Electricity 1 · Part B — Resistance and Ohm's law
⬅ Back to Electricity 1③④ Resistance & Ohm's law
Resistance and the colour code, Ohm's law (V = IR) used three ways, V–I graphs and temperature.
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L3 — Resistance & colour codes
Resistance is a measure of how difficult it is for charge to flow through a material. Symbol R, unit ohm (Ω).
Reading the colour code
The first two bands are digits, the third is the multiplier (how many zeros / power of ten), and the fourth is the tolerance.
Example — Orange, Orange, Brown, Gold
Orange = 3, Orange = 3 → digits give 33
Brown = ×10 → 33 × 10 = 330 Ω
Gold = ±5% tolerance
Tip — bands: Black0 Brown1 Red2 Orange3 Yellow4 Green5 Blue6 Violet7 Grey8 White9.
Maximum and minimum — using the tolerance
Tolerance tells you how far the real resistance is allowed to be from the quoted value. A ±5% resistor is guaranteed to be within 5% of what its bands say — no more, no less. Exam questions ask for the maximum and minimum in almost the same breath as the value, so always work them out the same way: find the percentage of the quoted value, then add it and subtract it.
⚠ Common mistake — tolerance
The percentage is of the quoted value, not a number of ohms you can guess. 5% of 1000 Ω = 50 Ω, so the range is 950 Ω to 1050 Ω.
Don't write "±5 Ω", and don't forget that a bigger resistor gets a bigger spread — ±10% of 10 kΩ is a whole 1000 Ω either way.
Resistor colour-band questions
Check your booklet resistance & colour-code table
Resistance: a measure of how difficult it is for charge to flow through a material.
| Bands | Quoted value | Tolerance |
|---|---|---|
| Orange Orange Brown Silver | 330 Ω | ±10% |
| Green Blue Red Silver | 5600 Ω = 5.6 kΩ | ±10% |
| Brown Green Orange Gold | 15 000 Ω = 15 kΩ | ±5% |
| Red Black Orange Gold | 20 000 Ω = 20 kΩ | ±5% |
Reverse examples: 470 Ω = Yellow Violet Brown; 820 Ω = Grey Red Brown; 390 kΩ = Orange White Yellow.
Can you turn a colour code into a value and a value back into colours?
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L4a — Ohm's Law: equations & calculations
Ohm's Law links the three quantities. Learn the one relationship, then rearrange it:
Find voltage
V = I × R
volts = amps × ohms
Find current
I = V ÷ R
amps = volts ÷ ohms
Find resistance
R = V ÷ I
ohms = volts ÷ amps
Explore · optional Formula-triangle helper
A memory aid only — SQA marks reward rearranging the relationship, so practise that above. V sits on top; I and R sit underneath (so V = I × R). Tap what you're solving for and the triangle shows the rearranged formula.
Top ÷ bottom, or bottom × bottom.
⚠ Common mistakes — Ohm's Law
To find current it's I = V ÷ R, not I = R ÷ V.
Convert units before you calculate: 2 mA = 0.002 A (not 2 A, not 0.2 A) and 4.7 kΩ = 4700 Ω.
Ohm's Law Demo and Graph
Check your booklet Ohm's law calculations
Q (4). A resistor gives 150 mA at 3.0 V. (a) Find its resistance. (b) Why repeat the readings? (c) Give one source of random uncertainty.
Mark scheme
(a) R = V ÷ I = 3.0 ÷ 0.150 = 20 Ω
(b) Repeats reduce the effect of random variation / improve reliability.
(c) Fluctuating meter reading / meter sensitivity / loose contact.
Can you use V = IR all three ways, watching units (mA → A, kΩ → Ω), laid out the exam way?
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L4b — V–I graphs, temperature & experiment
N5 · Going further V–I graphs and temperature effects stretch beyond the core S3 work — useful for N5 revision. The core S3 outcome is Ohm's law (Part B, L4a).
LearnThe V–I experiment
Measure the current through a component for several voltages, then plot V (up) against I (across). A steeper line means higher resistance, and the gradient = R.
Fixed resistor — straight line through the origin. Resistance is constant (ohmic).
Filament lamp — curve gets steeper. As it heats up its resistance rises (non-ohmic).
Q9-style questions give you a table and 3 marks for the graph itself. The marks are not for the physics — they are for the drawing. You get one mark each for:
- Axes labelled with the quantity and its unit, with a sensible scale. voltage (V) and current (A) — not just "V" and "I". A sensible scale uses at least half the graph paper and goes up in easy steps (1, 2, 5, 10 — never 3s or 7s).
- Points plotted accurately. Every point from the table, marked clearly with a dot or a small cross.
- A best-fit line or curve. One smooth line that follows the trend — not a dot-to-dot zig-zag, and not forced through the origin if the points don't go there.
A filament lamp's V–I graph — a curve, getting steeper as it heats.
⚠ Watch out — mA on the axis
Tables of lamp data are almost always in milliamps. If the question later asks for a resistance, you must convert: 250 mA = 0.250 A.
You may label the axis in mA and plot the mA values — that is fine and still earns the mark, as long as the unit on the axis says mA. What loses marks is labelling the axis "current (A)" and then plotting 250 on it.
Either way up is fine. Our practicals plot V up, I across, because then the gradient is the resistance. A table that lists voltage first invites you to plot it the other way round. Both are accepted as long as each axis is labelled with its quantity and unit — so plot it whichever way you were taught, and label it properly.
Temperature & resistance
- Wire / conductor / filament: hotter → resistance increases (atoms vibrate more and get in the way of the charge).
- Thermistor: hotter → resistance decreases (more charge carriers become free).
Explore · optional V–I explorer — draw the graph yourself
Resistance of a filament lamp
Non-ohmic conductor demonstration
Check your booklet graph & temperature answers
Graphs: a fixed resistor = a straight line through the origin (ohmic); a filament lamp / non-ohmic conductor = a curve that gets steeper as it heats.
Temperature: a fixed resistor / conductor — resistance rises as temperature rises; a thermistor — resistance falls as temperature rises.
Q1 (4). A thermistor warms and its resistance falls from 8 kΩ to 2 kΩ. (a) What happens to the current? (b) Explain using charge carriers.
Mark scheme
(a) Current increases (lower resistance, same voltage).
(b) As temperature rises, more charge carriers are released, so resistance drops.
Q2 (3). A filament lamp's resistance rises as the voltage across it rises. Explain why.
Mark scheme
More current heats the filament; the hotter filament's atoms vibrate more, opposing the flow, so resistance increases.
Can you read a V–I graph (gradient = R) and state both temperature rules?
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Sources & credits: The RS Electricity B1 booklet © R Stewart, 2025. Videos © Mr Bell – Practical Electronics and The Other Mr Stewart (YouTube @mrstewartphysics), via YouTube.