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Class 12 Physics (India)

Course: Class 12 Physics (India) > Unit 3

Lesson 4: Electric power and DC circuits
Science
Class 12 Physics (India)
Current electricity
Electric power and DC circuits
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DC Circuit and electrical power review

Google Classroom
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, equals, I, delta, VP is power, I is current, and 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 (start text, W, end text).

Circuit component definitions and symbols

Resistor (R)

Resistors are electrical components that resist current and expends voltage within a circuit.
Figure 1. Symbol for resistor.

Battery (\epsilon)

Batteries are electrical components that provide electrical energy.
Figure 2. Symbol for battery. The short end is the negative terminal and the long end is the positive terminal.
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).
Figure 3. Symbol for open switch. No current flows through this location because the conductive pathway has a gap.
When the switch is closed, current can flow because the circuit is continuous (Figure 4).
Figure 4. Symbol for closed switch. Current can pass through this location because the circuit pathway is continuous.

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).
Figure 5. A junction (highlighted in green) between 5 different electrical components.

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 ε (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.
Figure 6. Simple circuit diagram.
In a simple circuit, the voltage supplied by the battery ε 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.
Figure 7. Diagram of a closed 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.
Figure 8. Diagram of a open circuit.

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 9. The blue wire has no resistance and is a short in this circuit. Since there is no resistance, all the current I flows across the blue wire instead of going through the resistor R.
Figure 10 below shows how closing a switch S can divert all the current from resistor R, start subscript, 2, end subscript. 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, start subscript, 2, end subscript. When the switch is closed (see Figure 10B), it forms a short around resistor R, start subscript, 2, end subscript. Now, once the current I reaches N, the current bypasses R, start subscript, 2, end subscript and flows through the switch.
Figure 10. When the switch S changes from open (diagram A) to closed (diagram B), resistor R, start subscript, 2, end subscript is shorted out and the current I bypasses it to go 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:

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