Your revision route
What you’ll learn
- Draw and interpret circuits using standard symbols, with ammeters in series and voltmeters in parallel.
- Explain current as charge flow and use Q = It and V = IR with correct units.
- Apply the rules for current, potential difference and resistance in series, parallel and mixed circuits.
- Interpret I–V graphs and explain how lamps, diodes, thermistors and LDRs behave.
- Distinguish direct and alternating potential difference and explain three-core mains wiring safely.
- Calculate electrical power and energy, and explain appliance energy transfers and power ratings.
- Explain why the National Grid uses high transmission potential differences and step-up and step-down transformers.
- Plan, analyse and improve the resistance and I–V characteristics required practicals.
- Separate Physics only: explain static charging, forces, sparking and electric fields around charged spheres.
Build the big picture
Key ideas
A complete loop gives charge somewhere to go
A cell does not manufacture current. It supplies a potential difference that drives charge already present around a closed circuit.
- Current is the rate of flow of charge. In a single closed loop, its value is the same at every point because charge does not pile up.
- Use standard symbols. Put an ammeter in series to measure current and a voltmeter in parallel across the component.
- A cell is one electrical source; a battery is two or more cells. A switch opens or closes the conducting path.
- Recognise and draw cell, battery, open and closed switch, lamp, fuse, meters, fixed and variable resistor, thermistor, LDR, diode and LED symbols.
The punchline: Trace the closed path first, then decide what each meter measures.
Potential difference pays for each coulomb's journey
Potential difference tells you the energy transferred per unit charge; resistance tells you how strongly a component limits the current.
- For a fixed potential difference, a greater resistance produces a smaller current. Link the three quantities with V = IR.
- One volt means one joule transferred per coulomb. This connects circuit measurements to energy transfers.
- Resistance can be found from R = V/I, using the potential difference across and current through the same component.
The punchline: Measure across for potential difference, through for current, then calculate resistance.
The route decides the rule
Series gives charge one continuous route; parallel adds junctions and branches. That topology decides what stays the same, splits, adds or is shared.
- Series loopCurrent is the same throughout, potential difference is shared and component resistances add.
- Parallel branchesPotential difference is the same across each branch, while current divides at a junction and recombines.
- Mixed circuitApply the rules section by section: identify each series stretch and each pair of common junctions.
Series shares; parallel branches
A series circuit offers one route, while a parallel circuit gives charge a junction and a choice of routes.
- In series, current is the same everywhere, supply potential difference is shared, and resistances add: R_total = R₁ + R₂.
- In parallel, each branch has the same potential difference and the total current equals the sum of the branch currents.
- Adding a parallel branch lowers total resistance below that of the smallest branch because it provides another route for charge.
- The reciprocal-resistance equation for parallel resistors is not required; use the circuit rules and V = IR.
The punchline: Mark loops and junctions on the diagram before applying any circuit rule.
Components leave fingerprints on an I–V graph
Change the potential difference, measure the current, and the graph reveals whether resistance stays still or changes with conditions.
- At constant temperature, an ohmic resistor gives a straight line through the origin: current is directly proportional to potential difference.
- A filament lamp heats as current rises, so its resistance increases and the I–V graph curves with a decreasing gradient.
- A diode conducts readily in one direction but has very high resistance in reverse, so current is almost zero there.
- A thermistor's resistance falls as temperature rises, suiting thermostats; an LDR's falls as light intensity rises, suiting automatic lights.
The punchline: Describe the graph, identify the component mechanism, then state how current changes.
Make the model move
Interactive checkpoint
Touch the science. Change a state, build a route or test a relationship.
Trace before calculating
Match each circuit clue to its rule
Pair every circuit feature with the relationship it guarantees. Potential differences in series are shared, but not necessarily equally.
Circuit rules come from charge conservation and how components are connected. Once the loops and junctions are clear, V = IR supplies the numerical link.
Change the component
Predict current from voltage, resistance and conditions
Select a component. For the same applied potential difference, ask whether its resistance stays constant, rises or falls—and predict the current.
Current is directly proportional to potential difference, producing a straight I–V line through the origin.
1 of 5 states explored
V = IR still links the quantities at a chosen operating point. What changes between components is whether resistance remains constant as voltage, temperature or light changes.
The earth wire waits for trouble
UK mains is about 230 V ac at 50 Hz. Its wiring is designed so a fault current takes a safer path than a person.
- Live is brown and carries an alternating potential difference; the potential difference between live and earth is about 230 V.
- Neutral is blue and near 0 V; earth is green-and-yellow and normally carries no current.
- Live supplies the alternating potential difference, neutral completes the circuit, and earth carries fault current if a metal case becomes live.
- A live wire can remain dangerous when a switch is open. Any live-to-earth connection can create a large, potentially fatal current.
The punchline: Never describe the earth wire as part of the normal operating circuit.
Power tells you how fast the bill is ticking
An appliance's power rating is its energy-transfer rate, not the total energy it will transfer during its lifetime.
- Use P = VI or P = I²R. One watt is one joule transferred each second.
- Use E = Pt when power and time are known, or E = QV when charge moves through a potential difference.
- Appliances transfer energy from a battery or ac supply to useful stores, such as the kinetic store of a motor or thermal store of a heater.
- A higher-power appliance transfers energy faster; total energy also depends on how long it operates.
The punchline: Power is a rate; multiply by time to find an energy transfer.
Why the Grid sends power at high potential difference
The efficiency argument is a causal chain. Each arrow earns more than the vague claim that ‘high voltage wastes less energy’. Rules assume the same transmitted power.
- Step up VA transformer raises the transmission potential difference.
- Current fallsFor the same transferred power, a larger potential difference allows a smaller current.
- Cable heating fallsHeating power depends on I²R, so reducing current sharply reduces unwanted transfer to thermal stores.
- Step down VTransformers lower the potential difference before electricity reaches consumers.
Send the power high, keep the current low
The National Grid raises potential difference for transmission because the same power can then travel with less current and far less cable heating.
- Step-up transformers raise potential difference before transmission; step-down transformers reduce it before electricity reaches consumers.
- For a given power, a higher potential difference means a lower current. Since cable heating depends on I²R, lower current sharply reduces wasted energy.
- HT only, both routes: for an ideal transformer use VₚIₚ = VₛIₛ. Detailed transformer structure belongs elsewhere in the specification.
- Separate Physics HT only: transformer construction and the turns-ratio equation are covered in Magnetism and Electromagnetism.
The punchline: Explain Grid efficiency as high potential difference → low current → less I²R heating.
Separate Physics only: charge without a circuit
Rub two insulators and electrons can cross the boundary, leaving equal and opposite charges that act through electric fields.
- The material gaining electrons becomes negative; the one losing electrons becomes positive. Protons do not transfer between the materials.
- Like charges repel and unlike charges attract. These electrostatic forces act without contact and become stronger at smaller separations.
- A charge creates an electric field. Around a sphere, lines are radial: away from positive and towards negative, packed most closely where the field is strongest.
- Earthing lets excess electrons move to or from Earth; a spark is a sudden discharge through air when the electric field becomes strong enough.
The punchline: Use electron transfer to explain the charge, then use the field to explain the force or spark.
Words worth knowing
Key definitions
- electric current
- The rate of flow of electrical charge, measured in amperes.
- charge flow
- The quantity of electric charge that passes a point, measured in coulombs.
- potential difference
- The energy transferred per unit charge between two points, measured in volts.
- resistance
- A measure of how strongly a component opposes current, measured in ohms.
- ohmic conductor
- A conductor whose current is directly proportional to potential difference when its temperature is constant.
- direct potential difference
- A potential difference that maintains one polarity, producing current in one direction.
- alternating potential difference
- A potential difference that repeatedly reverses polarity, making current repeatedly reverse direction.
- power
- The rate at which energy is transferred or work is done, measured in watts.
- National Grid
- The network of cables and transformers that transfers electrical power from power stations to consumers.
- thermistor
- A resistor whose resistance changes with temperature; for the expected GCSE type, resistance falls as temperature rises.
- light-dependent resistor
- A resistor whose resistance decreases as light intensity increases.
- electric field
- A region where a charged object experiences a force.
Calculate with confidence
Equations
Charge flow
Q = I × t
The charge passing a point equals current multiplied by time.
| Symbol | Meaning | Unit |
|---|---|---|
| Q | charge flow | C |
| I | current | A |
| t | time | s |
Exam tip: Convert minutes to seconds before substituting.
Potential difference, current and resistance
V = I × R
The potential difference across a component equals current through it multiplied by its resistance.
| Symbol | Meaning | Unit |
|---|---|---|
| V | potential difference | V |
| I | current | A |
| R | resistance | Ω |
Exam tip: Use measurements from the same component: current through it and potential difference across it.
Total resistance in series
R_total = R₁ + R₂ + …
The total resistance of components in series is the sum of their individual resistances.
| Symbol | Meaning | Unit |
|---|---|---|
| R_total | total series resistance | Ω |
| R₁, R₂ | individual resistances | Ω |
Exam tip: Do not use this addition rule for components in parallel.
Electrical power from potential difference
P = V × I
Electrical power equals potential difference multiplied by current.
| Symbol | Meaning | Unit |
|---|---|---|
| P | power | W |
| V | potential difference | V |
| I | current | A |
Exam tip: Check whether a current given in milliamperes must be divided by 1,000.
Electrical power from current and resistance
P = I² × R
Electrical power equals current squared multiplied by resistance.
| Symbol | Meaning | Unit |
|---|---|---|
| P | power | W |
| I | current | A |
| R | resistance | Ω |
Exam tip: Square the current, not the resistance.
Energy from power and time
E = P × t
Energy transferred equals power multiplied by operating time.
| Symbol | Meaning | Unit |
|---|---|---|
| E | energy transferred | J |
| P | power | W |
| t | time | s |
Exam tip: Watts multiplied by seconds give joules.
Energy from charge and potential difference
E = Q × V
Energy transferred equals charge flow multiplied by potential difference.
| Symbol | Meaning | Unit |
|---|---|---|
| E | energy transferred | J |
| Q | charge flow | C |
| V | potential difference | V |
Exam tip: One volt is one joule per coulomb, so C × V gives J.
HT only: ideal transformer power
Vₚ × Iₚ = Vₛ × Iₛ
For an ideal transformer, electrical power entering the primary equals electrical power leaving the secondary.
| Symbol | Meaning | Unit |
|---|---|---|
| Vₚ, Vₛ | primary and secondary potential differences | V |
| Iₚ, Iₛ | primary and secondary currents | A |
Exam tip: This equation is required at Higher tier on both Combined Science and separate Physics routes.
Follow it step by step
Processes to remember
How to read an unfamiliar circuit
- Trace each complete loop from one terminal of the supply back to the other.
- Mark junctions and identify which components are in series and which lie on parallel branches.
- Apply the current rules: unchanged around one loop, split at a junction, and recombined afterwards.
- Apply the potential-difference rules: shared in series and equal across parallel branches.
- Use R_total for series sections and V = IR only after the circuit relationships are clear.
Exam tip: A component drawn beside another is not necessarily in series; follow the conducting path.
How to interpret an I–V graph
- Check the axes: current should be on the vertical axis and potential difference on the horizontal axis for this practical.
- Decide whether the graph is straight or curved and whether it passes through the origin.
- Check positive and negative regions to see whether current behaves symmetrically or flows mainly one way.
- Calculate resistance at a chosen point with R = V/I; do not call the graph gradient resistance.
- Link the shape to constant temperature, heating or one-way conduction.
Exam tip: For an I-against-V graph, gradient is I/V, which is the reciprocal of resistance for an ohmic conductor.
How to explain the National Grid
- State that a step-up transformer increases transmission potential difference.
- For the same transferred power, explain that the transmission current therefore decreases.
- Use P_loss = I²R qualitatively: a smaller current produces much less heating in the cables.
- State that step-down transformers reduce potential difference before electricity reaches consumers.
Exam tip: Do not say high potential difference is efficient by itself; include the lower-current and reduced-heating chain.
See the thinking
Worked example
Worked example: a heater's electrical trail
A heater is connected to a 230 V supply and draws a current of 5.0 A for 120 s. Calculate its resistance, power, charge flow and energy transferred.
- Rearrange V = IR: R = V/I = 230/5.0 = 46 Ω.
- Calculate power: P = VI = 230 × 5.0 = 1,150 W.
- Calculate charge flow: Q = It = 5.0 × 120 = 600 C.
- Calculate energy: E = Pt = 1,150 × 120 = 138,000 J.
- Check with E = QV: 600 × 230 = 138,000 J = 138 kJ.
Answer: The resistance is 46 Ω, power is 1,150 W, charge flow is 600 C and energy transferred is 138 kJ.
Each result describes the same two-minute event from a different angle. The matching energy answers from E = Pt and E = QV provide a useful arithmetic check.
Protect the marks
Common mistakes
Watch out: Saying current is used up by a component.
Do this instead: Charge is conserved; in one loop the current is the same before and after the component.
Watch out: Connecting an ammeter in parallel or a voltmeter in series.
Do this instead: An ammeter measures current through a component in series; a voltmeter measures across it in parallel.
Watch out: Adding parallel resistances as though they were in series.
Do this instead: Adding a parallel branch lowers total resistance; the simple sum applies only in series.
Watch out: Calling any curved I–V graph an ohmic conductor.
Do this instead: An ohmic conductor at constant temperature gives a straight line through the origin.
Watch out: Saying a thermistor or LDR produces current by itself.
Do this instead: It changes resistance with temperature or light; a source still provides the potential difference.
Watch out: Describing the neutral or earth wire as completely safe to touch.
Do this instead: Never assume a wire is safe; mains circuits can be faulty or incorrectly connected.
Watch out: Explaining Grid efficiency only by saying the potential difference is high.
Do this instead: Link high potential difference to lower current and therefore less I²R heating in cables.
Watch out: Saying positive charge moves from one insulator to another when rubbed.
Do this instead: Separate Physics only: electrons transfer; gaining electrons makes an object negative.
Plan it like the exam
Required practicals
Investigate factors affecting resistance
Combined Science and separate Physics
Aim: Investigate wire length at constant temperature and compare resistor combinations in series and parallel.
Method
- Build a low-voltage circuit with the test wire and ammeter in series, and a voltmeter in parallel across the measured wire length.
- Include a variable resistor to keep the current small and protect the wire from appreciable heating.
- Change the wire length across a suitable range, measure V and I for each length, and calculate R = V/I.
- Switch off between readings where needed so the wire remains at approximately constant temperature.
- Repeat readings, calculate means and plot resistance against wire length.
- Build series and parallel resistor combinations, measure total V and I, and calculate each total resistance for comparison.
Variables
- Independent
- length of test wire, or resistor arrangement in the combinations investigation
- Dependent
- resistance calculated from the measured potential difference and current
- Controls
- wire material and cross-sectional area
- wire temperature
- supply setting
- meters and connection points
Analysis: A straight resistance–length graph through the origin supports direct proportionality for a uniform wire at constant temperature. Compare series and parallel totals using circuit rules, and identify anomalies.
Safety
- Use only the teacher-approved low-voltage laboratory supply.
- Keep current small and switch off between readings because the wire can become hot.
- Switch off the supply before changing connections and keep leads tidy.
Improvements
- Use a fine current control and monitor temperature so heating does not change resistance.
- Measure from fixed contact points and read the ruler at eye level to reduce length uncertainty.
- Repeat each reading and use a best-fit line rather than joining points dot to dot.
Investigate I–V characteristics
Combined Science and separate Physics
Aim: Measure and compare the current–potential-difference characteristics of a resistor, filament lamp and diode.
Method
- Build a low-voltage circuit with the test component, ammeter and variable resistor in series, and a voltmeter across the component.
- Adjust the variable resistor to obtain a sensible range of positive V and I readings at regular intervals.
- For the resistor, keep current low or switch off between readings so its temperature remains approximately constant.
- Reverse the supply or component and repeat to obtain negative readings; use a suitable meter range for the diode.
- Repeat for the filament lamp and diode, then plot I vertically against V horizontally for each component.
- Compare shapes and calculate R = V/I at selected points where appropriate.
Variables
- Independent
- potential difference across the test component
- Dependent
- current through the test component
- Controls
- component being tested for each data set
- resistor temperature where constant temperature is required
- meter ranges and circuit arrangement
Analysis: Expect a straight line through the origin for the constant-temperature resistor, a curved filament-lamp graph, and appreciable diode current in one direction only. Investigate anomalies rather than smoothing them away.
Safety
- Use only the teacher-approved low-voltage laboratory supply.
- Limit current to protect the components and avoid touching a hot filament lamp.
- Switch off before reversing connections.
Improvements
- Take more readings where the graph changes shape rapidly.
- Use digital sensors or data logging to collect paired V and I values consistently.
- Repeat readings and keep the resistor at constant temperature.
Try it before you move on
Quick check
Say your answer first, then open the card to check it.
A current of 0.40 A flows for 30 s. How much charge passes?
Answer: 12 C
Q = It = 0.40 × 30 = 12 C.
Two resistors of 3 Ω and 7 Ω are connected in series. What is their total resistance?
Answer: 10 Ω
Series resistances add: R_total = 3 + 7 = 10 Ω.
A 0.20 A current enters a junction and 0.08 A leaves along one branch. What leaves along the other branch?
Answer: 0.12 A
Current is conserved at a junction: 0.20 − 0.08 = 0.12 A.
Why does a filament lamp's resistance rise as its current increases?
Answer: The filament becomes hotter, so its resistance increases.
The changing temperature makes the lamp non-ohmic and curves its I–V graph.
Why does the National Grid transmit power at a high potential difference?
Answer: It allows the same power to be transferred with a lower current, reducing cable heating.
Cable heating depends on I²R, so reducing current makes the energy loss much smaller.
Separate Physics only: an insulator gains electrons when rubbed. What charge does it gain?
Answer: Negative charge
Electrons carry negative charge, so gaining electrons makes the object negatively charged.
Good questions, clear answers
Frequently asked questions
What is the difference between current and potential difference?
Current is the rate at which charge flows through a point. Potential difference is the energy transferred per unit charge between two points. They are related, but they are not two names for the same thing.
Why is current the same around a series circuit?
There is only one route and charge is conserved. In steady operation, charge cannot continuously accumulate in a component, so the same charge per second passes every point in the loop.
Why does adding a parallel resistor lower total resistance?
The extra branch gives charge another conducting route. For the same potential difference, the total current increases, so R_total = V/I_total decreases.
What is the difference between dc and ac?
Direct potential difference keeps one polarity and drives current in one direction. Alternating potential difference repeatedly reverses polarity, so the current also reverses direction.
Is the neutral wire always safe?
No. It is normally close to 0 V, but faults or incorrect wiring can make conductors dangerous. Never touch mains wiring or rely on wire colour alone; mains work belongs to trained adults.
Does an electric field show the path a charge must follow?
No. A field line shows the direction of force on a positive test charge at each point. An actual charge's motion also depends on its sign, mass, initial motion and other forces.
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