Speca › Topics › GCSE Combined Science

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• Assessment mark schemeWord, editable
• End-of-topic assessmentWord, editable
• HomeworkWord, editable
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• Tiered worksheetWord, editable
• Worksheet mark schemeWord, editable

Foundation and Higher tier

• Draw and interpret circuit diagrams using standard circuit symbols.
• Explain that a closed circuit needs a source of potential difference for charge to flow, define electric current as the rate of flow of electrical charge, and use Q = I × t (charge flow in coulombs; current in amperes; time in seconds): knowing current has the same value at any point in a single closed loop.
• Explain that a component's current depends on its resistance and the potential difference across it (greater resistance means smaller current for a given potential difference), and use V = I × R (potential difference in volts; current in amperes; resistance in ohms): questions may use either "potential difference" or "voltage," and both are creditable.
• Explain that some resistors (ohmic conductors, at constant temperature) have constant resistance, where current is directly proportional to potential difference, while others' resistance changes with current: a filament lamp's resistance increases as its filament heats up; a diode conducts in one direction only, with very high resistance in reverse; a thermistor's resistance decreases as temperature increases (used in circuits such as thermostats); an LDR's resistance decreases as light intensity increases (used in circuits such as automatic lighting), explain how to design a circuit to measure a component's resistance (via current and potential difference), draw the correct circuit diagram, and use I–V graphs to judge whether a component is linear or non-linear and relate that curve shape to its function.
• Describe the difference between series and parallel circuits, and explain qualitatively why adding resistors in series increases total resistance while adding them in parallel decreases it. For components in series: the current is the same through each component, the supply's total potential difference is shared between them, and total resistance is the sum of the individual resistances (R_total = R₁ + R₂). For components in parallel: the potential difference across each is the same, total current is the sum of the currents through each branch, and total resistance is less than the smallest individual resistance (calculating the actual total parallel resistance value is not required). Construct and check series/parallel circuits from diagrams, explain the design of dc series circuits for measurement/testing, calculate current/potential difference/resistance in dc series circuits, and solve series-circuit problems using equivalent resistance.
• Explain that mains electricity is an ac supply (UK domestic supply: 50 Hz, about 230 V), and explain the difference between direct and alternating potential difference.
• Describe three-core mains cable's colour-coded wires (live, brown; neutral, blue; earth, green/yellow stripes), explain that the live wire carries the alternating supply potential difference (~230 V relative to earth's 0 V), the neutral wire completes the circuit (at or near 0 V), and the earth wire is a safety wire only carrying current if a fault occurs, explain why a live wire can be dangerous even with the mains switch open, and the dangers of any connection between live and earth.
• Explain how a circuit device's power relates to the potential difference across it and current through it, and to energy change over time, using P = V × I and P = I² × R (power in watts; potential difference in volts; current in amperes; resistance in ohms).
• Describe how everyday appliances transfer energy, from batteries or the ac mains to a motor's kinetic energy or a heating device's thermal energy, explain that the energy an appliance transfers depends on how long it runs and its power, and that work is done when charge flows in a circuit; use E = P × t and E = Q × V (energy transferred in joules; power in watts; time in seconds; charge flow in coulombs; potential difference in volts); explain how a device's power relates to potential difference/current and to energy transferred over time, and describe the relationship, with examples, between an appliance's power rating and how its stored energy changes in use.
• Describe the National Grid as the network of cables and transformers connecting power stations to consumers, explain that step-up transformers raise potential difference for efficient transmission while step-down transformers lower it for safe domestic use, and explain why the National Grid is an efficient way to transfer energy overall: detailed knowledge of a transformer's internal structure is not required.

Higher tier only

• Select and use the transformer equation (supplied on the equation sheet): potential difference across the primary coil × current in the primary coil = potential difference across the secondary coil × current in the secondary coil.

Required practicals

• Required practical 15: use circuit diagrams to build and check circuits investigating what affects resistance, including wire length at constant temperature, and combinations of resistors in series and in parallel.
• Required practical 16: use circuit diagrams to build circuits investigating the current–potential-difference (I–V) characteristics of different components, including a filament lamp, a diode, and a resistor at constant temperature.

Where students go wrong

• Confusing current and potential difference, particularly treating them as the same measurable "amount of electricity," rather than distinguishing current (charge flow rate) from potential difference (energy transferred per unit charge, driving that flow).
• Assuming current is "used up" as it passes around a series circuit, rather than understanding current is the same at every point in a single loop, while potential difference (not current) is what's shared between series components.
• Applying the series resistance-adds rule to a parallel circuit or vice versa: these two behave in genuinely opposite ways for both current and resistance, and mixing them up is one of the most common electricity errors.
• Believing the neutral wire is always completely safe because it sits near 0 V, rather than understanding only the earth wire is a dedicated safety conductor, and that live-to-neutral or live-to-earth contact can both still be dangerous.
• Mixing up step-up and step-down transformers' roles in the National Grid, or forgetting the purpose of stepping up voltage for transmission is to reduce transmission energy losses, not simply "to travel further."

How it gets asked in the exam

"Calculate the current/potential difference/resistance in this circuit", "Explain why the resistance of [component] changes as...", "Describe the difference between series and parallel circuits", "Explain why the National Grid uses transformers", "Explain why a live wire can still be dangerous even when a switch is open", "Calculate the potential difference/current in the secondary coil" (Higher).

Key vocabulary

Current, potential difference, resistance, ohmic conductor, thermistor, LDR, diode, series circuit, parallel circuit, alternating current, direct current, live wire, neutral wire, earth wire, power, National Grid, transformer (HT).

Assumed prior knowledge

• Comfort rearranging and substituting into formulae: reused constantly across this topic's many equations.
• Basic familiarity with the idea of a circuit and simple series/parallel wiring, from KS3 science.
• Power and energy-transfer equations from Energy (this taxonomy's topic 18): P = E ÷ t is directly reused and extended here.

How Speca scaffolds this topic

• A single, always-visible circuit symbol key (cell, resistor, variable resistor, lamp, diode, thermistor, LDR, ammeter, voltmeter) used consistently across every circuit diagram in this topic, since correctly reading symbols is a prerequisite for every other skill here.
• A fixed side-by-side series/parallel comparison table (current, potential difference, resistance: one row each) built and referred back to throughout, directly targeting the topic's single most common confusion.
• I–V graphs for the four non-ohmic components (filament lamp, diode, thermistor, LDR) benefit from being shown together on one reference sheet, each paired with a one-line "what this shape means" explanation, so the shape-to-behaviour link is explicit rather than left for students to infer.
• The mains-safety content (three-core cable, live/neutral/earth) benefits from a single consistently coloured, labelled diagram used every time it's referenced, both because the colour-coding itself is content to be learned and because consistent visuals reduce the real-world safety stakes of misremembering which wire is which.

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