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Course: AP®︎/College Physics 1>Unit 11

Lesson 2: Electric power and DC circuits

DC Circuit and electrical power review

Review the components of a circuit and their symbols such as battery, resistor, and switch. Analyze how the power of a resistor is related to the current and electric potential difference across the resistor.

Equations

EquationSymbolsMeaning in words
$P=I\mathrm{\Delta }V$$P$ is power, $I$ is current, and $\mathrm{\Delta }V$ is electric potential differenceThe rate at which energy is transferred from a resistor is equal to the product of the electric potential difference across the resistor and the current through the resistor. Scalar quantity with units of Watts ($\text{W}$).

Circuit component definitions and symbols

Resistor ($R$‍ )

Resistors are electrical components that resist current and expends voltage within a circuit.

Battery ($ϵ$‍ )

Batteries are electrical components that provide electrical energy.
Batteries have positive and negative terminals. The negative terminal is drawn with a short line, and the positive terminal is shown as a long line.

Switch ($S$‍ )

Switches turn the flow of current through a circuit pathway on and off. When the switch is open, no current flows because there is a gap in the circuit (Figure 3).
When the switch is closed, current can flow because the circuit is continuous (Figure 4).

Node

A node (or junction) is a place where two or more circuit elements join together. Figure 5 below shows a single node (the black dot) formed by the junction of five electrical components (abstractly represented by orange rectangles).

DC circuit types

Simple circuit

A simple circuit contains the minimum amount of components that allow it to be a functional electric circuit: a voltage source $\epsilon$ (battery), a resistor $R$, and a loop of wires for current $I$ to flow around (see Figure 6 below). We usually ignore any resistance from the wires.
In a simple circuit, the voltage supplied by the battery $\epsilon$ is the voltage expended by the resistor $R$, and there is only one current $I$ in the circuit.

Closed circuit

A closed circuit has a continuous pathway for current to flow through. In other words, there are no gaps in the circuit.

Open circuit

An open circuit has a gap in the circuit that does not allow current to flow through. The gap can be caused by an open switch, a broken component, or broken wire.

Short circuit

A short is a pathway of zero resistance within a circuit (see the blue wire in Figure 9). When there is a short circuit, all the current flows across the short because the current prefers the path of least resistance.
Figure 10 below shows how closing a switch $S$ can divert all the current from resistor ${R}_{2}$. When switch $S$ is open (see Figure 10A), the current $I$ flows out of the positive terminal of the battery towards node $N$. Since the switch is open, no current flows through the switch and all the current flows through resistor ${R}_{2}$. When the switch is closed (see Figure 10B), it forms a short around resistor ${R}_{2}$. Now, once the current $I$ reaches $N$, the current bypasses ${R}_{2}$ and flows through the switch.

For deeper explanations on electrical power, see our video on power used by a resistor.
To check your understanding and work toward mastering these concepts, check out our exercises:

Want to join the conversation?

• Why doesn't some of the current flow through resistor R2 in Figure 10B?