4.3 Basic Circuits and Electrical Cause-and-Effect

Circuits on the Mechanical Reasoning battery

The same candidate pack that names kinetic energy also names circuits. The Online Cognitive Ability Test is still two components — Mechanical Reasoning and Core Abilities — and QFD does not publish circuit item counts or a percentage pass mark. This section is independent OpenExamPrep teaching of qualitative electrical cause-and-effect: what lights, what goes dark, and what a short does. It uses appliance and station-electrical flavour. It does not claim to be an official QFD paper, and it does not reteach Chapter 3 levers, pulleys, or gears. If a generator is belt-driven, Chapter 3 covers the belt; this section covers the wires.

You do not need an electrician's licence for these items. You need a path model. Current needs a closed loop from one side of a supply to the other. Break the loop and current in that loop stops. Give current an easy extra path around a lamp and the lamp may go dark while the extra path carries a large current.

Current, a supply, and a load

A circuit is a path for electric current. A battery, generator, or station supply is the energy source. A load is what you want the current to do work in: a bulb, a fan motor, a charger, a thermal-camera battery indicator, a locker light. Wires connect them. A switch is a deliberate break or join in the path.

Current is the flow of charge. It does not jump a gap in an ordinary aptitude diagram. An open switch is a gap. A closed switch is a joined path.

Qualitative reminder, not an algebra dump: for a given supply, more resistance means less current, and less resistance means more current. A lamp is a moderate resistance. A thick wire is a low resistance. A short circuit is an unintended very-low-resistance path. Tiny worked example: a 12 volt supply across a 6 ohm lamp would drive 2 amperes (current = voltage ÷ resistance). If a near-zero-ohm wire sits across the same 12 volt supply, the current tries to become huge. You will not be asked to compute 2 amperes, and Pearson VUE / OnVUE rules still allow no calculator. You will be asked which path the current prefers, and which bulb stays lit.

Two switches can combine in the same qualitative way as fluid valves, with one vocabulary trap: an open electrical switch stops current; a closed hydrant valve stops water. Read the noun. Do not copy the open/closed word from a hose item onto a wiring item without flipping the meaning.

• Two switches in series: both must be closed or the loop is dead (logical AND).
• Two switches in parallel: either closed switch can complete a path around that pair (logical OR).

Series circuits

In a series circuit there is one path. Every load and switch sits on that same loop. The same current has to pass through each device in turn.

Cause-and-effect:

• Open any one switch, or remove any one bulb so the loop breaks, and every load on that loop goes out.
• Add more bulbs in series and they typically glow dimmer (more total resistance, less current), if the supply is unchanged.
• There is no “other way around” for the current.

If a stem shows two bulbs and one switch all in a single loop, one open switch darkens both bulbs. Distance along the wire does not save the far bulb. Copper still needs a closed loop. Current is not “used up” so that the last bulb is dark because it is last; if the loop is complete, the same series current goes through each lamp. They may all be dim together. They do not take turns.

Parallel circuits

In a parallel circuit there are two or more branches from the same supply points. Each branch is its own path.

Cause-and-effect:

• Open a switch on one branch, or remove a bulb on that branch, and that branch goes dark.
• Other complete branches can stay lit.
• Each branch can take current from the supply independently (up to what the supply can give).

This is why household lights are wired in parallel: one blown bulb does not darken the whole station kitchen. A Mechanical Reasoning picture with two loops sharing a battery is almost always asking: does the other loop still have a closed path?

Which bulb stays lit

Use this checklist:

1. Trace from one side of the battery with your eye. Can you reach the other side without a break, passing through the bulb you care about?
2. If the only path through that bulb is broken, that bulb is out.
3. If another bulb has its own unbroken path, that other bulb can stay lit even if the first is out.
4. If both bulbs share a single series path, they live and die together.

A common mixed picture is a series pair on one branch, sitting in parallel with a third bulb. If the third bulb's branch is intact, it stays lit when the series pair opens. If the shared battery connection is cut, everything goes out. Trace the shared wire before you guess.

Short paths

A short is a low-resistance extra path that bypasses the intended load. Current prefers the easier path. Effects you should expect:

• The intended bulb can go dim or out because current is stolen by the short.
• The shorted wire can carry a large current (overheat risk in real life; on the test, “large current in the short path”).
• A fuse or breaker, if shown, is the device meant to open when current is too large. An aptitude diagram may show a fuse blown after a short: then the whole protected loop is open.

A short is not “a longer path.” It is usually the shorter, easier electrical path. It is not a fluid kink, and it is not a hose. Water and electricity both follow paths, but a kink reduces water flow while a short increases electrical flow in the unintended path. An extra wire that still goes through a lamp is just another load path, not a short. A short skips the lamp.

Appliance electrics on a fireground-flavoured stem

Station and appliance pictures stay qualitative:

• A thermal imaging camera on charge: if the charger lead is unplugged, that path is open; the camera may still run on its battery (a second source) if the diagram shows one.
• A locker light and a bench light sharing a fuse: a short in one appliance can blow the shared fuse and darken both.
• A vehicle work-lamp switch in series with the lamp: open switch, lamp out. A second lamp in parallel with the first can stay on if its own switch is closed.
• A BA-set compartment lamp on a door switch: opening the door closes the switch in many real fittings, but on the test you follow the drawn switch, not a remembered locker. If the drawn switch is open, the lamp is out.

Do not drag gears or pulleys into a wiring diagram unless the stem actually shows a driven generator. The tested idea is the electrical path.

The diagram below is the decision tree those pictures are testing: one path versus separate branches, then what an open switch does in each case.

Traps that cost easy marks

• Believing current “uses up” so the last bulb in series must be darker because it is last, rather than because the whole series current is lower.
• Assuming an open switch only affects nearby bulbs. In series it affects the whole loop.
• Treating parallel as if it were series: one bulb out does not have to kill the others.
• Calling any extra wire a short even when it still goes through a lamp. A short bypasses the load.
• Mixing electrical “pressure” (voltage) with hose pressure. The analogy can help (“push”), but a closed hydrant and an open switch are opposites in everyday language: a closed valve stops fluid; an open switch stops current. Watch the vocabulary.
• Re-solving a circuit as a lever or gear problem. Chapter 3 stays on the machine. This section stays on the wires.

On a live item, cover the battery with your finger and ask whether each lamp still has a way home. If the only way home is the series loop and that loop is open, every series lamp is out. If a second branch still joins both supply terminals through its own lamp, that lamp can stay lit. If a wire jumps around a lamp, treat that jump as a short and expect the bypassed lamp to lose current.