AeroLab Reading room

Physics / Foundations

Simple electric circuits

Connect charge flow, potential difference and resistance.

Learning objectives

The big idea

Current measures the flow of electric charge. A battery maintains a potential difference that can transfer energy to charges in a complete circuit. Conventional current is defined in the direction positive charge would move.

A resistor opposes current. For an ohmic resistor at constant temperature, current is proportional to potential difference. In a series loop the same current passes through every component.

1. A loop is a shared system

A lamp needs a complete conducting path through a source and back. The battery transfers energy to charges; the wire already contains mobile charges before the switch closes. Opening one place in a simple series loop stops steady current throughout the loop.

2. Current is not used up

Current tells us how much charge crosses a point each second. In a steady single loop, the same charge rate passes before and after the lamp. The lamp transfers energy from the charges to light and thermal stores; it does not consume the charges themselves.

3. Voltage and resistance

Potential difference measures transferred energy per charge. For an ohmic resistor at fixed temperature, I = V/R. A 6 V source across 3 Ω gives 2 A. Increasing resistance at the same voltage reduces current. An ammeter belongs in series; a voltmeter measures between two points.

A method you can reuse

  1. Draw the complete circuit before calculating.
  2. Identify which component voltage is given.
  3. Use the appropriate resistance and units.
  4. Check current continuity around a single loop.
Why can current be equal on both sides of a glowing lamp?

The lamp transfers energy but does not use up charge. Charge conservation requires equal steady current in that series loop.

Equations and relationships

I=QtI=\frac Qt

Current is charge transferred per unit time.

V=IRV=IR

Ohm’s law for an ohmic component under fixed conditions.

Worked example

A 3 Ω resistor is connected across 6 V. What current flows?

Follow the solution

I = V/R = 6/3 = 2 A.

Investigate and explain

Use only low-voltage cells in a classroom circuit. Compare the current for two resistor values at the same voltage.

Use only simulated or teacher-approved low-voltage circuits. Move meters deliberately and compare current with voltage rather than treating both as flow rates.

Open the interactive investigation

Check your understanding

1. Where should an ammeter be connected?

  1. In series with the component
  2. Directly across a battery with no load
  3. Outside the conducting circuit
Check the answer and reasoning

In series with the component

An ammeter measures charge flowing through its branch.

2. A 3 Ω resistor is connected across 6 V. What current flows?

  1. 18 A
  2. 2 A
  3. 0.5 A
Check the answer and reasoning

2 A

I = V/R = 6/3 = 2 A.

3. Why can a lamp stop glowing when a switch opens?

  1. All the charge has been used up.
  2. The battery’s voltage necessarily becomes zero.
  3. The complete path for current is broken.
Check the answer and reasoning

The complete path for current is broken.

An open switch interrupts the conducting loop.