Circuits, resistance and electrical power
Circuit questions reduce to three quantities — current, potential difference and resistance — and the rules that connect them in series and parallel.
Lesson overview
What you'll learn in this lesson
Calculate energy transfers and analyse electrical circuits at GCSE standard.
Key learning points
- • The core relationships
- • Series and parallel
- • Components and safety
This lesson at a glance
- 30 minutes
- 16 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 16
Watch first
Circuits, resistance and electrical power
An English teaching video on this topic from BBC Bitesize - GCSE Revision Support. Watch it, then scroll down for the lesson.
In a nutshell
Calculate energy transfers and analyse electrical circuits at GCSE standard.
Learning cycle
Part 2 of 16
Learning cycle 1 of 2
Part 1 · The core relationships
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 3 of 16
Learn
The core relationships
Current is the rate of flow of charge, Q = It, and potential difference is the energy transferred per unit charge. Ohm's law, V = IR, holds for an ohmic conductor at constant temperature; a filament lamp's resistance rises as it heats, so its graph curves.
Explore the idea
Part 4 of 16
Learn
Series and parallel
In series the current is the same everywhere and potential differences add, so total resistance is the sum. In parallel each branch has the full potential difference and currents add, so adding branches reduces total resistance. This is why household circuits are wired in parallel.
Quick check
Part 5 of 16
Quick check
Ohm's law states
Quick check
Part 6 of 16
Quick check
In a series circuit the current is
Reset break
Part 7 of 16
Pause
That's sitting 1 of 2 done
Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.
Learning cycle
Part 8 of 16
Learning cycle 2 of 2
Part 2 · Components and safety
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 9 of 16
Learn
Components and safety
A thermistor's resistance falls as temperature rises, and a light-dependent resistor's falls as light increases, making both useful in sensing circuits. Mains electricity is alternating at 230 V and 50 Hz; the earth wire and fuse or circuit breaker together prevent a fault from making a metal case live.
Quick check
Part 10 of 16
Quick check
Adding a parallel branch makes total resistance
Quick check
Part 11 of 16
Quick check
A thermistor's resistance
Explore the idea
Part 12 of 16
Worked example
Worked answer: a 12 V supply drives 0.4 A through a resistor. Find the resistance and the power (4 marks)
Rearranging V = IR gives R = V ÷ I (1). R = 12 ÷ 0.4 = 30 Ω (1). Power P = IV (1), so P = 0.4 × 12 = 4.8 W (1). Checking with P = I²R gives 0.4² × 30 = 4.8 W, confirming the answer, which is a quick way to catch a slip in either calculation.
Challenge round
Part 13 of 16
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Challenge round
Part 14 of 16
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
Current is the rate of flow of charge, Q = It, and potential difference is the ____ transferred per unit charge.
This is why household ____ are wired in parallel.
Mastery quiz
Part 15 of 16
Marked quiz
End of lesson quiz: Circuits, resistance and electrical power
4 questions, marked with the reasoning shown. No timer.
1. Ohm's law states
2. In a series circuit the current is
3. Adding a parallel branch makes total resistance
4. A thermistor's resistance
Lesson round-up
Part 16 of 16
Lesson round-up
Ready when you are
Quiz score
Not sat
Games
Not played
Points this lesson
0
Best run
0 in a row
Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?
Ask LunaDo the work
Your written work for this lesson
This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.
Why the school greenhouse failed
A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.
The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.
The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.
Written for Good Learning Co.
Reading questions
- Name the variable the club changed the second time, and one variable they kept the same.
- Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
- Write a conclusion for the club's report, saying how far the evidence supports it.
Method and results
WritingWrite the aim, what you changed, what you measured and what you kept the same, then record your results in a table.
1 page + results table
Conclusion
WritingExplain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.
6–8 sentences
Labelled diagram
DiagramDraw the apparatus or the process in pencil, label it fully, and photograph it.
1 labelled diagram
Reading and notes
ReadingRead the lesson text again and write the key ideas in your own words.
Half a page of notes
Saved as you type.
Part 1 of 16 · Watch & discover