Current, Voltage and Resistance

§Text Passage

1Summary 重點摘要

Current is the rate of charge flow (amperes, A), voltage the push driving it (volts, V), and resistance the opposition (ohms, Ω). More voltage means more current; more resistance means less.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Current: how much charge flowsElectric current tells us how much charge passes a point in the circuit each second — picture a water pipe: current is like the amount of water flowing through each second; the larger the flow, the more water is delivered, and the larger the current, the brighter the bulb. Adding another cell strengthens the push and the current grows. The instrument for measuring current is the ammeter, and the unit is the ampere (A): a small lab bulb usually carries less than one ampere, while an electric kettle draws around ten.
2. Voltage: the driving pushVoltage is the force that drives charge along a circuit, like the height of a water tower: the taller the tower, the higher the pressure and the faster the flow; the larger the voltage, the harder the charge is pushed and the larger the current. One dry cell gives about 1.5 V and two in series about 3 V, which is why two cells make a bulb brighter. Voltage is measured with a voltmeter in volts (V), a unit named after the physicist Volta.
3. Resistance: the oppositionResistance is the part of a circuit that opposes current: connect a long thin iron wire into a circuit and the bulb visibly dims, because a large resistance makes the current small; cut the wire shorter and the bulb brightens again, showing that a longer wire has greater resistance. Material matters too: at the same size, copper's resistance is far lower than iron's, which is why wires are usually copper. The unit of resistance is the ohm (Ω), named after the German physicist Ohm.
4. How the three relateCurrent, voltage and resistance are tightly linked: current = voltage ÷ resistance. For example, with 3 V across 3 Ω the current is 3 ÷ 3 = 1 A; double the voltage to 6 V and the current doubles to 2 A; double the resistance to 6 Ω instead and the current halves to 0.5 A. Knowing any two quantities, substitute into the formula to find the third — the most basic tool for analysing circuits.
5. Connecting an ammeterAn ammeter must be connected in series so that all the current passes through it: like a toll booth on the only road, every vehicle must pass, and only then does the ammeter read the true current. Wiring it in parallel by mistake would drive too large a current into the meter and could damage it. When reading, remember the unit is the ampere, not the volt, and choose a suitable range — start from the largest and step down to protect the instrument.
6. Connecting a voltmeterA voltmeter must be connected in parallel across the component being tested: to measure a bulb's voltage, attach the voltmeter's two terminals to the two sides of the bulb and it shows the difference between them. Wired in series by mistake, its very large resistance would leave almost no current in the circuit — the bulb goes out and the reading becomes meaningless. Before wiring, decide whether you are measuring current or voltage, then choose series or parallel accordingly.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• Current measures how much charge flows, in amperes (A).
• Voltage is the push driving charge, in volts (V).
• Resistance opposes current, in ohms (Ω).
• More voltage gives more current; more resistance gives less (current = voltage ÷ resistance).
• Ammeter in series; voltmeter in parallel across the component.