7.2 Circuits

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7. Electronics Information

Circuits

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Circuit diagram

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. These arrows show conventional current, which by convention flows from positive to negative; the electrons themselves move the opposite way, from the negative terminal to the positive. 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.

Definitions

Charge: The electrical property carried by the particles in a circuit.
Current: The rate at which that charge flows through a wire, measured in amps (A).
Voltage: The push that drives current around the circuit, measured in volts (V).
Resistance: Opposition to the flow of current, measured in ohms (Ω).
Load: What consumes the electricity, such as a lamp, a motor, a TV, or a heater.

These are tied together by Ohm’s law from Electricity Basics: $V = I \times R$ , so $I = \frac{V}{R}$ and $R = \frac{V}{I}$ .

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 is the same through every component, 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.

Example: Series resistance

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

For a series circuit, we just add them: $2 + 3 + 5 = 10Ω$ .

Answer: $10Ω$

To find the current in a series circuit, add the resistances, then use Ohm’s law with the battery’s voltage. Because there is only one path, that current is the same through every resistor.

Example: Series current

A $9$ V battery is connected to a $1Ω$ resistor and a $2Ω$ resistor in series. What current flows through the $2Ω$ resistor?

Total resistance: $1 + 2 = 3Ω$

$I = \frac{V}{R} = \frac{9}{3} = 3$ amps

Answer: $3$ amps, the same through each resistor

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, only its branch goes dark; the other branches stay on because each still forms its own closed loop.

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

Example: Parallel resistance

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

$1/ R = 1/ R_{1} + 1/ R_{2} = 1/4 + 1/4 = 1/2$

$R = 2Ω$

Answer: $2Ω$

Shortcuts: for $n$ equal resistors in parallel, $R_{t o t a l} = R \div n$ ; for any two resistors, $R_{t o t a l} = (R_{1} \times R_{2}) \div (R_{1} + R_{2})$ . Here, $16 \div 8 = 2Ω$ , matching the answer above.

Let’s look at a more complex example that combines what we’ve learned. A circuit like the one below, with some parts in series and others in parallel, is called a series-parallel (or combination) circuit.

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. Current flows through every available path, but most of it takes the path with the least resistance. When an unintended low-resistance path forms, nearly all the current rushes through it instead of through the load.

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 current 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:

Example: Short-circuit current

Normal path: $I = \frac{V}{R} = \frac{120}{10} = 12$ amps

Short-circuit path: $I = \frac{V}{R} = \frac{120}{0.01} = 12, 000$ amps

Answer: the short circuit draws about a thousand times more current than the normal path.

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 most current follows the lowest-resistance path, 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.

Basic circuit concepts

Charge: electrical property carried by particles; Current (amps): rate of charge flow; Voltage (volts): push driving current; Resistance (ohms): opposition to flow; Load: device consuming electricity
Ohm’s law ties these together: $V = I \times R$ , so $I = \frac{V}{R}$ and $R = \frac{V}{I}$
Conventional current flows positive to negative; electrons actually flow negative to positive

Open vs. closed circuits

Closed circuit = complete loop, current flows
Open circuit = broken loop, no current flows
Switches control this: “on” closes circuit, “off” opens it

Series circuits

Single path for current; same current through all components, voltage divides
Total resistance = sum of resistances (increases as components added): $R_{t o t a l} = R_{1} + R_{2} + R_{3} ...$
One broken component stops entire circuit (loop opened)
Find current using total resistance and Ohm’s law; same current flows through every resistor

Parallel circuits

Multiple paths; voltage same across all branches, current divides
Total resistance decreases as components added: $\frac{1}{R _{t o t a l}} = \frac{1}{R _{1}} + \frac{1}{R _{2}}$ …
Shortcuts: equal resistors, $R_{t o t a l} = R \div n$ ; two resistors, $R_{t o t a l} = (R_{1} \times R_{2}) \div (R_{1} + R_{2})$
If one branch breaks, other branches stay powered (independent closed loops)
Switch position matters: switches before a branch split affect all branches; switches within one branch only affect that branch

Short circuits

Caused by unintended low-resistance path; current overwhelmingly follows path of least resistance
Example: normal path (10Ω) draws 12A at 120V, short path (0.01Ω) draws 12,000A — ~1000x spike
Dangerous: can melt wires, cause fires, destroy devices

Safety devices

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

Definitions

Charge: The electrical property carried by the particles in a circuit.
Current: The rate at which that charge flows through a wire, measured in amps (A).
Voltage: The push that drives current around the circuit, measured in volts (V).
Resistance: Opposition to the flow of current, measured in ohms (Ω).
Load: What consumes the electricity, such as a lamp, a motor, a TV, or a heater.

These are tied together by Ohm’s law from Electricity Basics: $V = I \times R$ , so $I = \frac{V}{R}$ and $R = \frac{V}{I}$ .

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.

Example: Series resistance

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

For a series circuit, we just add them: $2 + 3 + 5 = 10Ω$ .

Answer: $10Ω$

To find the current in a series circuit, add the resistances, then use Ohm’s law with the battery’s voltage. Because there is only one path, that current is the same through every resistor.

Example: Series current

A $9$ V battery is connected to a $1Ω$ resistor and a $2Ω$ resistor in series. What current flows through the $2Ω$ resistor?

Total resistance: $1 + 2 = 3Ω$

$I = \frac{V}{R} = \frac{9}{3} = 3$ amps

Answer: $3$ amps, the same through each resistor

Parallel circuits

If one component breaks, only its branch goes dark; the other branches stay on because each still forms its own closed loop.

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

Example: Parallel resistance

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

$1/ R = 1/ R_{1} + 1/ R_{2} = 1/4 + 1/4 = 1/2$

$R = 2Ω$

Answer: $2Ω$

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

Example: Short-circuit current

Normal path: $I = \frac{V}{R} = \frac{120}{10} = 12$ amps

Short-circuit path: $I = \frac{V}{R} = \frac{120}{0.01} = 12, 000$ amps

Answer: the short circuit draws about a thousand times more current than the normal path.

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

Circuits

8 min read

Font

Discuss

Feedback

Circuit diagram

Definitions

Charge: The electrical property carried by the particles in a circuit.
Current: The rate at which that charge flows through a wire, measured in amps (A).
Voltage: The push that drives current around the circuit, measured in volts (V).
Resistance: Opposition to the flow of current, measured in ohms (Ω).
Load: What consumes the electricity, such as a lamp, a motor, a TV, or a heater.

These are tied together by Ohm’s law from Electricity Basics: $V = I \times R$ , so $I = \frac{V}{R}$ and $R = \frac{V}{I}$ .

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.

Example: Series resistance

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

For a series circuit, we just add them: $2 + 3 + 5 = 10Ω$ .

Answer: $10Ω$

To find the current in a series circuit, add the resistances, then use Ohm’s law with the battery’s voltage. Because there is only one path, that current is the same through every resistor.

Example: Series current

A $9$ V battery is connected to a $1Ω$ resistor and a $2Ω$ resistor in series. What current flows through the $2Ω$ resistor?

Total resistance: $1 + 2 = 3Ω$

$I = \frac{V}{R} = \frac{9}{3} = 3$ amps

Answer: $3$ amps, the same through each resistor

Parallel circuits

If one component breaks, only its branch goes dark; the other branches stay on because each still forms its own closed loop.

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

Example: Parallel resistance

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

$1/ R = 1/ R_{1} + 1/ R_{2} = 1/4 + 1/4 = 1/2$

$R = 2Ω$

Answer: $2Ω$

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

Example: Short-circuit current

Normal path: $I = \frac{V}{R} = \frac{120}{10} = 12$ amps

Short-circuit path: $I = \frac{V}{R} = \frac{120}{0.01} = 12, 000$ amps

Answer: the short circuit draws about a thousand times more current than the normal path.

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.”

Basic circuit concepts

Charge: electrical property carried by particles; Current (amps): rate of charge flow; Voltage (volts): push driving current; Resistance (ohms): opposition to flow; Load: device consuming electricity
Ohm’s law ties these together: $V = I \times R$ , so $I = \frac{V}{R}$ and $R = \frac{V}{I}$
Conventional current flows positive to negative; electrons actually flow negative to positive

Open vs. closed circuits

Closed circuit = complete loop, current flows
Open circuit = broken loop, no current flows
Switches control this: “on” closes circuit, “off” opens it

Series circuits

Single path for current; same current through all components, voltage divides
Total resistance = sum of resistances (increases as components added): $R_{t o t a l} = R_{1} + R_{2} + R_{3} ...$
One broken component stops entire circuit (loop opened)
Find current using total resistance and Ohm’s law; same current flows through every resistor

Parallel circuits

Multiple paths; voltage same across all branches, current divides
Total resistance decreases as components added: $\frac{1}{R _{t o t a l}} = \frac{1}{R _{1}} + \frac{1}{R _{2}}$ …
Shortcuts: equal resistors, $R_{t o t a l} = R \div n$ ; two resistors, $R_{t o t a l} = (R_{1} \times R_{2}) \div (R_{1} + R_{2})$
If one branch breaks, other branches stay powered (independent closed loops)
Switch position matters: switches before a branch split affect all branches; switches within one branch only affect that branch

Short circuits

Caused by unintended low-resistance path; current overwhelmingly follows path of least resistance
Example: normal path (10Ω) draws 12A at 120V, short path (0.01Ω) draws 12,000A — ~1000x spike
Dangerous: can melt wires, cause fires, destroy devices

Safety devices

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

Definitions

Charge: The electrical property carried by the particles in a circuit.
Current: The rate at which that charge flows through a wire, measured in amps (A).
Voltage: The push that drives current around the circuit, measured in volts (V).
Resistance: Opposition to the flow of current, measured in ohms (Ω).
Load: What consumes the electricity, such as a lamp, a motor, a TV, or a heater.

These are tied together by Ohm’s law from Electricity Basics: $V = I \times R$ , so $I = \frac{V}{R}$ and $R = \frac{V}{I}$ .

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.

Example: Series resistance

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

For a series circuit, we just add them: $2 + 3 + 5 = 10Ω$ .

Answer: $10Ω$

To find the current in a series circuit, add the resistances, then use Ohm’s law with the battery’s voltage. Because there is only one path, that current is the same through every resistor.

Example: Series current

A $9$ V battery is connected to a $1Ω$ resistor and a $2Ω$ resistor in series. What current flows through the $2Ω$ resistor?

Total resistance: $1 + 2 = 3Ω$

$I = \frac{V}{R} = \frac{9}{3} = 3$ amps

Answer: $3$ amps, the same through each resistor

Parallel circuits

If one component breaks, only its branch goes dark; the other branches stay on because each still forms its own closed loop.

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

Example: Parallel resistance

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

$1/ R = 1/ R_{1} + 1/ R_{2} = 1/4 + 1/4 = 1/2$

$R = 2Ω$

Answer: $2Ω$

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

Example: Short-circuit current

Normal path: $I = \frac{V}{R} = \frac{120}{10} = 12$ amps

Short-circuit path: $I = \frac{V}{R} = \frac{120}{0.01} = 12, 000$ amps

Answer: the short circuit draws about a thousand times more current than the normal path.

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.”