7.2 Circuits

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7. Electronics Information

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Circuits

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Circuit Diagram

Reading a circuit diagram is crucial for electronics. Let’s start with the basics.

Here we can see the battery, a switch, a light bulb, and the wire connecting everything. The arrows show the direction current would flow if the switch were closed — from the positive end of the battery, through the switch, to the light bulb, and back. Right now, though, the switch is drawn open, so the loop is broken and no current is actually flowing. We’ll talk more about open and closed switches in a moment.

Charge is the electrical property carried by the particles in a circuit. Current is the rate at which that charge flows through a wire.

Load is what consumes the electricity. This could be a lamp, a motor, a TV, or a heater.

Open vs. closed circuits

For electrical charge to flow through a circuit, the circuit must form a closed loop. If the loop is ever broken, the current will stop flowing.

An open circuit is not a closed loop, so electric current cannot flow through it.

A closed circuit is a complete loop through which electric current can flow.

To easily open or close a circuit, we often use a switch. When a switch is flipped to the “on” position, it will close the circuit, allowing electrical current to flow and turning the device on.

When a switch is flipped to the “off” position, it opens the circuit, stopping electrical current from flowing and turning the device off.

Types of Circuits

The two ways of wiring a circuit are series and parallel.

Series and parallel circuit. — Series and parallel circuit

Series circuits

In a series circuit, there is only one path for the electricity to flow. The current will remain the same across all components, and the voltage is divided among them. The resistance increases as components are added.

If one component breaks, the entire circuit stops, because the loop has been opened.

Let’s look at an example with the resistance in a series circuit.

What is the total resistance in a series circuit if the resistors are $2Ω$ , $3Ω$ , and $5Ω$ ?

For a series, we would just add them, so

$2 + 3 + 5 = 10Ω$ .

Parallel circuits

In a parallel circuit, there are multiple paths for the electricity to flow. The current will be divided amongst the parallel branches, while the voltage stays the same across every component. The resistance decreases as components are added.

If one component breaks, the entire circuit will remain on, because the loop is still closed.

Now let’s look at an example with the resistance in a parallel circuit.

If two $4Ω$ resistors are wired in parallel, what is the total resistance?

$1/ R = 1/ R_{1} + 1/ R_{2}$

$1/ R = 1/4 + 1/4$

$1/ R = 1/2$

$R = 2Ω$

Let’s look at a more complex example that combines what we’ve learned.

This circuit has two switches and two branches. $S 1$ sits before the branches split, so opening it cuts power to both $L 1$ and $L 2$ .

$S 2$ , however, sits only in the branch with $R 1$ and $L 1$ , so opening it only turns off $L 1$ .

$L 2$ stays on, powered through the $R 2$ branch, since that loop is still closed.

Short circuits

A short in a circuit is caused by electricity traveling an unintended path. Electrons will flow through the path of least resistance, so if something interferes with the intended loop, the electrons will take another route.

Short circuit caused by an unintended low-resistance path. — Short circuit

We can use Ohm’s Law to see exactly why this is so dangerous. In the diagram above, the normal load path has a resistance of about $10Ω$ , while the short circuit path (created by electrons flowing through the metal screwdriver instead of through the circuit) has almost no resistance at all, about $0.01Ω$ . Assuming a standard household voltage of $120$ V:

Normal path: $I = \frac{V}{R}$

$I = \frac{120}{10}$

$I = 12$ amps

Short circuit path: $I = \frac{V}{R}$

$I = \frac{120}{0.01}$

$I = 12, 000$ amps

That’s a thousand-fold spike in current in an instant — more than enough to melt wires, start a fire, or destroy the device. This is exactly the kind of current spike a fuse or breaker is designed to catch before it causes damage.

Safety

Grounding

Grounding works on the same principle as shorts in circuits. Because electricity will follow the path of least resistance, we can use that knowledge to safely dissipate dangerous charges. Grounding literally uses the earth as a conductor for the electricity, away from people or equipment. A lightning rod is a familiar example: it gives a lightning strike an easy, low-resistance path into the ground instead of through a building.

Fuses and breakers

Fuses and breakers serve the same purpose: to disconnect power when the current gets too high, protecting people or equipment.

Fuses contain a small metal link that melts when the current exceeds the rated value, breaking the circuit. They are a one-time-use protection and must be replaced once “blown.”

Breakers contain a thermal strip or magnetic coil, which releases—or “trips”—when the current is overloaded. This opens the circuit and shuts off the power. Unlike fuses, breakers can usually be reset for repeated use.

Circuit Basics

Charge: electrical property carried by particles; current: rate of charge flow
Load: component that consumes electricity (lamp, motor, TV, heater)
Circuit diagrams show direction current flows from positive battery terminal through components back to battery

Open vs. Closed Circuits

Closed circuit: complete loop, current flows
Open circuit: broken loop, current cannot flow
Switches control this: “on” closes circuit (current flows), “off” opens circuit (current stops)

Series Circuits

Single path for current; current same throughout, voltage divides among components
Total resistance increases as components added: $R_{t o t a l} = R_{1} + R_{2} + R_{3} ...$
If one component fails, entire circuit stops (loop broken)

Parallel Circuits

Multiple paths for current; current divides among branches, voltage same across each component
Total resistance decreases as components added: $1/ R_{t o t a l} = 1/ R_{1} + 1/ R_{2} ...$
If one component fails, rest of circuit stays on (loop still closed elsewhere)
Switch placement matters: a switch before a branch split affects all branches; a switch within one branch only affects that branch

Short Circuits

Caused by electricity taking unintended, low-resistance path
Ohm’s Law ( $I = V / R$ ) shows danger: tiny resistance drop causes massive current spike (e.g., $12 A \to 12, 000 A$ )
Can melt wires, cause fires, destroy devices

Safety: Grounding, Fuses, and Breakers

Grounding: uses earth as low-resistance path to safely dissipate dangerous charge (e.g., lightning rods)
Fuses: metal link melts when current exceeds rating; one-time use, must be replaced
Breakers: thermal/magnetic trip mechanism opens circuit on overload; resettable, reusable

Next | 7.3 Electrical Power

Circuits

Circuit Diagram

Reading a circuit diagram is crucial for electronics. Let’s start with the basics.

Charge is the electrical property carried by the particles in a circuit. Current is the rate at which that charge flows through a wire.

Load is what consumes the electricity. This could be a lamp, a motor, a TV, or a heater.

Open vs. closed circuits

For electrical charge to flow through a circuit, the circuit must form a closed loop. If the loop is ever broken, the current will stop flowing.

An open circuit is not a closed loop, so electric current cannot flow through it.

A closed circuit is a complete loop through which electric current can flow.

To easily open or close a circuit, we often use a switch. When a switch is flipped to the “on” position, it will close the circuit, allowing electrical current to flow and turning the device on.

When a switch is flipped to the “off” position, it opens the circuit, stopping electrical current from flowing and turning the device off.

Types of Circuits

The two ways of wiring a circuit are series and parallel.

Series circuits

If one component breaks, the entire circuit stops, because the loop has been opened.

Let’s look at an example with the resistance in a series circuit.

What is the total resistance in a series circuit if the resistors are $2Ω$ , $3Ω$ , and $5Ω$ ?

For a series, we would just add them, so

$2 + 3 + 5 = 10Ω$ .

Parallel circuits

If one component breaks, the entire circuit will remain on, because the loop is still closed.

Now let’s look at an example with the resistance in a parallel circuit.

If two $4Ω$ resistors are wired in parallel, what is the total resistance?

$1/ R = 1/ R_{1} + 1/ R_{2}$

$1/ R = 1/4 + 1/4$

$1/ R = 1/2$

$R = 2Ω$

Let’s look at a more complex example that combines what we’ve learned.

This circuit has two switches and two branches. $S 1$ sits before the branches split, so opening it cuts power to both $L 1$ and $L 2$ .

$S 2$ , however, sits only in the branch with $R 1$ and $L 1$ , so opening it only turns off $L 1$ .

$L 2$ stays on, powered through the $R 2$ branch, since that loop is still closed.

Short circuits

Normal path: $I = \frac{V}{R}$

$I = \frac{120}{10}$

$I = 12$ amps

Short circuit path: $I = \frac{V}{R}$

$I = \frac{120}{0.01}$

$I = 12, 000$ amps

Safety

Grounding

Fuses and breakers

Fuses and breakers serve the same purpose: to disconnect power when the current gets too high, protecting people or equipment.

Fuses contain a small metal link that melts when the current exceeds the rated value, breaking the circuit. They are a one-time-use protection and must be replaced once “blown.”

Achievable ASVAB

7. Electronics Information

Our ASVAB course is currently in development and is a work-in-progress.

Circuits

7 min read

Font

Discuss

Feedback

Circuit Diagram

Reading a circuit diagram is crucial for electronics. Let’s start with the basics.

Charge is the electrical property carried by the particles in a circuit. Current is the rate at which that charge flows through a wire.

Load is what consumes the electricity. This could be a lamp, a motor, a TV, or a heater.

Open vs. closed circuits

For electrical charge to flow through a circuit, the circuit must form a closed loop. If the loop is ever broken, the current will stop flowing.

An open circuit is not a closed loop, so electric current cannot flow through it.

A closed circuit is a complete loop through which electric current can flow.

To easily open or close a circuit, we often use a switch. When a switch is flipped to the “on” position, it will close the circuit, allowing electrical current to flow and turning the device on.

When a switch is flipped to the “off” position, it opens the circuit, stopping electrical current from flowing and turning the device off.

Types of Circuits

The two ways of wiring a circuit are series and parallel.

Series circuits

If one component breaks, the entire circuit stops, because the loop has been opened.

Let’s look at an example with the resistance in a series circuit.

What is the total resistance in a series circuit if the resistors are $2Ω$ , $3Ω$ , and $5Ω$ ?

For a series, we would just add them, so

$2 + 3 + 5 = 10Ω$ .

Parallel circuits

If one component breaks, the entire circuit will remain on, because the loop is still closed.

Now let’s look at an example with the resistance in a parallel circuit.

If two $4Ω$ resistors are wired in parallel, what is the total resistance?

$1/ R = 1/ R_{1} + 1/ R_{2}$

$1/ R = 1/4 + 1/4$

$1/ R = 1/2$

$R = 2Ω$

Let’s look at a more complex example that combines what we’ve learned.

This circuit has two switches and two branches. $S 1$ sits before the branches split, so opening it cuts power to both $L 1$ and $L 2$ .

$S 2$ , however, sits only in the branch with $R 1$ and $L 1$ , so opening it only turns off $L 1$ .

$L 2$ stays on, powered through the $R 2$ branch, since that loop is still closed.

Short circuits

Normal path: $I = \frac{V}{R}$

$I = \frac{120}{10}$

$I = 12$ amps

Short circuit path: $I = \frac{V}{R}$

$I = \frac{120}{0.01}$

$I = 12, 000$ amps

Safety

Grounding

Fuses and breakers

Fuses and breakers serve the same purpose: to disconnect power when the current gets too high, protecting people or equipment.

Fuses contain a small metal link that melts when the current exceeds the rated value, breaking the circuit. They are a one-time-use protection and must be replaced once “blown.”

Circuit Basics

Charge: electrical property carried by particles; current: rate of charge flow
Load: component that consumes electricity (lamp, motor, TV, heater)
Circuit diagrams show direction current flows from positive battery terminal through components back to battery

Open vs. Closed Circuits

Closed circuit: complete loop, current flows
Open circuit: broken loop, current cannot flow
Switches control this: “on” closes circuit (current flows), “off” opens circuit (current stops)

Series Circuits

Single path for current; current same throughout, voltage divides among components
Total resistance increases as components added: $R_{t o t a l} = R_{1} + R_{2} + R_{3} ...$
If one component fails, entire circuit stops (loop broken)

Parallel Circuits

Multiple paths for current; current divides among branches, voltage same across each component
Total resistance decreases as components added: $1/ R_{t o t a l} = 1/ R_{1} + 1/ R_{2} ...$
If one component fails, rest of circuit stays on (loop still closed elsewhere)
Switch placement matters: a switch before a branch split affects all branches; a switch within one branch only affects that branch

Short Circuits

Caused by electricity taking unintended, low-resistance path
Ohm’s Law ( $I = V / R$ ) shows danger: tiny resistance drop causes massive current spike (e.g., $12 A \to 12, 000 A$ )
Can melt wires, cause fires, destroy devices

Safety: Grounding, Fuses, and Breakers

Grounding: uses earth as low-resistance path to safely dissipate dangerous charge (e.g., lightning rods)
Fuses: metal link melts when current exceeds rating; one-time use, must be replaced
Breakers: thermal/magnetic trip mechanism opens circuit on overload; resettable, reusable

Next | 7.3 Electrical Power

Circuits

Circuit Diagram

Reading a circuit diagram is crucial for electronics. Let’s start with the basics.

Charge is the electrical property carried by the particles in a circuit. Current is the rate at which that charge flows through a wire.

Load is what consumes the electricity. This could be a lamp, a motor, a TV, or a heater.

Open vs. closed circuits

For electrical charge to flow through a circuit, the circuit must form a closed loop. If the loop is ever broken, the current will stop flowing.

An open circuit is not a closed loop, so electric current cannot flow through it.

A closed circuit is a complete loop through which electric current can flow.

To easily open or close a circuit, we often use a switch. When a switch is flipped to the “on” position, it will close the circuit, allowing electrical current to flow and turning the device on.

When a switch is flipped to the “off” position, it opens the circuit, stopping electrical current from flowing and turning the device off.

Types of Circuits

The two ways of wiring a circuit are series and parallel.

Series circuits

If one component breaks, the entire circuit stops, because the loop has been opened.

Let’s look at an example with the resistance in a series circuit.

What is the total resistance in a series circuit if the resistors are $2Ω$ , $3Ω$ , and $5Ω$ ?

For a series, we would just add them, so

$2 + 3 + 5 = 10Ω$ .

Parallel circuits

If one component breaks, the entire circuit will remain on, because the loop is still closed.

Now let’s look at an example with the resistance in a parallel circuit.

If two $4Ω$ resistors are wired in parallel, what is the total resistance?

$1/ R = 1/ R_{1} + 1/ R_{2}$

$1/ R = 1/4 + 1/4$

$1/ R = 1/2$

$R = 2Ω$

Let’s look at a more complex example that combines what we’ve learned.

This circuit has two switches and two branches. $S 1$ sits before the branches split, so opening it cuts power to both $L 1$ and $L 2$ .

$S 2$ , however, sits only in the branch with $R 1$ and $L 1$ , so opening it only turns off $L 1$ .

$L 2$ stays on, powered through the $R 2$ branch, since that loop is still closed.

Short circuits

Normal path: $I = \frac{V}{R}$

$I = \frac{120}{10}$

$I = 12$ amps

Short circuit path: $I = \frac{V}{R}$

$I = \frac{120}{0.01}$

$I = 12, 000$ amps

Safety

Grounding

Fuses and breakers

Fuses and breakers serve the same purpose: to disconnect power when the current gets too high, protecting people or equipment.

Fuses contain a small metal link that melts when the current exceeds the rated value, breaking the circuit. They are a one-time-use protection and must be replaced once “blown.”