Electric Circuits

Fundamental Quantities

Three key quantities in circuits:

Current (I) — the rate of flow of electric charge. Measured in amperes A using an ammeter (connected in series).

• Symbol equation: I = Q/t (charge Q in coulombs, time t in seconds)
• Conventional current flows from + to − (opposite to electron flow, which is from − to +)

Potential difference (p.d. / voltage, V) — the energy transferred per unit charge. Measured in volts (V) using a voltmeter (connected in parallel).

• Symbol equation: V = W/Q (W = energy in joules, Q = charge in coulombs)
• Also: V = E/Q (E = energy)

Resistance (R) — opposition to current flow. Measured in ohms (Ω).

Ohm's Law

Ohm's Law: For a conductor at constant temperature, the current through it is directly proportional to the potential difference across it.

$$V = IR$$

Where:

• V = potential difference (V)
• I = current A
• R = resistance (Ω)

Rearranged forms:

• I = V/R (current)
• R = V/I (resistance)

Worked example: A resistor has a current of 2 A flowing through it when p.d. is 12 V. Resistance = 12/2 = 6 Ω.

IV Characteristics

An IV characteristic graph plots current (y-axis) against potential difference (x-axis) for a component.

Ohmic resistor (e.g., fixed resistor):

• Straight line through the origin
• Gradient = 1/R (resistance is constant)
• Obeys Ohm's law at constant temperature

Filament lamp:

• Curve that gets less steep as V increases
• As current increases → lamp gets hotter → resistance increases (filament metals have higher resistance at higher temperatures) → does NOT obey Ohm's law

Diode:

• Allows current to flow in one direction only (forward bias)
• Very high resistance in reverse direction (virtually zero current)
• Threshold voltage of ~0.7 V (for silicon diode) before significant current flows

Power and Energy

Electrical power: $$P = IV$$ Also: P = I²R and P = V²/R

Energy transferred: $$E = Pt = IVt$$

Example: A kettle with resistance 25 Ω draws a current of 9.2 A from 230 V mains.

• Power = IV = 9.2 × 230 = 2116 W ≈ 2.1 kW
• Or P = I²R = 9.2² × 25 = 84.64 × 25 = 2116 W ✓

Charge

$$Q = It$$

Example: A current of 0.5 A flows for 3 minutes = 180 s. Q = 0.5 × 180 = 90 C (coulombs)

WJEC Practical Requirement

WJEC Eduqas required practical: investigating IV characteristics of a resistor, lamp and diode. You may be asked to describe the method (variable resistor in series to change p.d., ammeter in series, voltmeter in parallel) and explain the shape of each IV graph.