Action and use of circuit components

1. Overview

This topic explores how different electrical components can be used to control the flow of current and the distribution of voltage within a circuit. Understanding these principles is essential for designing practical electronic systems, such as light-sensitive switches, temperature alarms, and volume controls.

Key Definitions

• Potential Difference (p.d.): The energy transferred per unit charge between two points in a circuit, measured in Volts (V).
• Resistance: The opposition to the flow of electrical current, measured in Ohms (Ω).
• Potential Divider: A simple circuit which uses two or more resistors in series to split (divide) the voltage of a supply into smaller parts.
• Potentiometer: A type of variable resistor with three terminals used as a variable potential divider to provide a manually adjustable output voltage.
• LDR (Light Dependent Resistor): A component whose resistance decreases as the intensity of light falling on it increases.
• Thermistor: A component whose resistance decreases as its temperature increases (specifically Negative Temperature Coefficient or NTC thermistors).

Core Content

Relationship between Resistance and Potential Difference In a series circuit, the total voltage from the power supply is shared between the components. The way this voltage is shared depends directly on the resistance of each component.

• Rule: For a constant current flowing through a circuit, the potential difference across a conductor increases as its resistance increases.
• This is derived from Ohm’s Law: $V = I \times R$. If $I$ is constant, then $V$ is directly proportional to $R$ ($V \propto R$).
• If you have two resistors in series, the one with the higher resistance will take a larger "share" of the total voltage.

Worked Example: A 12V battery is connected to a 4Ω resistor and an 8Ω resistor in series.

• Because the 8Ω resistor has twice the resistance of the 4Ω resistor, it will receive twice the voltage.
• The 12V is split: 4V goes across the 4Ω resistor, and 8V goes across the 8Ω resistor.

Extended Content (Extended Curriculum Only)

The Variable Potential Divider A potential divider consists of two resistors in series. By changing the resistance of one of these resistors, we can change the output voltage ($V_{out}$) across it.

• Variable Resistors: Using a potentiometer allows you to manually adjust $V_{out}$ from 0V up to the full supply voltage.
• Sensors: We can replace a fixed resistor with a sensor like an LDR or a Thermistor.
  • Light Sensors: In an LDR circuit, as light intensity increases, LDR resistance decreases. This causes the p.d. across the LDR to decrease, while the p.d. across the fixed resistor increases.
  • Temperature Sensors: In a thermistor circuit, as temperature increases, thermistor resistance decreases, changing the p.d. shared across it.

The Potential Divider Equation To calculate the specific output voltage ($V_{out}$) across one resistor ($R_2$) in a pair, use:

$$V_{out} = \left( \frac{R_2}{R_1 + R_2} \right) \times V_{in}$$

• $V_{in}$ = Supply voltage
• $R_1$ = Resistance of the first component
• $R_2$ = Resistance of the component you are measuring the voltage across

Worked Example: A potential divider circuit has a $V_{in}$ of 10V. $R_1$ is a fixed 200Ω resistor and $R_2$ is an LDR. In bright light, the LDR resistance is 50Ω. Calculate $V_{out}$ across the LDR.

• $V_{out} = (50 / (200 + 50)) \times 10$
• $V_{out} = (50 / 250) \times 10 = 0.2 \times 10 = 2\text{V}$

Key Equations

• Ohm’s Law: $V = I \times R$
  • $V$: Potential Difference (V)
  • $I$: Current (A)
  • $R$: Resistance (Ω)
• Potential Divider Formula: $V_{out} = \frac{R_2}{R_{total}} \times V_{in}$
  • $R_{total} = R_1 + R_2$

Common Mistakes to Avoid

• ❌ Wrong: Decreasing a component's resistance decreases the current in a series circuit.
• ✓ Right: Decreasing resistance decreases the total resistance, which increases the current ($I = V/R$).
• ❌ Wrong: LDRs have low resistance when it is dark.
• ✓ Right: LDR resistance is high when dark and low when light (Remember: Light Up, Resistance Down).
• ❌ Wrong: In a potential divider, the larger resistance gets a smaller share of the voltage.
• ✓ Right: The larger the resistance, the larger the share of the total voltage it receives.
• ❌ Wrong: Assuming a change in one branch of a parallel circuit automatically changes the voltage in other branches.
• ✓ Right: In a standard parallel circuit with a constant voltage source, the p.d. across each branch remains equal to the source voltage regardless of changes in other branches.

Exam Tips

1. Identify the "Fixed" vs "Variable": When looking at a potential divider, identify which resistor is fixed and which is the sensor (LDR/Thermistor). If the sensor resistance goes down, its share of the voltage goes down.
2. Check the Output: Always check which component the $V_{out}$ (or the voltmeter) is connected across. If the voltmeter is across the fixed resistor, it will show a higher voltage when the sensor's resistance decreases.
3. Diode Orientation: If a diode is in the circuit, check the arrow direction. Current only flows in the direction of the arrow. If the battery is trying to push current "against" the arrow, the resistance is effectively infinite and no current flows.