VCE Physics (Year 11) · Unit 1 — Electricity · 11m read

Electric Circuits — Charge, Current, Voltage

Ever wonder why your phone needs different chargers than your laptop? It all comes down to three invisible forces working in your circuits: charge (the stuff flowing), current (how fast it flows), and voltage (the push behind it). Understanding these three quantities is essential for VCE physics and explains everything from why batteries die to how your devices actually work.

Learning objective: Define current, voltage and resistance.

By the end of this lesson you can:

• I define charge, current, potential difference.
• I draw basic circuits with correct symbols.
• I relate Q = It.

Current is water flow; voltage is pressure. Charge flows like water in pipes, pushed by the pressure difference (voltage).

Charge, current, voltage

CHARGE (Q):
• Unit: coulomb (C).
• 1 electron = −1.6 × 10⁻¹⁹ C.
• 1 C = 6.25 × 10¹⁸ electrons.

CURRENT (I):
• Rate of charge flow.
• Unit: ampere (A) = C/s.
• I = Q / t.
• Direction: CONVENTIONAL current flows + → − (opposite to electron motion).

VOLTAGE (V) / POTENTIAL DIFFERENCE:
• Energy per unit charge.
• Unit: volt (V) = J/C.
• V = W/Q (work per charge).

EMF:
• Electromotive force — voltage provided by a source.
• Battery EMF = energy per coulomb given to charges.

CIRCUIT SYMBOLS:
• Cell (long line = +, short = −).
• Battery (multiple cells).
• Resistor (rectangle).
• Ammeter (A circle) — in SERIES, zero resistance ideally.
• Voltmeter (V circle) — in PARALLEL, infinite resistance ideally.
• Switch (open/closed).
• Lamp (circle with X).

SIMPLE CIRCUIT RULES:
• Current needs a complete loop.
• Voltmeter across component (parallel).
• Ammeter in line (series).

CONDUCTORS vs INSULATORS:
• Conductors: metals, allow electron flow.
• Insulators: plastics, wood; very high resistance.

ELECTRON MODEL:
• Electrons drift slowly (~10⁻⁴ m/s).
• Signal propagates at ~speed of light.

RESISTANCE (R) PREVIEW:
• V = IR (Ohm's law — next topic).
• Resistance opposes current flow.
• Units: ohm (Ω).

WORK AND POWER:
• W = VQ = VIt.
• P = VI (power in watts).

SAFETY:
• Electric shock hazard ≥ 50 V AC (20 V DC).
• Current > 100 mA through heart can be fatal.

Circuit basics

• Current: I = Q/t.
• Voltage: V = W/Q.
• Meters: A series, V parallel.

Charge, current, voltage

• Charge: Q = It
• Current: I = \dfrac{Q}{t}
• Voltage: V = \dfrac{W}{Q}
• Elementary charge: e = 1.6 \times 10^{-19}\,C

Quick reference

| Quantity | Formula | Unit |
|---|---|---|
| Charge | Q = It | Coulomb (C) |
| Current | I = Q/t | Ampere (A) |
| Voltage | V = W/Q | Volt (V) |
| Electron charge | 1.6 × 10⁻¹⁹ C | |

Worked example (Easy)

How many electrons pass through a 2 A current in 5 seconds?

1. Total charge. Q = It = 2 × 5 = 10 C.
2. Electrons per C. 1 C = 6.25 × 10¹⁸ electrons.
3. Total. 10 × 6.25 × 10¹⁸ = 6.25 × 10¹⁹ electrons.

Answer: Approximately 6.25 × 10¹⁹ electrons.

Worked example (Hard)

A device transfers 540 J of energy when 45 C of charge passes through it. Calculate the potential difference across the device and the current if the process takes 30 seconds.

1. Potential difference. $V = W/Q = 540/45 = 12$ V.
2. Current. $I = Q/t = 45/30 = 1.5$ A.
3. Verify with power. $P = VI = 12 \times 1.5 = 18$ W. Check: $P = W/t = 540/30 = 18$ W. ✓

Answer: Potential difference = 12 V, current = 1.5 A.

Common mistakes

• Confusing conventional current direction with electron flow Students mix up which direction current "actually" flows Fix: Conventional current flows from + to − (positive charge direction). Electrons actually flow from − to +. VCAA uses conventional current unless stated otherwise.
• Connecting an ammeter in parallel or a voltmeter in series Forgetting the correct connection for each meter Fix: Ammeter = in SERIES (measures current flowing through). Voltmeter = in PARALLEL (measures potential difference across).
• Forgetting that 1 C is a huge number of electrons Students don't appreciate the scale of charge carriers Fix: 1 C = $6.25 \times 10^{18}$ electrons. Even 1 A (1 C/s) involves billions of billions of electrons per second.

The water analogy and its limits

The water analogy is often used to introduce circuits: voltage is like water pressure, current is like flow rate, and resistance is like pipe narrowness. While helpful for building intuition, this analogy has important limitations.

Voltage (potential difference) is the energy per unit charge — it represents how much "push" the battery gives to each coulomb of charge. A 12 V battery gives 12 J of energy to every coulomb that passes through it. Unlike water pressure, voltage exists even when no current flows (an open circuit still has a voltage across its terminals).

Current is the rate of charge flow: $I = Q/t$. In metals, the charge carriers are free electrons which drift slowly (~$10^{-4}$ m/s) through the lattice. Despite this slow drift, the electric field propagates at near light speed, which is why a light turns on instantly when you flip a switch — you don't wait for electrons to travel from the switch to the bulb.

EMF (electromotive force) is not actually a force — it's the energy per coulomb that a source (battery, generator) provides to the circuit. The term is historical but the concept is crucial: EMF drives current around the complete circuit.

Understanding these quantities is foundational for all of electricity in VCE Physics. Every circuit problem reduces to applying $V = IR$ (Ohm's law), $P = VI$ (power), and conservation of energy and charge (Kirchhoff's laws). Mastering the definitions now makes Units 3–4 electromagnetism much more accessible.

Practice questions

1. A voltmeter is connected to a resistor in:

   1. Series
   2. Parallel
   3. Opposite direction
   4. Short circuit

   (1 marks)

2. A battery delivers 12 C through a bulb in 4 seconds. Calculate the current. If the battery provides 9 V, how much energy does it deliver to the bulb in this time?

   (3 marks)

Summary

• Define charge, current, potential difference.
• Draw basic circuits with correct symbols.
• Relate Q = It.