out from the centre like the spokes of a bicycle wheel.
4. For a positively charged sphere the arrows point outwards, and for a negatively charged sphere they point inwards.
5. Where field lines are close together the field is strong, and where they are spread out the field is weak.
6. Radial field lines spread apart as they get further from the sphere, which shows the field becoming weaker with distance.
7. Field lines never cross, because a charge at any point can only be pushed in one direction.
8. Between two parallel metal plates with opposite charges, the field lines are straight, parallel and equally spaced, running from the positive plate to the negative plate.
9. This is a uniform field, so a charge between the plates feels the same force wherever it is placed.
10. A positively charged particle between the plates is pushed along the field lines towards the negative plate.
11. A negatively charged particle, such as an electron, is pushed against the field lines towards the positive plate.

How fields explain static effects

1. Two charged objects each sit in the field of the other, so each one feels a force.
2. Near two positive objects, the field of each object points away from it, and the other positive object is pushed along that field, so the objects are pushed apart.
3. Near two negative objects, the field of each object points towards it, and the other negative object is pushed against that field, so these objects are also pushed apart.
4. Near a positive and a negative object, the field of the positive object points away from it, and the negative object is pushed against that field, so it is pulled towards the positive object.
5. These field rules give the same results as like charges repel and unlike charges attract, and they also explain how the force reaches across the gap.
6. The field also explains why the force between charged objects weakens as they move apart, because each object is then in a weaker part of the other's field.
7. The same field acts on the electrons inside a nearby neutral object, which is how a charged balloon can attract an uncharged wall or scraps of paper.
8. Fields explain sparking too, because as more charge builds up on an object, the field near its surface becomes stronger.
9. When the field is strong enough, it pulls electrons off the air molecules near the object, and the air is then able to conduct.
10. Charge rushes across the gap as a spark, with a flash of light and a crack.
11. A spark only jumps from your hand to a door handle once your hand is very close, because the field in a small gap is much stronger than the field across a wide gap.

3.1.5 Current as a flow of charge

Current as a rate of flow of charge

1. An electric current is the rate of flow of charge, which means it measures how much charge passes a point each second.
2. The word rate matters, because current tells you how quickly charge is flowing, not how much charge there is in total.
3. A large current means a lot of charge passes a point every second, and a small current means only a little charge passes each second.
4. Current is measured in amperes, usually shortened to amps, with the unit symbol $A$.
5. In a metal wire, the charges that flow are electrons.
6. A metal is made of positive metal ions held in a fixed arrangement, surrounded by electrons that are not attached to any particular atom.
7. These free electrons drift through the metal when a current flows, while the positive ions stay where they are.
8. The free electrons are already in every part of the wire before the circuit is switched on, so a cell does not supply the charge but makes the charge that is already there move.
9. Electrons are negative, so they drift from the negative terminal of a cell, round the circuit, to the positive terminal.
10. Circuit diagrams show current flowing the opposite way, from positive to negative, and this agreed direction is called conventional current.

Conditions for charge to flow

1. Charge only flows continuously when two conditions are both met.
2. The first condition is a source of potential difference, such as a cell, a battery or a power supply.
3. The source provides the push that makes the free electrons drift in one direction, in a similar way to a pump driving water round a central heating system.
4. The second condition is a closed circuit, a complete loop of conducting material with no gaps that runs from one terminal of the source, through the components and back to the other terminal.
5. The loop must be made of conductors, such as copper wire, because insulators such as plastic have no free electrons to carry the charge.
6. If the loop has a gap, charge cannot cross it, so the flow of charge stops.
7. An open switch is a deliberate gap, so opening a switch stops the current and closing it completes the circuit again.
8. A broken wire, a loose connection or a broken lamp filament also leaves a gap and stops the current.
9. A complete loop without a source of potential difference has no current, because nothing pushes the free electrons in one direction.
10. A source connected to an incomplete loop has no current either, because the charge has no complete path to flow around.
11. A flat cell can no longer provide a potential difference, so a torch with flat cells stays dark even when its switch is closed.

3.1.6 Current in a single closed loop

The same current at every point

1. A single closed loop is a circuit with only one path for the charge, so charge leaving the source passes through every component in turn before returning.
2. A torch, with its cells, switch and lamp joined in one ring, is a single closed loop.
3. In a single closed loop, the current has the same value at every point.
4. The current just after the cell, the current between two components and the current just before the charge returns to the cell are all equal.
5. The current is not used up by lamps, motors, resistors or any other component.
6. The amount of charge that enters a component each second is exactly equal to the amount of charge that leaves it each second.
7. This follows from the conservation of charge, because charge cannot be destroyed inside a component and, in a steady circuit, it does not pile up there either.
8. A component transfers energy from the circuit, but the charge flowing through it is not lost, so the current after a component equals the current before it.
9. Every part of the loop is already full of free electrons, so when the switch closes, electrons all the way round the loop start drifting at the same moment.
10. A bicycle chain behaves in a similar way, because every link moves round at the same rate and no link is used up as it passes the gears.
11. If the loop is changed, for example by adding another lamp, the size of the current may change, but it changes everywhere in the loop at once and is still the same at every point.
12. This rule applies only when there is a single path for the charge.

Measuring current in a loop

1. Current is measured with an ammeter, which is connected in series.
2. Connecting in series means the loop is opened at one point and the ammeter is placed in the gap, so that all of the charge flows through the meter.
3. In a single closed loop, an ammeter gives the same reading wherever it is placed, whether that is next to the cell, before a lamp, after a lamp or between two components.
4. Several ammeters placed in the same loop all show the same reading, apart from tiny differences caused by the resolution of the meters.
5. Two different components in the same loop can behave very differently, such as one lamp glowing brightly and another glowing dimly, yet the ammeter shows the same current through each.
6. Brightness is therefore not a reliable way to judge where the current is largest in a single loop.

3.1.7 Charge, current and time

Linking charge, current and time

1. Current is the rate of flow of charge, so the charge that passes a point depends on both the current and the time for which it flows.
2. The relationship is $charge\; flow=current\times time$.
3. In symbols this is $Q=It$, where $Q$ is the charge in coulombs ($C$), $I$ is the current in amperes ($A$) and $t$ is the time in seconds ($s$).
4. You need to learn this equation and be able to use it without being given it.
5. A current of $1A$ means that $1C$ of charge passes a point every second, so $1A=1C/s$.
6. In the same way, one coulomb is the charge that passes a point when a current of $1A$ flows for $1s$, so $1C=1As$.
7. A larger current moves more charge each second, so for a fixed time, doubling the current doubles the charge that flows.
8. Leaving the same current flowing for longer also moves more charge, so for a fixed current, doubling the time doubles the charge.
9. For a steady current, charge flow is directly proportional to time, so a graph of charge against time is a straight line through the origin with a gradient equal to the current.
10. Dividing both sides of $Q=It$ by $t$ gives $I=\frac{Q}{t}$, which finds the current.
11. Dividing both sides of $Q=It$ by $I$ gives $t=\frac{Q}{I}$, which finds the time.

Using the right units

1. The equation only gives the right answer when charge is in coulombs, current is in amperes and time is in seconds.
2. Convert minutes to seconds by multiplying by $60$, so $5min=5\times 60=300s$.
3. Convert hours to seconds by multiplying by $3600$, so $2h=2\times 3600=7200s$.
4. Convert milliamperes to amperes by dividing by $1000$, so $250mA=0.25A$.
5. Convert kilocoulombs to coulombs by multiplying by $1000$, so $3.6kC=3600C$.
6. Give the final answer with its unit, rounded to the same number of significant figures as the least precise value you were given.