To measure the current in a circuit:

Question

BlackTom AI · Accepted Answer

An ammeter must be placed in series with the circuit elements so the same current flows through the meter as through the element being measured.

1. Relevant knowledge points and concepts
- Electric current is the flow of charge and in a series branch the current is the same through every element in that branch (Kirchhoff’s current law applied to a single series path).
- An ammeter is designed with very low internal resistance so it does not significantly change the circuit current when inserted.
- A voltmeter is designed with very high internal resistance and is connected in parallel to measure potential difference without drawing significant current.

2. Step-by-step reasoning
- To measure current through a particular element you must place the measuring device so that all charge flowing through the element also flows through the meter. That requires placing the meter in series with the element.
- If the meter were placed in parallel, the current would split and the meter would not read the element’s actual current (and a low-resistance ammeter in parallel would short the element).
- A voltmeter in parallel measures voltage, not current; a voltmeter in series would drastically reduce current because of its high resistance.

3. Simple illustrative calculation (intermediate steps)
- Example: battery V = 9 V, expected circuit resistance R = 10 Ω → expected I = V/R = 9/10 = 0.9 A.
- If an ammeter of low internal resistance r_a = 0.1 Ω is accidentally placed in parallel with the 10 Ω element, the parallel branch current through the ammeter would be I_a = V / r_a = 9 / 0.1 = 90 A, drastically altering the circuit and likely damaging the meter or source.

4. Why the correct answer is correct
- The ammeter in series ensures the meter carries the same current as the element without significantly changing the circuit current (due to low meter resistance).

5. Why other options are incorrect
- b: ammeter in parallel may short-circuit the element and give wrong reading.
- c: voltmeter in parallel measures voltage, not current.
- d: voltmeter in series blocks current (high resistance), so it cannot measure current properly.

6. Conclusion
The correct choice is a. an ammeter placed in series with the circuit elements is required. This matches the provided answer.

To measure the current in a circuit:单项选择题

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