S2 Science | Electricity and Circuits: Lesson-Prep Guide for Tutors (current, voltage, series and parallel, charge, fuses)

2026-11-12

Tutor lesson-prep guides · Junior Science

Students explain a dimmer second bulb by saying the current ran out, and expect every bulb in parallel to dim. This lesson fixes three things: current is charge per second, each circuit type keeps one quantity constant, and total resistance is checked against the smallest resistor.

Objectives — Students can: (1) define current as charge flowing per second in amperes and voltage as energy released per unit charge in volts, and use Q equals It to get 180 coulombs from 1.5 amperes over 120 seconds; (2) state that series keeps current the same and shares voltage, while parallel keeps voltage the same and shares current, and compute 9 ohms in series and 2 ohms in parallel from 6 ohms and 3 ohms; (3) solve a 9-volt supply to 1 ampere with 6 volts and 3 volts across the resistors in series, and 4.5 amperes total with 1.5 and 3 amperes in the branches in parallel; (4) explain that each parallel branch sees the full supply voltage so brightness is unchanged, and why a fuse belongs on the live wire.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Units and decimals	How many milliamperes are in one ampere?	One thousand; reteach that milli means a thousandth
Reciprocals	What is one sixth plus one third?	One half; reciprocal arithmetic is the only hard step in parallel
Circuit symbols	How do you draw a cell, a lamp, a switch and an ammeter?	Long and short lines, a crossed circle, a broken line, a circle labelled A

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Collect the wrong idea	Ask why the second bulb is dimmer and write the answer on the left of the board	Say what you think, wrong is useful
5-14 min	Current and voltage defined	Use the water-pipe picture, then Q equals It for 180 coulombs	Work two examples with units
14-25 min	Series shares voltage	Wire 6 and 3 ohms to 9 volts and derive 1 ampere, 6 volts, 3 volts	Compute the split and check the sum
25-35 min	Parallel shares current	Rewire in parallel, get 2 ohms, then 4.5 amperes and both branches	Explain why brightness is unchanged
35-41 min	Power and safety	Use the 1.35-watt bulb for power, then place the fuse on the live wire	State the consequence of the wrong position
41-45 min	Exit check	Collect three answers, mark live, name the act to reteach	Hand in three answers

03 Teaching script (read it out)

Scene 1: Act one: current flows, it is not used up

“Start with the most expensive error: the current ran out, so the second bulb is dim. Current is not consumed. In one path the same charge passes every point each second, so two bulbs in series carry the same current. What is used up is energy: each charge releases energy as it passes a bulb, and voltage is the energy released per unit charge. So keep two sentences. Current is how much charge passes per second, written Q equals I times t. Voltage is how much energy each charge carries. 1.5 amperes for 120 seconds is 180 coulombs. A missing unit here is a concept error, not carelessness.”

Current is the same everywhere; energy is what gets used.

Scene 2: Act two: series adds resistance, parallel reduces it

“Series gives one path, so charges pass both resistors in turn and the resistances add: 6 plus 3 is 9 ohms. Parallel gives two paths at once, which is like opening extra gates, so the total must be smaller than the smallest resistor: 6 and 3 in parallel is 2 ohms, less than 3. The method is reciprocals, add, then invert: one sixth plus one third is one half, inverted is 2. The common mistake is adding and halving to get 4.5, which is neither series nor parallel. The moment you see it, ask how many paths there are.”

Series adds, parallel inverts, and the result is smaller than the smallest.

Scene 3: Act three: voltage sets brightness, so parallel bulbs match

“Brightness follows voltage, not current. In series the 9 volts must be shared: 6 volts across 6 ohms and 3 volts across 3 ohms, so one bulb is brighter. In parallel each branch connects straight across the supply, so each gets 9 volts and both match, but the battery must supply 1.5 plus 3, which is 4.5 amperes, so it drains faster. That is why household appliances are all in parallel: switching one off leaves the others working, and each still receives 220 volts. A fuse goes on the live wire because that is the wire carrying current into the appliance. On the neutral wire the appliance stays live, and touching it completes the circuit.”

Voltage sets brightness; parallel branches share one voltage.

04 Question types and worked steps

Type	Example	Steps	Answer
Series resistance and current	A 6-ohm and a 3-ohm resistor in series across 9 volts. Find total resistance, current and both voltages.	(1) Add to 9 ohms (2) 9 divided by 9 gives 1 ampere (3) voltage is current times resistance (4) check the two voltages add to 9	9 ohms, 1 ampere, 6 volts and 3 volts
Parallel resistance and branch currents	The same pair in parallel across 9 volts. Find total resistance, total current and both branch currents.	(1) Reciprocals give one half, inverted gives 2 ohms (2) 9 divided by 2 gives 4.5 amperes (3) each branch is 9 divided by its own resistance (4) check 1.5 plus 3 equals 4.5	2 ohms, 4.5 amperes, 1.5 and 3 amperes
Charge and time	A branch carries 1.5 amperes for 2 minutes. How much charge passes?	(1) Convert to 120 seconds (2) use Q equals I times t (3) multiply 1.5 by 120 (4) answer in coulombs	180 coulombs
Power and bulb resistance	Three 1.5-volt cells in series light a bulb drawing 0.3 amperes. Find resistance and power.	(1) Supply is 4.5 volts (2) resistance is 4.5 divided by 0.3, giving 15 ohms (3) power is voltage times current (4) 1.35 watts	15 ohms and 1.35 watts

Sanity check (teach this): Consistency check: (1) a series total must exceed every resistor and a parallel total must be below the smallest; a parallel answer of 4.5 ohms means the wrong formula. (2) Series voltages must add to the supply and parallel branch currents must add to the total, so both sums are free checks. (3) Units are ohms, amperes, volts and coulombs, and a wrong unit is a wrong idea. (4) Resistance does not change with current unless the question says temperature changes it. (5) Convert minutes to seconds first, because substituting 2 for 120 seconds is the single most common lost mark.

05 Independent practice (3 questions)

Question	Answer	What to watch
4 ohms and 12 ohms in parallel across 12 volts. Find total resistance and total current.	Reciprocals give one third, so 3 ohms, and 12 divided by 3 is 4 amperes	Whether the reciprocal was inverted and the branches 3 plus 1 checked
Find the power in each resistor above.	12 times 3 is 36 watts in the 4-ohm branch, 12 times 1 is 12 watts in the other	Whether both use the full 12 volts rather than a share
A 1.5-ampere circuit runs for 5 minutes. How many coulombs?	300 seconds times 1.5 is 450 coulombs	Whether minutes were substituted without conversion

06 Common mistakes and what to say

Mistake	Student writes	Say this
Current is used up	The second bulb is dim because only half the current is left	Current is the same everywhere in series. Energy is what gets used, and voltage is the energy per charge. Say voltage drop, not ran out.
Parallel by averaging	6 and 3 in parallel average to 4.5 ohms	Parallel uses reciprocals and gives 2 ohms. Two paths are easier than one, so the total must be below the smallest, which is 3.
Voltage and current swapped	Each parallel branch carries half the total current	Parallel keeps voltage equal and splits current by resistance. The smaller resistor takes more, so 1.5 and 3 amperes, not an even split.
Fuse on the neutral wire	The neutral wire works just as well because it still breaks the circuit	The fuse belongs on the live wire. On the neutral the appliance stays at 220 volts, so it looks safe with the switch off and is not.

07 Homework (paper, section and question numbers)

Junior Science chapter index — All chapters for the subject; the two S2 chapters work in order
Lesson-prep master index — Other subjects show the same four-step calculation pattern
Redo this lesson — Draw one circuit with two bulbs, once in series and once in parallel, and write total resistance, current and each bulb voltage with the sums checked

08 Exit check (last 5 minutes)

Question	Answer	Decision
Which quantity stays constant in series, and which in parallel?	Current in series, voltage in parallel	Reversed means reteach act two
What is the parallel total of 6 and 3 ohms?	2 ohms, below the smallest	Answering 4.5 means repeat reciprocal addition
Which wire carries the fuse?	The live wire	Answering neutral means return to act three

09 Tutor FAQ

How many formulas must students memorise?

Three: Q equals It, V equals IR, P equals VI. Everything else is derived. The two qualitative sentences about series and parallel earn more marks than a longer formula list.

Why not start with the circuit board?

Numbers and the diagram must fix which quantity is constant first. Start with hardware and students remember a lit bulb but cannot explain it.

What if the parallel total comes out larger than a resistor?

Do not give the answer. Ask them to draw two paths and say whether two paths are harder or easier to walk. Once they say easier, return to the arithmetic.

Find a tutor — free Tutor Registration →

A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.