Simple Circuits
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AP Physics C: Electricity and Magnetism › Simple Circuits
Which of the following best defines an electrical loop within a circuit?
Any straight-line segment of wire connecting two circuit components.
Any closed path within a circuit through which electric charge can flow and return to its starting point.
The entire set of components connected to a single battery, excluding the battery itself.
A point in the circuit where three or more wires connect, which is also known as a junction.
Explanation
An electrical loop is, by definition, any closed conducting path in a circuit. A charge carrier that traverses a loop starts and ends at the same point, having experienced a net change in potential of zero. This concept is fundamental to applying Kirchhoff's loop rule.
When the switch $$S$$ is closed, how does the behavior of the circuit change compared to when the switch is open?
Both resistors are short-circuited, and the total current flowing from the battery becomes zero.
The resistor $$R_1$$ is short-circuited, and the total current flowing from the battery decreases.
The resistor $$R_2$$ is short-circuited, and the total current flowing from the battery increases.
The total resistance of the circuit increases because a new path is added, and the total current decreases.
Explanation
Closing the switch creates a zero-resistance path in parallel with $$R_2$$. This is a short circuit, and nearly all current will bypass $$R_2$$. The total resistance of the circuit decreases from $$R_1 + R_2$$ to just $$R_1$$. Since the total resistance decreases, the total current from the battery ($$I = V/R_{eq}$$) increases.
For a steady conventional current to exist in a circuit, which of the following conditions must be met?
The circuit must contain at least one closed conducting loop that includes a source of electromotive force.
The circuit must be connected to a ground point to allow for the continuous dissipation of charge.
The circuit must contain at least one switch that is in the open position to regulate the flow.
The net charge within the wires of the circuit must be constantly increasing over time.
Explanation
A steady current requires a continuous, unbroken path for charge to flow. This is known as a closed loop. Additionally, a source of electromotive force (like a battery) is needed to provide the potential difference that drives the current.
Which of the following correctly pairs a circuit component with its primary function in a simple circuit?
Switch: To provide a constant source of potential difference to the circuit when closed.
Resistor: To control the amount of current for a given potential difference and dissipate electrical energy.
Ideal Wire: To provide a specific, calibrated amount of potential drop between components.
Battery: To store electric charge on its plates and release it when the circuit is closed.
Explanation
The primary functions of a resistor are to impede the flow of charge (thus controlling current, per $$I = V/R$$) and to convert electrical potential energy into other forms, typically thermal energy. The other options misrepresent the functions of a switch, wire, and battery.
From the perspective of circuit analysis, why is this action considered dangerous for a real battery?
Connecting the wire in this way reverses the polarity of the battery, which can damage the internal chemical structure.
A real battery has internal resistance; a short circuit causes a very large current to flow, generating significant thermal energy.
The electric field inside the wire becomes zero, which causes a rapid buildup of static charge on the battery terminals.
The wire adds significant resistance to the circuit, causing the battery to overheat as it tries to push current through.
Explanation
A real battery has a small internal resistance. When its terminals are short-circuited by a low-resistance wire, the total resistance of the circuit is just the internal resistance, which is very small. This results in a very large current ($$I = \mathcal{E}/r_{int}$$). The power dissipated as heat inside the battery ($$P = I^2 r_{int}$$) becomes very large, which can cause overheating, leakage, or explosion.
In a simple series circuit containing a battery, a resistor, and a switch, what is a key difference between the switch when it is open and when it is closed?
When open, it has zero resistance to allow potential build-up; when closed, it has a significant resistance.
When open or closed, the potential difference across it is always equal to the battery's emf.
When open, current flows through it by jumping the gap; when closed, current is blocked by it.
When open, there is a potential difference across it; when closed, the potential difference across it is ideally zero.
Explanation
When a switch is closed, it acts as an ideal wire with zero resistance, so the potential difference across it is zero. When it is open, it breaks the circuit. No current flows, so there is no potential drop across the resistor, and the full potential difference of the source appears across the open contacts of the switch.
What is the most immediate consequence of connecting the wire in parallel with the lightbulb?
The total resistance of the circuit will increase, causing the current from the battery to decrease.
The current through the lightbulb will double because there are now two paths for the current.
The potential difference across the lightbulb will increase, causing it to become brighter.
The current from the battery will increase significantly, and the lightbulb will go out.
Explanation
The wire with negligible resistance creates a 'short circuit' across the lightbulb. Since current follows the path of least resistance, almost all the current will flow through the wire, bypassing the bulb. This causes the bulb to go out and the total resistance of the circuit to drop to near zero, resulting in a very large current from the battery.
If the switch is opened, what are the immediate effects on the current in the circuit and the potential difference across the switch's contacts?
The current remains unchanged for a brief moment, and the potential difference across the switch's contacts is zero.
The current becomes zero, and the potential difference across the switch's contacts remains zero.
The current becomes zero, and the potential difference across the switch's contacts becomes equal to the battery's emf.
The current is halved, and the potential difference across the switch's contacts is half the battery's emf.
Explanation
Opening the switch breaks the conducting loop, so the steady current immediately ceases and becomes zero. With no current flowing through the resistor, there is no potential drop across it ($$\Delta V = IR = 0$$). Therefore, the full potential difference of the battery's emf appears across the gap created by the open switch.
In the context of a simple direct current (DC) circuit, what is the primary function of an ideal battery?
To supply a constant amount of electric charge to the circuit.
To create new electrons and protons to flow through the circuit.
To provide a constant current to any circuit it is connected to.
To maintain a constant potential difference between its terminals.
Explanation
An ideal battery is a source of electromotive force (emf), which acts to maintain a constant potential difference across its terminals, regardless of the current it supplies. The current supplied depends on the external circuit's resistance. The battery moves existing charges; it does not create them or supply a fixed amount of charge.
A simple circuit consists of a resistor connected to the terminals of a battery. Which statement accurately describes the flow of charge?
Conventional current describes the flow of protons from the positive to the negative terminal, while electrons flow in the opposite direction.
Conventional current flows from the positive terminal to the negative terminal, while electrons flow from the negative to the positive terminal.
Both conventional current and electrons flow from the negative terminal to the positive terminal through the external circuit.
Both conventional current and electrons flow from the positive terminal to the negative terminal through the external circuit.
Explanation
By convention, the direction of current is defined as the direction that positive charge would flow, which is from a higher potential (positive terminal) to a lower potential (negative terminal). In metallic conductors, the actual charge carriers are electrons, which are negatively charged and thus flow from the negative terminal to the positive terminal.