Current, potential difference and resistance
Đã giới thiệu| English | Ý nghĩa |
|---|---|
| current/ˈkʌrənt/ | dòng điện |
| potential difference/pəˈtenʃl ˈdɪfrəns/ | hiệu điện thế |
What would explain this observation?
- A lamp becomes dimmer when another is added in series. Current 电流 and energy transfer depend on the whole circuit, not only one lamp.
Build the model
- Current is charge passing per time.
- Potential difference 电势差 is energy transferred per charge.
- Resistance is potential difference divided by current for a stated operating point.
- current: Rate of flow of charge.
- potential difference: Energy transferred per unit charge.
How is a voltmeter connected to measure voltage across a resistor?
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A voltmeter measures the potential difference between two points, so it connects in parallel across the component. In series it would add its very large resistance to the circuit and the current would collapse.
Match each technical term to its precise meaning.
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Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Current is the same through components in series.
- Potential differences add around the series path.
- In parallel, branches share the same potential difference, while branch currents sum at a junction.
In the lab: Place an ammeter in series and a voltmeter in parallel. For an I-V investigation, change voltage in steps, reverse polarity when appropriate and limit current to reduce heating.
Which two habits make the investigation or model in this case more defensible?
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Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: a resistor has 6.0 V across it and carries 0.30 A.
- Use R = V/I.
- R = 6.0/0.30 = 20 ohms.
- Power = VI.
- Power = 6.0×0.30 = 1.8 W.
- These quantities describe the same operating point.
Your turn: A component has 12 V across it and carries 0.40 A. Find resistance.
A component has 12 V across it and carries 0.40 A. Find resistance.
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The result is 30 Ω. Known: V = 12 V; I = 0.40 A. Use R = V/I. R = 12/0.40 = 30 ohms. This describes the component at that operating point; a filament lamp need not keep this resistance at other voltages.
Check the conclusion and its limits
- Current is not used up by a lamp.
- A filament heats up, so its resistance need not remain constant as voltage changes.
Not true: A lamp uses up electric current as charge passes through it. Current is a flow of charge, and charge is conserved: the same current enters and leaves a lamp. The lamp transfers energy from the charges to heat and light; it does not consume the charges.
- Current is charge passing per time.
- Current is the same through components in series.
- Watch for: current is not used up by a lamp.
Write from a resistance of 60 ohms
- A component has 15 V across it and carries 0.25 A.
- Find the resistance and the power. Current is not used up in a lamp.
A lamp uses up electric current as charge passes through it.
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Current is a flow of charge, and charge is conserved: the same current enters and leaves a lamp. The lamp transfers energy from the charges to heat and light; it does not consume the charges.
Rate of flow of charge: write the technical term.
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current means Rate of flow of charge.
A component has 15 V across it and carries 0.25 A. What are the resistance and the power, and what happens to the current through a lamp?
Resistance is voltage divided by current. Power is voltage times current. Charge is conserved.
What is the resistance?
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15 divided by 0.25 is 60 ohms. The 3.75 W figure is the power at the same operating point. Other options — 3.75 W (the power); 20 ohms (the worked resistance).