9.3 AS Level

Resistance and resistivity

9 flashcards to master this topic

Definition Flip

Define electrical resistance.

Answer Flip

Resistance (R) is the opposition to the flow of electric current in a circuit. It is defined as the ratio of voltage (V) across a component to the current (I) flowing through it: R = V/I.

Definition Flip

State Ohm's Law.

Answer Flip

Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature and other physical conditions remain constant. Mathematically: V = IR.

Key Concept Flip

Sketch the I-V characteristic for a metallic conductor at constant temperature.

Answer Flip

The I-V characteristic is a straight line passing through the origin. This indicates a linear relationship between voltage and current, obeying Ohm's Law.

Key Concept Flip

Sketch the I-V characteristic for a semiconductor diode.

Answer Flip

The I-V characteristic shows very little current flow for negative voltages (reverse bias). Current increases rapidly for positive voltages above a threshold voltage (forward bias).

Key Concept Flip

Sketch the I-V characteristic for a filament lamp.

Answer Flip

The I-V characteristic is a curve that starts linear, but the slope decreases as voltage/current increases. This indicates that resistance increases with temperature/current.

Key Concept Flip

Explain why the resistance of a filament lamp increases as current increases.

Answer Flip

As current increases, the temperature of the filament increases. This increased temperature causes greater vibrations in the metal lattice of the filament, hindering the flow of electrons and therefore increasing resistance.

Calculation Flip

How do you calculate the resistance of a wire given its resistivity, length and cross-sectional area?

Answer Flip

The resistance (R) is calculated using the formula: R = ρL/A, where ρ is resistivity, L is the length of the wire, and A is its cross-sectional area.

Example: A wire with resistivity 1.7 x 10⁻⁸ Ωm, length 2m, area 1 x 10⁻⁶ m² has R = 0.034Ω
Key Concept Flip

Describe how the resistance of a Light Dependent Resistor (LDR) changes with increasing light intensity.

Answer Flip

The resistance of an LDR decreases as light intensity increases. This is because more photons excite more electrons in the semiconductor material, leading to a greater number of charge carriers and therefore a lower resistance.

Key Concept Flip

Describe how the resistance of a thermistor changes with increasing temperature.

Answer Flip

The resistance of a thermistor (specifically an NTC thermistor as used at AS level) decreases as temperature increases. Higher temperatures provide more energy to charge carriers, allowing them to move more freely and thus lowering the resistance.

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9.2 Potential difference and power 10.1 Practical circuits