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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

Equation | Symbols | Meaning in words |
---|---|---|

The 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 ( |

## Circuit component definitions and symbols

### Resistor ($R$ )

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

### Battery ($\u03f5$ )

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.

## Learn more

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?(2 votes)
- Actually, there are charges flowing through R2; it's just that the path of least resistance will be preferred and so will have a considerably larger current compared to the path with the resistor.(13 votes)

- why does the voltage remains constant in a parallel combination circuit(4 votes)
- Because the battery (cell) which is generating this potential difference tries to maintain same potential difference across the two nodes (Starting and ending points) of the parallel combination.(1 vote)

- is power equal to thermal energy released by the resistor in joules per second(2 votes)
- In case the circuit only contains the resistor, the total power is indeed equal to the thermal energy released by the resistor. (J/s)(2 votes)

- In case the circuit only contains the resistor, the total power is indeed equal to the thermal energy released by the resistor. (J/s)(2 votes)