201902210854 Short Review I (Electric Current)

Definition.

An electric current is a flow of electric charge (through a conductor). We express electric current by the total electric charge flowing through a cross-sectional area per unit time .

One unit current is of one ampere ().

, so .

---

Concept Test

1. An electric current can result from
   1. the movement of atoms.
   2. the movement of electrons.
   3. the simultaneous movement of positive charges and electrons.
      1. I only
      2. III only
      3. I and II only
      4. II and III only
2. A direct current (d.c.) of flows through a wire. How much charge passes through the wire in 30 minutes? And how many electrons flow through it?
   1. ; electrons
   2. ; electrons
   3. ; electrons
   4. ; electrons
3. Suppose, on average, passengers arrive at Central station from Chai Wan station by MTR every hour. The journey takes up minutes, with average speed . The average train frequency is one every minutes. Which of the following statements is/are correct?
   1. The total number of passengers on this journey by MTR at any instant is .
   2. passengers arriving in Central per hour is analogous to a current passing through a cross-sectional area.
   3. Average train speed is analogous to the current in a conducting wire segment.
   4. The average train frequency is analogous to the current flowing through a circuit.
   1. I and II only
   2. I and III only
   3. II and III only
   4. III and IV only

 

---

Answers:

1. D
2. D
3. A

Explanation:

1. (I) is wrong because atoms are neutral and do not carry charge. Movement of atoms is not a flow of charge, viz. current. (II) is correct because electrons are charge-carriers. (III) is correct. An example is electrolytes with positive ions and negative ions as charge-carriers.
2. Charges passing through the wire (Caution: use SI-unit). An electron has of negative charge. So of charge consists in electrons. Thus corresponds to electrons. Remark: Options A and C do not make sense, how can an electron be split into pieces?
3. Draw an analogy like this: (i) trains charge carriers; (ii) passengers charge; and (iii) railway circuit. (2) is correct because current . (I) is correct because the total number of passengers on board a train at any instant is time needed for one journey . (III) is wrong because the average train speed the flow of charge carriers the flow of charge. (IV) is wrong because average train frequency density of charge carriers (i.e., electrical conductivity) the current.

---

Current Direction

By definition, current is the flow of charges (e.g., carried by electrons).

(Charge carriers can be positive, e.g., holes in semi-conductors, positive ions in electrolytes.)

In nature, current is due to the flow of negative electrons from the negative (-ve) terminal to the positive (+ve) terminal of a power source.

An old convention, which is wrong, lasts to date:

Conventional electric current is a flow of positive charge from the +ve terminal to the -ve terminal of a battery.

Conventional current direction is opposite to the direction of electron flow.

Measuring Current

By ammeter (also by a current sensor with data-logger, or by galvanometer, which is used for large current, large voltage, and any resistance, like a multimeter), connected in series to a component in a circuit. An ideal ammeter should have zero resistance.

Current in Series and Parallel Circuits

In a series circuit, the current is the same at all points.

In a parallel circuit, the sum of currents passing through each branch is equal to the current in the main circuit.