Resistors | Physics | MrBadmusAI

📖 In-Depth Theory

Ohmic and Non-Ohmic Components

OHMIC CONDUCTOR — constant resistance; I–V graph is a straight line through the origin.

FILAMENT LAMP:

As current increases → lamp heats up → resistance INCREASES.

I–V graph: curve that gets shallower at higher pd.

Not ohmic — resistance changes with temperature.

DIODE:

Allows current in ONE DIRECTION only.

Very high resistance in reverse.

I–V graph: flat near zero in reverse; steep rise forward (above ~0.6 V threshold).

Used in rectifiers to convert AC to DC.

LED (LIGHT-EMITTING DIODE):

Works like a diode but emits light when forward biased.

Very efficient — widely used for lighting and indicators.

Thermistors and LDRs

THERMISTOR (NTC — negative temperature coefficient):

Resistance DECREASES as temperature INCREASES.

Hotter → more charge carriers → lower resistance → more current.

Applications: thermostats, temperature sensors, fire alarms, ovens.

LDR (LIGHT-DEPENDENT RESISTOR):

Resistance DECREASES as light intensity INCREASES.

Bright light → more charge carriers available → lower resistance.

In darkness → very high resistance.

Applications: automatic street lights (switch on when dark), security lights, camera exposure meters.

EXAMPLE: Street light circuit — dark → LDR resistance high → voltage across LDR high → triggers switch → light activates.

I–V Characteristics and Required Practical

REQUIRED PRACTICAL (RP16) — Investigate I–V characteristics of:

1. A resistor at constant temperature (ohmic — straight line)

2. A filament lamp (non-ohmic — curve, resistance rises)

3. A diode (allows one direction — flat then steep)

METHOD:

Variable resistor changes pd across component.

Ammeter (series) and voltmeter (parallel) measure I and V.

Reverse connections to obtain negative values.

Plot I–V graph for each.

SAFETY: Keep current low to avoid overheating. Use a protective resistor in series with the diode.

⚠️ Common Mistake

Thermistor resistance DECREASES with higher temperature (NTC). LDR resistance DECREASES with more light. Both are the OPPOSITE of what students often assume. A filament lamp is NOT ohmic — its resistance increases as it heats up.

📐 Variables

RResistance (R) is measured in ohms (Ω)

ICurrent (I) is measured in amperes (A)

VPotential difference (V) is measured in volts (V)

📐 Key Equations

R = V ÷ I

📌 Key Note

Ohmic resistor: constant R, straight I–V. Filament lamp: R increases with temperature, curved I–V. Diode: one direction only. Thermistor (NTC): R decreases with temperature. LDR: R decreases with light. RP16: investigate I–V graphs for resistor, lamp, diode.

🎯 Matching Activity — Component Behaviour

Match each component to how its resistance changes. — drag the symbols on the right to match the component names on the left.

Ohmic resistor

Drop here

Filament lamp

Drop here

Diode

Drop here

Thermistor (NTC)

Drop here

LDR

Drop here

Resistance decreases as temperature increases

Resistance increases as it heats up — curved I–V graph

Very low resistance in forward direction only — blocks reverse current

Resistance decreases as light intensity increases

Resistance stays constant — straight I–V line through origin

🧪 Required Practical

🔬 RP16 (Physics) — Construct circuits to investigate I–V characteristics of a filament lamp, diode and resistor. Plot graphs and interpret each.

Know the method, variables, equipment and how to analyse results.

🎯 Test Yourself

Question 1 of 2

1. When temperature rises, what happens to a thermistor's resistance and the current through it?

2. An LDR is in series with a fixed resistor and battery. A light is switched on. What happens to the current?

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⚡ Resistors

Ohmic and Non-Ohmic Components

Thermistors and LDRs

I–V Characteristics and Required Practical

🔬 RP16 (Physics) — Construct circuits to investigate I–V characteristics of a filament lamp, diode and resistor. Plot graphs and interpret each.

Mr. Badmus AI