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Ohm's Law & Basic Circuits
V = IR, Current, Resistance, and Power in Simple Circuits
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Ohm's law connects three core ideas in electric circuits: voltage, current, and resistance. It helps students predict how charges move through wires and components when a battery is connected. This law is one of the most useful tools for analyzing simple circuits in physics and electronics. Understanding it makes it easier to explain why bulbs brighten, resistors heat up, and devices need the right power source.
In a basic circuit, a battery provides a potential difference that pushes charge through a closed path. The current depends on both the applied voltage and the total resistance in that path, which is summarized by V = IR. Series and parallel arrangements change the total resistance in different ways, so they also change current and voltage distribution. Measuring current with an ammeter and voltage with a voltmeter lets students test these relationships directly.
Key Facts
- Ohm's law: V = IR
- Current is charge flow rate: I = Q/t
- Electric power: P = IV
- Using Ohm's law in power formulas: P = I^2R and P = V^2/R
- Series resistors add directly: R_total = R1 + R2 + ...
- For parallel resistors: 1/R_total = 1/R1 + 1/R2 + ...
Vocabulary
- Voltage
- Voltage is the electric potential difference that provides energy per unit charge and drives current through a circuit.
- Current
- Current is the rate at which electric charge flows through a conductor, measured in amperes.
- Resistance
- Resistance is the opposition a material or component gives to the flow of electric current, measured in ohms.
- Series circuit
- A series circuit has components connected in a single path so the same current passes through each one.
- Parallel circuit
- A parallel circuit has components connected across the same two points so each branch has the same voltage.
Common Mistakes to Avoid
- Using V = IR without matching the correct resistor or branch, which is wrong because voltage and current must refer to the same component or total circuit section.
- Putting an ammeter across a component, which is wrong because an ammeter must be placed in series and has very low resistance.
- Assuming current is the same everywhere in a parallel circuit, which is wrong because current splits between branches based on their resistances.
- Adding parallel resistors directly like series resistors, which is wrong because parallel combinations use reciprocals and always give a total resistance smaller than the smallest branch resistance.
Practice Questions
- 1 A 12 V battery is connected to a 4 ohm resistor. What current flows in the circuit?
- 2 Two resistors of 3 ohms and 6 ohms are connected in series to a 9 V battery. Find the total resistance and the current.
- 3 A bulb in a simple circuit goes out when the switch is opened. Explain in terms of charge flow and circuit path why this happens.