The Current Flows One Way and the Electrons Go the Other
Learn to define charge and current and calculate with I equals Q by t, tell a primary cell from a secondary one, draw every standard circuit symbol, and see why conventional current runs opposite to the electrons.
Why do we say current flows the opposite way to the electrons?
Every circuit diagram marks the current flowing out of the positive terminal of the cell, round the circuit, and back into the negative terminal.
The electrons do exactly the reverse. Being negatively charged, they are repelled by the negative terminal and attracted to the positive one, so they drift from negative to positive — against the arrow on the diagram.
That sounds like a mistake waiting to be corrected. It is not, and the reason is worth knowing: the direction of current was agreed upon before anyone knew what was carrying it. By the time the electron was identified, every rule, every instrument and every textbook was written in terms of that direction — and, crucially, nothing measurable depends on the choice. A bulb lights the same, an ammeter reads the same, and every calculation comes out the same.
There is also a better reason for keeping it. In a liquid or a gas, the charge is carried by ions of both signs moving in opposite directions at once. There is no single "direction the carriers go", so a convention based on positive charge is the more general description.
This page covers the first part of the ICSE Class 9 Physics chapter on current electricity — charge and current, cells, the circuit symbols, and conductors and insulators.
The electrons do exactly the reverse. Being negatively charged, they are repelled by the negative terminal and attracted to the positive one, so they drift from negative to positive — against the arrow on the diagram.
That sounds like a mistake waiting to be corrected. It is not, and the reason is worth knowing: the direction of current was agreed upon before anyone knew what was carrying it. By the time the electron was identified, every rule, every instrument and every textbook was written in terms of that direction — and, crucially, nothing measurable depends on the choice. A bulb lights the same, an ammeter reads the same, and every calculation comes out the same.
There is also a better reason for keeping it. In a liquid or a gas, the charge is carried by ions of both signs moving in opposite directions at once. There is no single "direction the carriers go", so a convention based on positive charge is the more general description.
This page covers the first part of the ICSE Class 9 Physics chapter on current electricity — charge and current, cells, the circuit symbols, and conductors and insulators.
Formula
How do you calculate current from charge and time?
Current is the rate of flow of charge — the quantity of charge passing a point each second.
- Electric charge (Q) — the property of matter responsible for electrical effects, of two kinds, positive and negative. SI unit the coulomb (C)
- Electric current (I) — the rate of flow of charge. SI unit the ampere (A), and
Rearranged: and .
Worked example 1 — finding the current. A charge of C passes a point in s:
Worked example 2 — finding the charge. A current of A flows for minutes. Convert the time to seconds first:
Worked example 3 — finding the time. How long must a current of A flow to carry C?
Counting the electrons. The charge on a single electron is
so the number of electrons making up one coulomb is
Worked example 4. How many electrons pass a point each second when the current is A?
Worked example 5 — a charge from a number of electrons. What charge is carried by electrons?
The sub-units, because real currents are often small.
so a current of mA is A, and is A.
Charge comes in whole numbers of electrons and nothing smaller. Any charge is a whole multiple of C, so a charge of exactly C is impossible — it would be one and a half electrons. That is why the electron's charge is called the elementary charge, and a question offering such a value is testing whether you noticed.
Convert minutes to seconds before substituting. The ampere is coulombs per second, so a time given in minutes or hours must be converted first. **Using instead of in worked example 2 would give C instead of C** — a sixtyfold error, and the commonest slip in these numericals.
- Electric charge (Q) — the property of matter responsible for electrical effects, of two kinds, positive and negative. SI unit the coulomb (C)
- Electric current (I) — the rate of flow of charge. SI unit the ampere (A), and
Rearranged: and .
Worked example 1 — finding the current. A charge of C passes a point in s:
Worked example 2 — finding the charge. A current of A flows for minutes. Convert the time to seconds first:
Worked example 3 — finding the time. How long must a current of A flow to carry C?
Counting the electrons. The charge on a single electron is
so the number of electrons making up one coulomb is
Worked example 4. How many electrons pass a point each second when the current is A?
Worked example 5 — a charge from a number of electrons. What charge is carried by electrons?
The sub-units, because real currents are often small.
so a current of mA is A, and is A.
Charge comes in whole numbers of electrons and nothing smaller. Any charge is a whole multiple of C, so a charge of exactly C is impossible — it would be one and a half electrons. That is why the electron's charge is called the elementary charge, and a question offering such a value is testing whether you noticed.
Convert minutes to seconds before substituting. The ampere is coulombs per second, so a time given in minutes or hours must be converted first. **Using instead of in worked example 2 would give C instead of C** — a sixtyfold error, and the commonest slip in these numericals.
What is the difference between a primary and a secondary cell?
A primary cell cannot be recharged because its chemical reaction is irreversible; a secondary cell can, because its reaction runs both ways.
A primary cell.
- Converts chemical energy into electrical energy
- The reaction is not reversible, so once the chemicals are used up the cell is discarded
- Has a comparatively high internal resistance, so it cannot supply a large current
- Examples: the dry cell used in a torch or a clock, and the simple voltaic cell
A secondary cell, also called an accumulator or a storage cell.
- Stores energy chemically and can be recharged by passing a current through it in the reverse direction
- The reaction is reversible, so the cell can be used many times over
- Has a low internal resistance, so it can supply a large current
- Examples: the lead-acid accumulator in a car, the nickel-cadmium cell, the lithium-ion cell in a phone
A secondary cell does not store electricity; it stores chemical energy. Charging drives the chemical reaction backwards, and discharging lets it run forwards again. So a "charged" battery holds no electrons in reserve — it holds reactants, and the electrons it pushes round a circuit come from the circuit itself.
The function of a cell in a circuit.
- It maintains a potential difference between its two terminals
- That potential difference is what drives charge round the external circuit
- It converts chemical energy into electrical energy as the charge passes through it
A cell does not create or store the charge that flows. The charge — the free electrons — is already present throughout the metal of the wires. The cell's job is to push it, much as a pump pushes water already in a pipe rather than manufacturing water. Remove the cell and the electrons are still there, drifting randomly with no net flow.
Worked comparison — why a car uses a secondary cell. Starting an engine needs a very large current for a few seconds. A dry cell's high internal resistance makes that impossible, while a lead-acid accumulator's low internal resistance allows it — and the accumulator can then be recharged by the vehicle's own generator while the engine runs. Torch cells and car batteries are chosen by how much current is needed and whether recharging is possible, not by which is better.
A cell and a battery are not the same word. A single unit is a cell; two or more cells joined together form a battery. So the object in a torch is a cell and the object in a car is a battery, and the circuit symbols in the next section are different for the two.
A primary cell.
- Converts chemical energy into electrical energy
- The reaction is not reversible, so once the chemicals are used up the cell is discarded
- Has a comparatively high internal resistance, so it cannot supply a large current
- Examples: the dry cell used in a torch or a clock, and the simple voltaic cell
A secondary cell, also called an accumulator or a storage cell.
- Stores energy chemically and can be recharged by passing a current through it in the reverse direction
- The reaction is reversible, so the cell can be used many times over
- Has a low internal resistance, so it can supply a large current
- Examples: the lead-acid accumulator in a car, the nickel-cadmium cell, the lithium-ion cell in a phone
A secondary cell does not store electricity; it stores chemical energy. Charging drives the chemical reaction backwards, and discharging lets it run forwards again. So a "charged" battery holds no electrons in reserve — it holds reactants, and the electrons it pushes round a circuit come from the circuit itself.
The function of a cell in a circuit.
- It maintains a potential difference between its two terminals
- That potential difference is what drives charge round the external circuit
- It converts chemical energy into electrical energy as the charge passes through it
A cell does not create or store the charge that flows. The charge — the free electrons — is already present throughout the metal of the wires. The cell's job is to push it, much as a pump pushes water already in a pipe rather than manufacturing water. Remove the cell and the electrons are still there, drifting randomly with no net flow.
Worked comparison — why a car uses a secondary cell. Starting an engine needs a very large current for a few seconds. A dry cell's high internal resistance makes that impossible, while a lead-acid accumulator's low internal resistance allows it — and the accumulator can then be recharged by the vehicle's own generator while the engine runs. Torch cells and car batteries are chosen by how much current is needed and whether recharging is possible, not by which is better.
A cell and a battery are not the same word. A single unit is a cell; two or more cells joined together form a battery. So the object in a torch is a cell and the object in a car is a battery, and the circuit symbols in the next section are different for the two.
How do you draw a circuit from a written description?
Learn the standard symbols, then place them round a closed loop in the order the description gives.
The symbols.
- Cell — a long thin line for the positive terminal and a short thick line for the negative one, side by side
- Battery — two or more such cell symbols in a row, joined
- Key or switch — a break in the line with a hinged arm, closed or open as stated
- Ammeter — a circle enclosing the letter A
- Voltmeter — a circle enclosing the letter V
- Resistor — a plain rectangle in the line
- Rheostat — a rectangle with an arrow or a sliding contact across it
- Bulb — a circle with a cross inside it, or a circle with a filament loop
- Connecting wire — a plain straight line, drawn with right-angled corners
The two rules about where meters go.
- An ammeter measures the current through something, so it is joined in series with it. It must have a low resistance, so that inserting it barely changes the current it is trying to measure
- A voltmeter measures the potential difference across something, so it is joined in parallel with it. It must have a high resistance, so that it draws almost no current away from the component
Worked construction. Draw a circuit for the following description: a battery of two cells, a plug key, an ammeter and a bulb all connected in series, with a voltmeter across the bulb, and a rheostat in the main circuit.
- Draw a rectangle of wire as the main loop
- On the left side place the battery — two cell symbols in a row, with the positive terminal marked
- Along the top place the key, then the ammeter, then the rheostat, each in the line, so the current passes through all of them in turn
- On the right side place the bulb
- Draw a second, smaller loop from the two sides of the bulb out to a voltmeter, so the voltmeter sits across the bulb and not in the main line
- Mark the direction of the conventional current, from the positive terminal round the outside
A circuit must be a closed loop for any current to flow. Break it anywhere — open the key, loosen a wire — and the current stops everywhere at once, not just at the break. A key is therefore a deliberate break, and drawing a circuit with a gap in it is drawing a circuit in which nothing happens.
Putting an ammeter in parallel or a voltmeter in series is a real error and not just untidy. An ammeter has almost no resistance, so placed across a bulb it would short the bulb out and could be damaged by the large current. A voltmeter has a very high resistance, so placed in series it would nearly stop the current altogether and the bulb would not light. The connection follows from the resistance the instrument is built to have, which is why the two facts are worth learning as a pair.
The symbols.
- Cell — a long thin line for the positive terminal and a short thick line for the negative one, side by side
- Battery — two or more such cell symbols in a row, joined
- Key or switch — a break in the line with a hinged arm, closed or open as stated
- Ammeter — a circle enclosing the letter A
- Voltmeter — a circle enclosing the letter V
- Resistor — a plain rectangle in the line
- Rheostat — a rectangle with an arrow or a sliding contact across it
- Bulb — a circle with a cross inside it, or a circle with a filament loop
- Connecting wire — a plain straight line, drawn with right-angled corners
The two rules about where meters go.
- An ammeter measures the current through something, so it is joined in series with it. It must have a low resistance, so that inserting it barely changes the current it is trying to measure
- A voltmeter measures the potential difference across something, so it is joined in parallel with it. It must have a high resistance, so that it draws almost no current away from the component
Worked construction. Draw a circuit for the following description: a battery of two cells, a plug key, an ammeter and a bulb all connected in series, with a voltmeter across the bulb, and a rheostat in the main circuit.
- Draw a rectangle of wire as the main loop
- On the left side place the battery — two cell symbols in a row, with the positive terminal marked
- Along the top place the key, then the ammeter, then the rheostat, each in the line, so the current passes through all of them in turn
- On the right side place the bulb
- Draw a second, smaller loop from the two sides of the bulb out to a voltmeter, so the voltmeter sits across the bulb and not in the main line
- Mark the direction of the conventional current, from the positive terminal round the outside
A circuit must be a closed loop for any current to flow. Break it anywhere — open the key, loosen a wire — and the current stops everywhere at once, not just at the break. A key is therefore a deliberate break, and drawing a circuit with a gap in it is drawing a circuit in which nothing happens.
Putting an ammeter in parallel or a voltmeter in series is a real error and not just untidy. An ammeter has almost no resistance, so placed across a bulb it would short the bulb out and could be damaged by the large current. A voltmeter has a very high resistance, so placed in series it would nearly stop the current altogether and the bulb would not light. The connection follows from the resistance the instrument is built to have, which is why the two facts are worth learning as a pair.
Which materials let current through, and which do not?
A conductor has free electrons that can drift through it; an insulator has none, because its electrons are all bound to their atoms.
Conductors — allow current to pass:
- Metals — copper, aluminium, silver, iron. Silver is the best conductor and copper is used for wiring because it conducts well and costs less
- Graphite, the one common non-metal that conducts
- Mercury, a liquid metal
- Acids, alkalis and salt solutions — the electrolytes, where the carriers are ions rather than electrons
- The human body and the earth
Insulators — do not allow current to pass:
- Rubber, plastic, ebonite, bakelite
- Glass, porcelain, mica
- Dry wood, dry paper, dry cloth
- Pure (distilled) water
- Dry air
Why metals conduct. In a metal, each atom releases one or more of its outer electrons, which are then not attached to any particular atom. These free electrons move about randomly inside the metal, and when a cell is connected they acquire a slow drift in one direction on top of that random motion. That drift is the current.
Why insulators do not. In an insulator every electron is firmly bound to its own atom. There are no free carriers, so however hard the cell pushes, nothing moves through.
The everyday consequence. A wire is made of copper to carry the current and covered in plastic to keep it in — one conductor and one insulator in the same object, chosen for opposite properties.
Pure water is an insulator and tap water is not. Distilled water has no free ions and conducts hardly at all. Ordinary tap water has dissolved salts, which provide ions and make it a conductor. That is exactly why electrical appliances are dangerous with wet hands — the water on the skin, together with the sweat and salts in it, gives the current an easy path through the body.
The human body is a conductor, which is the reason for insulated handles, rubber-soled footwear for electricians, and the earth wire in a three-pin plug.
Dry air insulates and moist air conducts better, which is why a charged object loses its charge faster on a humid day than on a dry one.
The same material can be either, depending on its condition. Dry wood is an insulator and wet wood conducts; dry air insulates and very moist air conducts a little. So a material is not conducting or insulating by its name alone — moisture supplies ions, and ions carry current. That is the practical version of the whole distinction, and it is why every safety rule in this chapter mentions being dry.
Conductors — allow current to pass:
- Metals — copper, aluminium, silver, iron. Silver is the best conductor and copper is used for wiring because it conducts well and costs less
- Graphite, the one common non-metal that conducts
- Mercury, a liquid metal
- Acids, alkalis and salt solutions — the electrolytes, where the carriers are ions rather than electrons
- The human body and the earth
Insulators — do not allow current to pass:
- Rubber, plastic, ebonite, bakelite
- Glass, porcelain, mica
- Dry wood, dry paper, dry cloth
- Pure (distilled) water
- Dry air
Why metals conduct. In a metal, each atom releases one or more of its outer electrons, which are then not attached to any particular atom. These free electrons move about randomly inside the metal, and when a cell is connected they acquire a slow drift in one direction on top of that random motion. That drift is the current.
Why insulators do not. In an insulator every electron is firmly bound to its own atom. There are no free carriers, so however hard the cell pushes, nothing moves through.
The everyday consequence. A wire is made of copper to carry the current and covered in plastic to keep it in — one conductor and one insulator in the same object, chosen for opposite properties.
Pure water is an insulator and tap water is not. Distilled water has no free ions and conducts hardly at all. Ordinary tap water has dissolved salts, which provide ions and make it a conductor. That is exactly why electrical appliances are dangerous with wet hands — the water on the skin, together with the sweat and salts in it, gives the current an easy path through the body.
The human body is a conductor, which is the reason for insulated handles, rubber-soled footwear for electricians, and the earth wire in a three-pin plug.
Dry air insulates and moist air conducts better, which is why a charged object loses its charge faster on a humid day than on a dry one.
The same material can be either, depending on its condition. Dry wood is an insulator and wet wood conducts; dry air insulates and very moist air conducts a little. So a material is not conducting or insulating by its name alone — moisture supplies ions, and ions carry current. That is the practical version of the whole distinction, and it is why every safety rule in this chapter mentions being dry.
Exam tip
Exam tip: convert the time to seconds, and mind where the meters go
**Convert minutes and hours to seconds before using .** Four minutes is s — using gives an answer sixty times too small.
State the unit: charge in coulombs, current in amperes, and .
**Use C** and electrons per coulomb.
**Charge is always a whole multiple of ** — a value like C is impossible.
Ammeter in SERIES with low resistance; voltmeter in PARALLEL with high resistance. Give the resistance as the reason.
Draw the cell correctly: a long thin line for positive, a short thick line for negative.
A single unit is a cell; two or more make a battery. Use the right word and the right symbol.
Mark the conventional current from the positive terminal round the external circuit, and say that electrons go the other way.
Primary cells cannot be recharged (irreversible reaction, high internal resistance); secondary cells can (reversible, low internal resistance, large currents).
A cell does not store charge — it stores chemical energy and pushes charge already in the wires.
And remember pure water insulates while tap water conducts, because dissolved salts supply the ions.
State the unit: charge in coulombs, current in amperes, and .
**Use C** and electrons per coulomb.
**Charge is always a whole multiple of ** — a value like C is impossible.
Ammeter in SERIES with low resistance; voltmeter in PARALLEL with high resistance. Give the resistance as the reason.
Draw the cell correctly: a long thin line for positive, a short thick line for negative.
A single unit is a cell; two or more make a battery. Use the right word and the right symbol.
Mark the conventional current from the positive terminal round the external circuit, and say that electrons go the other way.
Primary cells cannot be recharged (irreversible reaction, high internal resistance); secondary cells can (reversible, low internal resistance, large currents).
A cell does not store charge — it stores chemical energy and pushes charge already in the wires.
And remember pure water insulates while tap water conducts, because dissolved salts supply the ions.
Did you know
Why electrons crawl while the bulb lights instantly
Switch on a light and it comes on at once, even if the switch is across the room from the bulb. It is natural to imagine electrons racing along the wire from the switch to the lamp.
They do nothing of the kind. The drift of electrons along a wire carrying an ordinary current is astonishingly slow — slower than a person walking, and in many cases slower than a snail. An individual electron would take hours to travel from the switch to the bulb.
So why is there no delay?
Because the wire is already full of free electrons, from end to end, before the switch is closed. Closing the switch does not send electrons on a journey; it sets up an electric field along the whole wire almost instantly, and every electron in the wire begins to drift at once — including the ones already inside the bulb's filament.
A pipe already full of water behaves the same way. Open the tap and water comes out of the far end immediately, not after the water from the tap has travelled the length of the pipe. The water at the outlet was already there and only needed a push.
There is a second surprise hiding in the numbers. Those drifting electrons also have a violent random motion, far faster than the drift — they are careering about in all directions and merely leaning very slightly in one of them. The current is that slight lean, not the speed of the electrons.
And it explains a piece of everyday physics. In a household circuit the current reverses direction many times a second, so the electrons never get anywhere at all; they shuffle back and forth about their starting positions. The energy travels the length of the circuit and the electrons barely leave home — which is the same relationship between energy and matter that the sound chapter found in a wave.
They do nothing of the kind. The drift of electrons along a wire carrying an ordinary current is astonishingly slow — slower than a person walking, and in many cases slower than a snail. An individual electron would take hours to travel from the switch to the bulb.
So why is there no delay?
Because the wire is already full of free electrons, from end to end, before the switch is closed. Closing the switch does not send electrons on a journey; it sets up an electric field along the whole wire almost instantly, and every electron in the wire begins to drift at once — including the ones already inside the bulb's filament.
A pipe already full of water behaves the same way. Open the tap and water comes out of the far end immediately, not after the water from the tap has travelled the length of the pipe. The water at the outlet was already there and only needed a push.
There is a second surprise hiding in the numbers. Those drifting electrons also have a violent random motion, far faster than the drift — they are careering about in all directions and merely leaning very slightly in one of them. The current is that slight lean, not the speed of the electrons.
And it explains a piece of everyday physics. In a household circuit the current reverses direction many times a second, so the electrons never get anywhere at all; they shuffle back and forth about their starting positions. The energy travels the length of the circuit and the electrons barely leave home — which is the same relationship between energy and matter that the sound chapter found in a wave.
Exam relevance
How do charge and current feed into JEE Main and NEET?
Because is the definition every later result is built on, and the conventional-current choice fixes the sign of every force law that follows.
This is the foundation for Class 12 Physics Current Electricity and Moving Charges and Magnetism, examined in JEE Main and NEET. The definition here becomes the instantaneous form
so a question giving the charge as a function of time and asking for the current at an instant is asking for a derivative. Questions of that shape are standard in JEE Main, and they are this page's formula with the average replaced by a rate at a point.
Drift velocity makes the microscopic picture quantitative. Class 12 derives
with the number of free electrons per unit volume, the cross-section and the drift speed — and the surprising slowness described in the previous section comes out as a number from it. Numericals on drift velocity are recurring in both papers, and they need the electron charge C from this page.
The conventional direction becomes essential rather than harmless. In Class 12 Moving Charges and Magnetism, the force on a current-carrying wire is , and the right-hand rules are all stated for conventional current. Using the electron direction flips every force and every field direction, so the convention that looked arbitrary here decides the sign of the answer there.
Quantisation of charge is examined directly in Class 12 Electric Charges and Fields: any charge is for a whole number . **The point made on this page — that C is impossible — is that principle, and it appears as a multiple-choice item.
Conductors, insulators and their electron structure** become Class 12 Semiconductor Electronics, where the band picture explains why some materials have free electrons and others do not, and where semiconductors sit between the two. **Electrolytes reappear in Class 12 Chemistry Electrochemistry, where ions carry the current and a cell's reversibility decides whether it is a primary or a secondary cell — the same distinction, treated chemically.
For NEET Biology, nerve impulses are carried by ion movement across membranes, and the conductivity of a salt solution described here is what makes that possible. For NEET Physics**, expect and drift-velocity numericals.
What the questions look like. For board work, expect define charge and current with units, **numericals on and on numbers of electrons, distinguish primary from secondary cells, draw the standard symbols, construct a circuit from a description, and classify materials. Neat labelled diagrams carry much of the credit. For JEE Main and NEET, expect drift velocity, quantisation of charge, and circuits where the meters' resistances matter.
How board and competitive emphasis differ. A board paper rewards the correct symbol and the stated reason for an ammeter being in series. A competitive paper assumes the circuit and asks for a number — often requiring you to notice that a real ammeter's resistance changes the current.
The single trap that costs the most marks.** Leaving the time in minutes. A current of A for minutes delivers C, not C, and the wrong answer is a clean-looking number that fails only on the unit. The defence is to write the conversion as its own line — s — before touching the formula, since the ampere is defined per second and nothing else.
This is the foundation for Class 12 Physics Current Electricity and Moving Charges and Magnetism, examined in JEE Main and NEET. The definition here becomes the instantaneous form
so a question giving the charge as a function of time and asking for the current at an instant is asking for a derivative. Questions of that shape are standard in JEE Main, and they are this page's formula with the average replaced by a rate at a point.
Drift velocity makes the microscopic picture quantitative. Class 12 derives
with the number of free electrons per unit volume, the cross-section and the drift speed — and the surprising slowness described in the previous section comes out as a number from it. Numericals on drift velocity are recurring in both papers, and they need the electron charge C from this page.
The conventional direction becomes essential rather than harmless. In Class 12 Moving Charges and Magnetism, the force on a current-carrying wire is , and the right-hand rules are all stated for conventional current. Using the electron direction flips every force and every field direction, so the convention that looked arbitrary here decides the sign of the answer there.
Quantisation of charge is examined directly in Class 12 Electric Charges and Fields: any charge is for a whole number . **The point made on this page — that C is impossible — is that principle, and it appears as a multiple-choice item.
Conductors, insulators and their electron structure** become Class 12 Semiconductor Electronics, where the band picture explains why some materials have free electrons and others do not, and where semiconductors sit between the two. **Electrolytes reappear in Class 12 Chemistry Electrochemistry, where ions carry the current and a cell's reversibility decides whether it is a primary or a secondary cell — the same distinction, treated chemically.
For NEET Biology, nerve impulses are carried by ion movement across membranes, and the conductivity of a salt solution described here is what makes that possible. For NEET Physics**, expect and drift-velocity numericals.
What the questions look like. For board work, expect define charge and current with units, **numericals on and on numbers of electrons, distinguish primary from secondary cells, draw the standard symbols, construct a circuit from a description, and classify materials. Neat labelled diagrams carry much of the credit. For JEE Main and NEET, expect drift velocity, quantisation of charge, and circuits where the meters' resistances matter.
How board and competitive emphasis differ. A board paper rewards the correct symbol and the stated reason for an ammeter being in series. A competitive paper assumes the circuit and asks for a number — often requiring you to notice that a real ammeter's resistance changes the current.
The single trap that costs the most marks.** Leaving the time in minutes. A current of A for minutes delivers C, not C, and the wrong answer is a clean-looking number that fails only on the unit. The defence is to write the conversion as its own line — s — before touching the formula, since the ampere is defined per second and nothing else.
Key takeaways
Charge, current, cells and circuit symbols: quick revision
- Charge (Q) is the property responsible for electrical effects, positive or negative, in coulombs.
- Current (I) is the rate of flow of charge: , in amperes, and .
- C in s gives A; A for ** s** gives C; C at A takes s.
- Convert minutes to seconds first — four minutes is s, not .
- ** C**, so one coulomb is electrons.
- A current of A carries electrons per second; electrons carry C.
- ** A** and A, so mA is A.
- Charge is quantised — always a whole multiple of , so C is impossible.
- Primary cell: reaction irreversible, cannot be recharged, high internal resistance, small currents. Examples: dry cell, voltaic cell.
- Secondary cell: reaction reversible, can be recharged by a reverse current, low internal resistance, large currents. Examples: lead-acid, nickel-cadmium, lithium-ion.
- A cell stores chemical energy, not charge — it pushes electrons already present in the wires.
- A cell's function: maintain a potential difference and convert chemical energy into electrical energy.
- One unit is a cell; two or more form a battery.
- Symbols: cell — long thin line positive, short thick line negative; battery — several cells; key — a hinged break; ammeter — A in a circle; voltmeter — V in a circle; resistor — a rectangle; rheostat — a rectangle with a slider; bulb — a circle with a cross.
- Ammeter in SERIES, and it must have low resistance. Voltmeter in PARALLEL, and it must have high resistance.
- A circuit must be a closed loop — break it anywhere and the current stops everywhere.
- An ammeter in parallel would short out the component; a voltmeter in series would nearly stop the current.
- Conventional current flows from the positive terminal through the external circuit; electrons drift the opposite way.
- The convention predates the electron, nothing measurable depends on it, and in liquids and gases the carriers are ions of both signs.
- Conductors: metals (silver best, copper used), graphite, mercury, acids, alkalis, salt solutions, the human body, the earth.
- Insulators: rubber, plastic, ebonite, glass, porcelain, mica, dry wood, pure water, dry air.
- Metals conduct because of free electrons; insulators have none, as every electron is bound.
- Pure water insulates and tap water conducts, because dissolved salts supply ions — which is why wet hands and appliances are dangerous.
Count how many cells are in the remote controls and torches around your home, then work out how much charge one of them delivers in an hour at a tenth of an ampere.
- Current (I) is the rate of flow of charge: , in amperes, and .
- C in s gives A; A for ** s** gives C; C at A takes s.
- Convert minutes to seconds first — four minutes is s, not .
- ** C**, so one coulomb is electrons.
- A current of A carries electrons per second; electrons carry C.
- ** A** and A, so mA is A.
- Charge is quantised — always a whole multiple of , so C is impossible.
- Primary cell: reaction irreversible, cannot be recharged, high internal resistance, small currents. Examples: dry cell, voltaic cell.
- Secondary cell: reaction reversible, can be recharged by a reverse current, low internal resistance, large currents. Examples: lead-acid, nickel-cadmium, lithium-ion.
- A cell stores chemical energy, not charge — it pushes electrons already present in the wires.
- A cell's function: maintain a potential difference and convert chemical energy into electrical energy.
- One unit is a cell; two or more form a battery.
- Symbols: cell — long thin line positive, short thick line negative; battery — several cells; key — a hinged break; ammeter — A in a circle; voltmeter — V in a circle; resistor — a rectangle; rheostat — a rectangle with a slider; bulb — a circle with a cross.
- Ammeter in SERIES, and it must have low resistance. Voltmeter in PARALLEL, and it must have high resistance.
- A circuit must be a closed loop — break it anywhere and the current stops everywhere.
- An ammeter in parallel would short out the component; a voltmeter in series would nearly stop the current.
- Conventional current flows from the positive terminal through the external circuit; electrons drift the opposite way.
- The convention predates the electron, nothing measurable depends on it, and in liquids and gases the carriers are ions of both signs.
- Conductors: metals (silver best, copper used), graphite, mercury, acids, alkalis, salt solutions, the human body, the earth.
- Insulators: rubber, plastic, ebonite, glass, porcelain, mica, dry wood, pure water, dry air.
- Metals conduct because of free electrons; insulators have none, as every electron is bound.
- Pure water insulates and tap water conducts, because dissolved salts supply ions — which is why wet hands and appliances are dangerous.
Count how many cells are in the remote controls and torches around your home, then work out how much charge one of them delivers in an hour at a tenth of an ampere.