Fix the mistake: Electric Current and Its Uses

Circuits, Current, and Voltage

The world of electricity begins with circuits. Make sure you understand exactly what current and voltage mean, as well as the difference between series and parallel circuits.

Basic Knowledge

Current is the amount of electricity flowing. Its unit is the ampere (A), and its symbol is I. It is the amount of electricity that flows per second, and can be compared to the “flow rate of water” in a waterway.
Voltage is the strength of the force that tries to make electricity flow. Its unit is the volt (V), and its symbol is V. It can be compared to the “difference in height” in a waterway.
There are two types of circuits:
① Series circuit: One path. The current is the same everywhere, while the voltage is divided.
② Parallel circuit: Branching paths. The current divides and joins again, while the voltage is the same everywhere.
The rule is to connect an ammeter in series and a voltmeter in parallel.

Important Terms
Current I (the amount of electricity flowing. Unit: A (ampere))
Voltage V (the force that tries to make electricity flow. Unit: V (volt))
Series circuit (components connected along one path. Current is common; voltage is divided)
Parallel circuit (components connected along branching paths. Voltage is common; current is divided)
Power source (a device that supplies electricity. Dry cell or outlet)
Circuit diagram (a diagram representing an electric circuit with symbols)

Further Exploration (Background and Significance)

The true nature of current is the flow of electrons. Electrons are particles carrying negative electric charge; they leave the negative terminal of a power source and flow toward the positive terminal. However, by convention, the direction of current is defined as positive → negative (opposite to electron movement). This convention was established before electrons were discovered.
Remember that household outlets in Japan are 100V, dry cells are 1.5V, car batteries are 12V, and approximately 50V or more is dangerous when applied to the human body.
In the past, Christmas tree lights were connected in series, so if one bulb burned out, all of them went out. Modern lights are connected in parallel, so the others remain lit even if one bulb burns out. Household wiring is also basically parallel, so other appliances can still be used if one appliance breaks.

Key Points
  • Current I [A], voltage V [V]
  • Direction of current = positive → negative (by convention)
  • Direction of electrons = negative → positive (actual movement)
  • Series: current is common, voltage is the sum
  • Parallel: voltage is common, current is the sum
  • Ammeter connected in series; voltmeter connected in parallel
  • Household wiring is basically “parallel”

Points to Note (Easily Confused)

① The directions of current and electrons are opposite. ② An ammeter is connected in series (the circuit is opened and the ammeter is inserted), while a voltmeter is connected in parallel (bridging both ends). Connecting them incorrectly may damage them. ③ Organize and remember the behavior of current and voltage in series and parallel circuits. ④ Mistakes in connecting ammeters and voltmeters frequently appear on middle-school term tests.

Practice

  1. Give the unit symbol for current.
  2. How should an ammeter be connected (series or parallel)?
  3. State the relationship between the direction of current and the direction in which electrons move.

Physics (Junior High): related lessons