Why Current Is Said to Flow the Wrong Way Round
Learn the difference between primary and secondary cells, draw standard circuit symbols, tell an open circuit from a closed one, understand conventional current, and test materials as conductors or insulators.
Why is current said to flow from positive to negative?
Because the direction was agreed upon before anyone knew what was actually moving. This agreed direction, from the positive terminal to the negative through the external circuit, is called conventional current, and every circuit diagram still uses it.
It is a convention, not a mistake — and all the standard rules are built on it. This page covers everything in the ICSE Class 7 Physics chapter's first half: sources of electricity, cells and batteries, circuit symbols, open and closed circuits, and conductors and insulators.
It is a convention, not a mistake — and all the standard rules are built on it. This page covers everything in the ICSE Class 7 Physics chapter's first half: sources of electricity, cells and batteries, circuit symbols, open and closed circuits, and conductors and insulators.
What is the difference between a primary and a secondary cell?
A cell converts chemical energy into electrical energy. The difference is whether it can be recharged.
A primary cell cannot be recharged. Once its chemicals are used up it is discarded — a dry cell of the kind used in a torch or a wall clock is the standard example.
A secondary cell, also called an accumulator, can be recharged by passing current through it in the reverse direction, and used many times. A mobile phone battery and a car lead-acid battery are secondary cells.
A battery is two or more cells joined together. They are connected in series, the positive terminal of one to the negative terminal of the next, so their voltages add. Two dry cells joined this way give
which is why a torch needs two cells stacked the same way round.
That direction matters, and it is the commonest practical error. Put the two cells in facing each other, positive to positive, and their effects cancel — the torch stays dark even though both cells are perfectly good.
A dry cell has a carbon rod as its positive terminal and a zinc container as its negative one, and is marked with a and a so it can be inserted correctly.
A primary cell cannot be recharged. Once its chemicals are used up it is discarded — a dry cell of the kind used in a torch or a wall clock is the standard example.
A secondary cell, also called an accumulator, can be recharged by passing current through it in the reverse direction, and used many times. A mobile phone battery and a car lead-acid battery are secondary cells.
A battery is two or more cells joined together. They are connected in series, the positive terminal of one to the negative terminal of the next, so their voltages add. Two dry cells joined this way give
which is why a torch needs two cells stacked the same way round.
That direction matters, and it is the commonest practical error. Put the two cells in facing each other, positive to positive, and their effects cancel — the torch stays dark even though both cells are perfectly good.
A dry cell has a carbon rod as its positive terminal and a zinc container as its negative one, and is marked with a and a so it can be inserted correctly.
What are the standard circuit symbols?
Circuit diagrams use agreed symbols so that any diagram can be read by anyone.
- Cell — one long thin line and one short thick line. The long line is the positive terminal.
- Battery — two or more cell symbols drawn side by side in a row.
- Bulb — a circle with a cross inside it.
- Switch (key) — a gap in the wire with a small hinged line; drawn touching when closed and lifted away when open.
- Connecting wires — plain straight lines, drawn with right-angled corners.
- Ammeter — a circle containing the letter A, used to measure current.
A simple closed circuit joins a cell, a switch and a bulb in a single loop with connecting wires. Drawn properly, the lines are straight, the corners square, and the components spaced around the loop rather than crowded into one corner.
The detail examiners look for is the long line marked positive on the cell symbol, since the direction of conventional current in the diagram depends on it.
And the polarity of an ammeter is not optional: its positive terminal must be connected towards the positive terminal of the cell, or the needle is pushed backwards off the scale.
- Cell — one long thin line and one short thick line. The long line is the positive terminal.
- Battery — two or more cell symbols drawn side by side in a row.
- Bulb — a circle with a cross inside it.
- Switch (key) — a gap in the wire with a small hinged line; drawn touching when closed and lifted away when open.
- Connecting wires — plain straight lines, drawn with right-angled corners.
- Ammeter — a circle containing the letter A, used to measure current.
A simple closed circuit joins a cell, a switch and a bulb in a single loop with connecting wires. Drawn properly, the lines are straight, the corners square, and the components spaced around the loop rather than crowded into one corner.
The detail examiners look for is the long line marked positive on the cell symbol, since the direction of conventional current in the diagram depends on it.
And the polarity of an ammeter is not optional: its positive terminal must be connected towards the positive terminal of the cell, or the needle is pushed backwards off the scale.
What is the difference between an open and a closed circuit?
A closed circuit is a complete, unbroken loop, so current flows and the bulb lights. An open circuit has a break somewhere, so no current flows and the bulb stays dark.
A switch is simply a deliberate break you can control. Pressing it closes the gap and completes the loop; releasing it opens the circuit again.
Breaks can also be accidental: a loose connection at a terminal, a broken filament inside the bulb, or a snapped wire. This is what to check first when a torch fails — a cell inserted the wrong way round, a loose contact, or a blown bulb.
In a closed circuit, conventional current flows from the positive terminal of the cell, through the external circuit, to the negative terminal, and continues inside the cell from negative back to positive. Arrows on a circuit diagram must follow that direction.
The subtlety behind the opening question is worth stating. The particles that actually move in a metal wire are electrons, and they travel the opposite way, from negative to positive. Both statements are correct: conventional current is the agreed direction, and electron flow is the physical movement.
So a question asking for the direction of current wants positive to negative, while one asking about electrons wants the reverse.
A switch is simply a deliberate break you can control. Pressing it closes the gap and completes the loop; releasing it opens the circuit again.
Breaks can also be accidental: a loose connection at a terminal, a broken filament inside the bulb, or a snapped wire. This is what to check first when a torch fails — a cell inserted the wrong way round, a loose contact, or a blown bulb.
In a closed circuit, conventional current flows from the positive terminal of the cell, through the external circuit, to the negative terminal, and continues inside the cell from negative back to positive. Arrows on a circuit diagram must follow that direction.
The subtlety behind the opening question is worth stating. The particles that actually move in a metal wire are electrons, and they travel the opposite way, from negative to positive. Both statements are correct: conventional current is the agreed direction, and electron flow is the physical movement.
So a question asking for the direction of current wants positive to negative, while one asking about electrons wants the reverse.
How do you test whether a material conducts electricity?
Build a test circuit: a cell, a bulb and two free ends of wire. Touch the two free ends to the material being tested, and watch the bulb.
If the bulb lights, the material completes the circuit and is a conductor. If it stays dark, the material breaks the circuit and is an insulator.
Conductors of electricity include all metals — copper, aluminium, iron, silver — as well as graphite, and also salt solutions, acids and the human body.
Insulators include rubber, plastic, glass, dry wood, paper, cloth, ebonite and dry air.
Every electrical fitting in a home uses both together, on purpose. A plug's pins are brass because it must conduct, while its body is plastic because that must not. Wires have a copper core inside a rubber or PVC sleeve, and an electrician's screwdriver and pliers have insulated handles.
Two cases are easy to get wrong. Graphite conducts although it is a non-metal, which is why a pencil lead lights the test bulb. And water is a poor conductor when pure but a good one once salts are dissolved in it — tap water conducts, and that is precisely why electrical switches must never be touched with wet hands.
If the bulb lights, the material completes the circuit and is a conductor. If it stays dark, the material breaks the circuit and is an insulator.
Conductors of electricity include all metals — copper, aluminium, iron, silver — as well as graphite, and also salt solutions, acids and the human body.
Insulators include rubber, plastic, glass, dry wood, paper, cloth, ebonite and dry air.
Every electrical fitting in a home uses both together, on purpose. A plug's pins are brass because it must conduct, while its body is plastic because that must not. Wires have a copper core inside a rubber or PVC sleeve, and an electrician's screwdriver and pliers have insulated handles.
Two cases are easy to get wrong. Graphite conducts although it is a non-metal, which is why a pencil lead lights the test bulb. And water is a poor conductor when pure but a good one once salts are dissolved in it — tap water conducts, and that is precisely why electrical switches must never be touched with wet hands.
Exam tip
Exam tip: marking the long line and the current arrow
Circuit-diagram questions are marked on details that take seconds to add.
Draw the cell with its long line as the positive terminal, label and , and put an arrow on the wire showing conventional current leaving the positive terminal.
Use a ruler. Straight lines with right-angled corners and correctly drawn symbols carry marks of their own, and a freehand sketch loses them even when the circuit is right.
For cells joined into a battery, show them positive to negative in a row and add the voltages: two cells give .
And read the direction question carefully. Current flows positive to negative outside the cell; electrons flow negative to positive. Answering one when the question asked for the other is the single commonest error in this chapter.
Draw the cell with its long line as the positive terminal, label and , and put an arrow on the wire showing conventional current leaving the positive terminal.
Use a ruler. Straight lines with right-angled corners and correctly drawn symbols carry marks of their own, and a freehand sketch loses them even when the circuit is right.
For cells joined into a battery, show them positive to negative in a row and add the voltages: two cells give .
And read the direction question carefully. Current flows positive to negative outside the cell; electrons flow negative to positive. Answering one when the question asked for the other is the single commonest error in this chapter.
Did you know
Why does a pencil lead light the test bulb when it is not a metal?
Because conduction needs free charges to move, and graphite has them even though it is a non-metal.
In graphite, the carbon atoms are arranged in flat sheets, and some electrons are free to travel along those sheets rather than being locked to individual atoms. A current can therefore pass through the pencil lead.
That makes graphite the useful exception to "only metals conduct" — and the reason it is used for the brushes inside electric motors, where a conductor that also slides smoothly is exactly what is needed.
In graphite, the carbon atoms are arranged in flat sheets, and some electrons are free to travel along those sheets rather than being locked to individual atoms. A current can therefore pass through the pencil lead.
That makes graphite the useful exception to "only metals conduct" — and the reason it is used for the brushes inside electric motors, where a conductor that also slides smoothly is exactly what is needed.
Key takeaways
Cells, circuits and conductors: quick revision
- A cell converts chemical energy to electrical energy; a primary cell such as a dry cell cannot be recharged, while a secondary cell or accumulator can.
- A battery is two or more cells joined positive to negative in series, so two cells give — facing them the same way round matters.
- Know the symbols for cell, battery, bulb, switch, connecting wires and ammeter, and draw the cell's long line as positive.
- A closed circuit is an unbroken loop and the bulb lights; an open circuit has a break, so no current flows.
- Conventional current flows from positive to negative in the external circuit, while electrons physically move the opposite way.
- Test with a cell-and-bulb circuit: metals and graphite conduct, while rubber, plastic, glass and dry wood insulate — and salty or tap water conducts.
You will remember all of this far better after answering five questions on it than after reading it twice.
- A battery is two or more cells joined positive to negative in series, so two cells give — facing them the same way round matters.
- Know the symbols for cell, battery, bulb, switch, connecting wires and ammeter, and draw the cell's long line as positive.
- A closed circuit is an unbroken loop and the bulb lights; an open circuit has a break, so no current flows.
- Conventional current flows from positive to negative in the external circuit, while electrons physically move the opposite way.
- Test with a cell-and-bulb circuit: metals and graphite conduct, while rubber, plastic, glass and dry wood insulate — and salty or tap water conducts.
You will remember all of this far better after answering five questions on it than after reading it twice.