Electricity Fundamentals
Understand electric charge, current, voltage, and resistance, and apply Ohm's law to analyse simple circuits.
Electric Charge and Current
Electric charge (Q) is a fundamental property of matter. Electrons carry negative charge, protons carry positive charge. Electric current (I) is the rate of flow of charge through a conductor.
Electric Charge
Q = It
Charge (coulombs, C) = current (amperes, A) × time (seconds, s). One coulomb = the charge carried by 6.25 × 1018 electrons.
Electric Current
I = Q / t
Current (A) is the amount of charge passing a point per second. Conventional current flows from positive to negative; electrons flow the opposite way.
DC vs AC: Direct current (DC) flows in one direction (e.g., batteries). Alternating current (AC) periodically reverses direction (e.g., mains electricity in Australia at 50 Hz, 230 V).
Voltage and Resistance
Voltage (V), also called potential difference, is the energy per unit charge provided by a source or used by a component. Resistance (R) is a measure of how much a component opposes the flow of current.
Water Analogy for Electricity
Voltage = Water pressure
The "push" that drives the flow. Unit: volt (V)
Current = Water flow rate
The rate charges flow through. Unit: ampere (A)
Resistance = Pipe narrowing
Opposition to flow. Unit: ohm (Ω)
Factors affecting resistance: Length (longer = more resistance), cross-sectional area (thinner = more resistance), material (copper has low resistance), and temperature (usually increases with temperature for metals).
Ohm's Law
Ohm's law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance, provided temperature remains constant.
V = IR
V = IR
Find voltage
I = V / R
Find current
R = V / I
Find resistance
Electrical Power
Power (P) is the rate of energy transfer. P = VI = I²R = V²/R. Unit: watt (W).
Energy (E) = Pt = VIt. Unit: joule (J). Electricity bills use kilowatt-hours (kWh): 1 kWh = 3.6 × 106 J.
Example: A 60 W light bulb on a 230 V supply draws I = P/V = 60/230 = 0.26 A.
Key Vocabulary
Current (I)
The rate of flow of electric charge past a point in a circuit. Measured in amperes (A). I = Q/t.
Voltage (V)
The potential difference (energy per unit charge) between two points in a circuit. Measured in volts (V).
Resistance (R)
The opposition to current flow in a circuit. Measured in ohms (Ω). Higher resistance means less current for the same voltage.
Ohm's Law
V = IR. The voltage across a conductor is proportional to the current through it, at constant temperature.
Worked Examples
A 12 V battery is connected to a 4 Ω resistor. Calculate the current.
Step 1: Known: V = 12 V, R = 4 Ω. Find I.
Step 2: Using I = V/R = 12/4.
Answer: I = 3 A.
A current of 2 A flows through a circuit for 30 s. How much charge passes through?
Step 1: Q = It = 2 × 30.
Answer: Q = 60 C (60 coulombs of charge).
A toaster uses 2000 W on a 230 V supply. What is its resistance?
Step 1: Find current: I = P/V = 2000/230 = 8.70 A.
Step 2: Find resistance: R = V/I = 230/8.70 = 26.4 Ω.
Answer: R = 26.4 Ω. (Alternatively: R = V²/P = 230²/2000 = 26.5 Ω.)
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
What is the SI unit of electric current?
Question 2
A 6 V battery drives a current of 0.5 A through a resistor. What is the resistance?
Question 3
If you double the resistance in a circuit while keeping voltage constant, the current will:
Question 4
What is the power dissipated by a 10 Ω resistor with 2 A flowing through it?
Question 5
Australian mains electricity is supplied at:
Key Concepts Summary
- ●Current (I) = charge (Q) per unit time. Unit: ampere (A).
- ●Voltage (V) is the energy per unit charge (potential difference). Unit: volt (V).
- ●Resistance (R) opposes current flow. Unit: ohm (Ω).
- ●Ohm's law: V = IR. Current is proportional to voltage and inversely proportional to resistance.
- ●Power: P = VI = I²R = V²/R. Unit: watt (W).