Direct current circuits

1. A d.c. circuit is a circuit in which the current always flows in the same direction, supplied by a cell, a battery or a d.c. power supply.
2. Every d.c. supply has two terminals, a positive terminal ($+$) and a negative terminal ($-$).
3. In a metal wire, the moving charges are electrons, which are negatively charged, so they leave the negative terminal, travel around the circuit and return to the positive terminal.
4. The direction of current was chosen before electrons were discovered, and scientists agreed to show current flowing from positive to negative.
5. This agreed direction, called conventional current, is still used on every circuit diagram.
   1. Conventional current flows out of the positive terminal, around the circuit and back into the negative terminal, which is the opposite direction to the flow of electrons.
6. The positive and negative terminals matter whenever a component only works one way round, such as a diode, or whenever a meter has its own positive and negative terminals.

Circuit diagrams

1. A circuit diagram uses standard symbols instead of pictures, so that anyone can read it in the same way whatever the real components look like.
2. Connecting wires are drawn as straight lines, usually horizontal or vertical, so that most circuit diagrams look like a set of rectangles.
3. A junction, where wires are joined, is shown by a solid dot, while two lines that cross without a dot are not connected.
4. A circuit diagram shows how the components are connected, not where they sit on the bench or how long the wires are.
   1. Connecting wires are treated as having no resistance, so a long wire on a diagram behaves exactly like a short one.
5. A complete circuit is a closed loop of conductors running from one terminal of the supply, through the components, back to the other terminal.
   1. Any gap in the loop, apart from a switch that is meant to be open, means that no current can flow.

Supplies and switches

1. A cell is drawn as two parallel lines of different lengths, where the long thin line is the positive terminal and the short thick line is the negative terminal.
2. A battery is drawn as two or more cell symbols joined together, sometimes with a dashed line between the first and last cell.
   1. When the cells all face the same way, the potential difference of the battery is the sum of the potential differences of the cells, so two $1.5V$ cells give $3.0V$.
3. A d.c. power supply is usually drawn as two small open circles marked $+$ and $-$, sometimes labelled with the potential difference it is set to.
   1. On a laboratory power supply, the positive terminal is normally red and the negative terminal is normally black.
4. An open switch is drawn as a short line hinged at one end and lifted away from the contact, showing a gap that breaks the circuit.
5. A closed switch is drawn with the line lying flat against the contact, showing that the circuit is complete.

Component symbols

1. A fixed resistor is drawn as a rectangle, and it has a set resistance that limits the current in a circuit.
2. A variable resistor is drawn as a rectangle with a diagonal arrow through it, and moving its slider changes its resistance and so changes the current.
3. A filament lamp is drawn as a circle with a cross inside it.
4. A diode is drawn as a triangle pointing at a short bar, with the wire passing straight through both.
   1. The triangle points in the direction in which conventional current is allowed to flow through the diode.
5. A light-dependent resistor (LDR) is drawn as a resistor rectangle, usually inside a circle, with two arrows pointing towards it to show light falling on it.
6. An NTC thermistor is drawn as a resistor rectangle with a diagonal line through it that ends in a short horizontal tail.
7. An ammeter is drawn as a circle containing the letter A, and a voltmeter is drawn as a circle containing the letter V.
   1. An ammeter is drawn in the line of the circuit, while a voltmeter is drawn on its own short loop that joins the circuit on either side of the component it measures.

Drawing and reading diagrams

1. Draw circuit diagrams with a ruler and a sharp pencil, using only the standard symbols.
2. Draw the wires so that each loop is closed, with no gaps except at an open switch.
3. Mark every junction with a dot, and draw the cell or battery so that its long and short lines are clear.
4. To read a diagram, start at the positive terminal and trace the path of conventional current around the circuit back to the negative terminal.
   1. Components that the current passes through one after another, with no junction between them, are in series.
   2. Components on separate branches between the same two junctions are in parallel.
5. Check which way round any diode is drawn, because a diode pointing against the conventional current from the supply blocks the current in that part of the circuit.