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Theme B — The Particulate Nature of Matter

B.5.7 I-V Characteristics — IB Physics HL Revision Notes & Topical Questions

5 min read 11 parts Lesson 48 of 53 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

B.5.7 I-V Characteristics — IB PHYSICS SL notes: Sketch and interpret I–V graphs for ohmic resistors, filament lamps, diodes, thermistors and LDRs.Ohm's law — for a component at constant temperature, the current through it is proportional to the p.d.

  • What the symbols mean
  • Test for obeying it
  • Ohmic components
  • I–V characteristic
  • At a point
  • Gradient on an I–V graph

Part 1 of 9Ohm's law and the I–V graph

Ohm's law and the I–V graph
Ohm's law
  • Ohm's law for a component at constant temperature, the current through it is proportional to the p.d. across it
  • V = IR
    What the symbols mean: V = potential difference (V) · I = current (A) · R = resistance (Ω)
Test for obeying it
  • Test for obeying it the graph of current against p.d. is a straight line through the origin
Ohmic components
  • Ohmic components wires at constant temperature and fixed resistors
Non-ohmic components
  • Non-ohmic components filament lamps, LEDs and other diodes, thermistors and LDRs
I–V characteristic
  • I–V characteristic a graph of current (y-axis) against p.d. across a component
  • It summarises how the component behaves electrically.

Part 2 of 9Reading resistance from a graph

At a point
  • At a point resistance is R = V/I using the coordinates of that point
  • It is not the gradient of the tangent, except for a straight line through the origin.
Gradient on an I–V graph
  • Gradient on an I–V graph the gradient of a straight line is 1/R, so a steeper line means lower resistance
Gradient on a V–I graph
  • Gradient on a V–I graph the gradient is R
Power at a point
  • Power at a point multiply the coordinates, P = VI
    What the symbols mean: P = power (W) · I = current (A) · V = potential difference (V)

Part 3 of 9Circuit to obtain an I–V characteristic

1
Connect the component in series with a cell, ammeter and variable resistor or potential divider.
2
Connect a voltmeter in parallel across the component.
3
Adjust the variable resistor so the current changes.
4
Record V for each value of I.
5
Reverse the supply to take readings in both directions and plot I against V.
  • Small currents
    • keep the current small so heating does not change the resistance

Part 4 of 9Ohmic resistor

Shape
  • Shape a straight line through the origin, so doubling V doubles I
Symmetry
  • Symmetry reversing the supply reverses the current by the same amount
Gradient
  • Gradient the gradient is 1/R
Heating
  • Heating if the current is large enough to heat the resistor, R drifts and the line begins to curve

Part 5 of 9Filament lamp

Shape
  • Shape an S-shaped curve through the origin that is symmetric about it
  • At low p.d. it is nearly straight and behaves approximately ohmic.
  • As V increases, I rises less than proportionally, so the line bends toward the V-axis.
Cause
  • Cause the current heats the tungsten filament, so lattice ions vibrate more and collide more often with electrons
  • The resistance rises, so V/I increases with p.d.
  • The change is due to temperature, not to a change of material.

Part 6 of 9Semiconductor diode

Semiconductor diode
Function
  • Function a diode allows current in one direction only
Forward bias
  • Forward bias current is almost zero until the threshold p.d., then rises very steeply
  • The threshold is about 0.6 V for silicon and about 2 V for many LEDs.
  • A series resistor is needed to limit the current.
Reverse bias
  • Reverse bias the diode does not conduct, so the current is negligible
  • It is a few microamps or less until a very high breakdown p.d. is reached.
Resistance
  • Resistance it is very high in reverse bias and falls rapidly in forward bias above the threshold
LED
  • LED its I–V characteristic has the same form as other diodes
Use
  • Use diodes are used for rectification, converting alternating to direct current

Part 7 of 9Thermistors and LDRs

NTC thermistor
  • NTC thermistor its resistance falls as temperature rises, because more charge carriers are released
  • With increasing p.d., self-heating makes the current rise faster than proportionally.
  • Its I–V line therefore curves toward the I-axis.
  • It is used in temperature sensors.
LDR
  • LDR its resistance drops as light intensity rises, because photons liberate extra carriers
  • At fixed illumination it behaves like an ohmic resistor.
  • Brighter light gives a steeper line, so there is a family of straight lines.

Part 8 of 9Identifying an unknown component

Identifying an unknown component
Straight line through the origin
  • Straight line through the origin the component is ohmic
Symmetric or not
  • Symmetric or not a lamp and resistor are symmetric, a diode is not
Gradient trend
  • Gradient trend an increasing gradient means falling resistance and a decreasing gradient means rising resistance
Sketching
  • Sketching label both axes with quantity and unit, mark the origin and show behaviour for negative values
  • A lamp curve flattens as p.d. increases.
  • A diode has a flat axis in reverse bias, a sharp knee near 0.6 V, then an almost vertical rise.

Part 9 of 9More from the specification

More from the specification
Electromotive Force and Terminal Potential Difference
  • The electromotive force (EMF, epsilon) of a battery or chemical cell is defined as the total electrical energy transferred per unit charge; terminal voltage V across the circuit load is given by the equation V = epsilon - I * r,
    • (epsilon = W / Q)
  • where r is internal resistance.
Ohm's Law and I-V Graphs
  • An ohmic conductor displays a straight-line I-V graph through the origin at constant temperature, indicating that current is directly proportional to potential difference (V = IR),
  • whereas filament lamps show decreasing gradient due to thermal resistance increases.
Electrical Resistivity of Conductor Materials
  • The resistance of a uniform wire depends on the intrinsic resistivity rho of the material according to the equation R = rho * L / A;
    • good conductors have low resistivity, facilitating charge transfer.
Direction of Current and Electron Drift
  • In an electrical circuit, conventional current flows from the positive terminal to the negative terminal of the DC supply,
  • whereas mobile conduction electrons drift in the opposite direction.
Conventional current
  • Current drawn from the positive to the negative terminal.
Battery
  • Two or more cells joined together.
Resistivity
  • Property of a material defined by ρ = RA/L.
Terminal
  • Connection point of a cell, labelled + or −.
Source
  • Reservoir at TH providing heat QH
Electrical energy
  • Energy carried by electrons in a circuit.
Energy transferred
  • W = V Q, in joules.
    What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
Combined resistance
  • Single resistance that could replace several resistors.
Circuit diagram
  • Drawing of a circuit using standard symbols.

Key terms

TermMeaning
I–V characteristicGraph showing the current through a component as the p.d. across it changes.
OhmicComponent with a constant resistance; its I–V line is straight through the origin.
Forward biasDiode connection in which current flows after the threshold p.d.
Reverse biasDiode connection in which almost no current flows.
Threshold voltageForward p.d. above which a diode conducts strongly, about 0.6 V for silicon.
NTC thermistorResistor whose resistance decreases as temperature increases.
LDRLight-dependent resistor whose resistance falls as light intensity rises.

Avoid these mistakes

1
Using the gradient of a curved I–V graph to give resistance — use V/I at the point.
2
Reading an I–V graph as a V–I graph, so a steeper line is wrongly taken as higher resistance.
3
Saying a filament lamp is non-ohmic because of a different material — the cause is temperature.
4
Saying a reverse-biased diode carries exactly zero current — it is negligible but not exactly zero.
5
Saying the resistance of an NTC thermistor increases with temperature — it decreases.

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