AQA GCSE (9-1) Physics: Electricity

Key Takeaways for GCSE Electricity

1. Current, Potential Difference & Resistance

• Current (II): Flow of charge (electrons in metals). Measured in amps (A).
  Q=I×t(Charge = Current × Time)Q=I×t(Charge = Current × Time)
  Example: A current of 0.1A for 30 mins:
  Q=0.1×(30×60)=180 CQ=0.1×(30×60)=180C
• Potential Difference (Voltage, VV): Energy transferred per charge. Measured in volts (V).
• Resistance (RR): Opposes current flow. Measured in ohms (Ω).
  V=I×R(Ohm’s Law)V=I×R(Ohm’s Law)
  Example: A 12V battery across a 240Ω resistor:
  I=12240=0.05 AI=24012​=0.05A

Key Rules:

• Series Circuits:
  • Same current everywhere.
  • Total VV splits across components.
  • Total R=R1+R2+…R=R1​+R2​+…
• Parallel Circuits:
  • Same VV across all branches.
  • Total II splits.
  • Total RR is less than smallest individual resistor.

Tips:

• Use the triangle method to rearrange V=IRV=IR.
• Ammeters are in series; voltmeters are in parallel.

2. Components & Circuit Symbols

• Resistors: Fixed, variable, LDRs, thermistors.
• Diodes: Allow current in one direction only (forward bias ≈ 0.7V).
• LDRs: Resistance ↓ in bright light. Used in security lights.
• Thermistors: Resistance ↓ as temperature ↑. Used in thermostats.

I-V Characteristics:

• Ohmic Conductor (e.g., fixed resistor): Straight-line graph through origin.
• Filament Lamp: Curve (resistance ↑ with temperature).
• Diode: Current only flows above 0.7V in forward bias.

Example:

• For a filament lamp at 1V: R=10.15=6.67 ΩR=0.151​=6.67Ω
• At 3V: R=30.25=12 ΩR=0.253​=12Ω (resistance increases!).

3. Domestic Electricity & Safety

• UK Mains: 230V AC, 50Hz.
• Three Wires:
  • Live (Brown): 230V. Dangerous if touched.
  • Neutral (Blue): Completes circuit.
  • Earth (Green/Yellow): Safety wire to prevent shocks.

Safety Features:

• Fuses: Melt if current exceeds rating (e.g., 3A, 13A).
• Circuit Breakers: Automatically switch off for faults.
• Earthing: Redirects fault current to Earth.

Example: A 1800W heater on 230V:
I=1800230≈7.8 A(Use 13A fuse)I=2301800​≈7.8A(Use 13A fuse)

4. Energy & Power

• Energy Transfer:
  E=V×QorE=P×tE=V×QorE=P×t
  Example: A 6V battery transferring 250C:
  E=6×250=1500 JE=6×250=1500J
• Power:
  P=V×IorP=I2×RP=V×IorP=I2×R
  Example: A 30Ω resistor with 4.7A:
  P=4.72×30≈660 WP=4.72×30≈660W

National Grid:

• High voltage (400,000V) reduces energy loss (P=I2RP=I2R).
• Transformers step voltage up/down.

5. Static Electricity

• Charging by Friction: Electrons transfer (e.g., balloon rubbed on hair becomes negative).
• Electric Fields:
  • Direction: Away from +ve, towards -ve.
  • Strength ↓ with distance.

Applications:

• Lightning: Discharge between cloud and Earth.
• Photocopiers: Use static to attract toner.

Example: Lightning with 5C charge in 0.002s:
I=50.002=2500 AI=0.0025​=2500A

6. Practical Investigations

• Resistance of a Wire:
  • Resistance ∝ length (directly proportional).
  • Resistance ∝ 1/cross-sectional area.
• Errors: Width of crocodile clips → random error in length measurement.

Graphs:

• For resistance vs. length: Straight line through origin.
• For thermistors/LDRs: Curves showing resistance changes.

Exam Tips

1. Unit Conversions:
   • mA → A: Divide by 1000.
   • Hours → Seconds: Multiply by 3600.
2. Diode Questions: Check direction and threshold voltage.
3. Parallel Resistors: Total RR always < smallest resistor.
4. Safety First: Always mention earthing/fuses in fault scenarios.

Common Mistakes:

• Confusing series and parallel rules.
• Forgetting to convert units (e.g., minutes → seconds).

50 GCSE Electricity Questions

Section A: Basic Concepts & Definitions

1. Define electric current and state its unit.
2. What is potential difference? Give its unit.
3. Explain the difference between conductors and insulators. Provide three examples of each.
4. State the equation linking charge, current, and time. Rearrange it to solve for time.
5. What is the function of a resistor in a circuit?

Section B: Circuit Symbols & Components

6. Identify the circuit symbol for a light-dependent resistor (LDR).
7. Draw the circuit symbol for a diode. How does it differ from an LED?
8. Label the components in a circuit diagram containing a cell, ammeter, variable resistor, and lamp.
9. What is the purpose of a fuse in a plug?
10. Explain why voltmeters are connected in parallel with components.

Section C: Series & Parallel Circuits

11. Are household lights wired in series or parallel? Justify your answer.
12. Calculate the total resistance of two 10 Ω resistors connected in series.
13. Two resistors (5 Ω and 15 Ω) are connected in parallel. Calculate the total resistance.
14. In a series circuit with a 12 V battery and two lamps, the potential difference across one lamp is 4 V. What is the p.d. across the second lamp?
15. Explain why adding more lamps in parallel decreases the total resistance of the circuit.

Section D: Ohm’s Law & Calculations

16. A resistor has a potential difference of 6 V and a current of 0.5 A. Calculate its resistance.
17. A 240 Ω heater operates at 230 V. Calculate the current it draws.
18. Complete the table:

19. A 12 V battery delivers 300 C of charge. Calculate the energy transferred.
20. A 60 W lamp is left on for 2 hours. Calculate the energy consumed in joules.

Section E: I–V Characteristics

21. Sketch the I–V graph for an ohmic conductor and label the axes.
22. Why does the resistance of a filament lamp increase as the current rises?
23. A diode allows current to flow only in one direction. Explain how this is shown on its I–V graph.
24. The resistance of a thermistor decreases as temperature increases. Give one practical use for this component.
25. An LDR has a resistance of 200 Ω in bright light and 10 kΩ in darkness. Explain how it could be used in a security light circuit.

Section F: Domestic Electricity & Safety

26. Explain the difference between AC and DC. Which is used in UK mains supply?
27. State the colour coding for live, neutral, and earth wires in a UK plug.
28. Why are appliances with metal casings earthed?
29. A toaster draws 4 A from a 230 V supply. Calculate its power rating.
30. A fuse rated at 3 A is used in a 230 V lamp. Explain why this is unsafe if the lamp’s power is 100 W.

Section G: National Grid & Transformers

31. Why is electricity transmitted at high voltages in the National Grid?
32. A power station generates 25,000 V. A step-up transformer increases this to 400,000 V. If the initial current is 1000 A, calculate the new current.
33. Calculate the power loss in a 200 Ω transmission cable carrying a current of 50 A.
34. Explain how step-down transformers make electricity safe for household use.

Section H: Static Electricity

35. Explain how a nylon comb becomes negatively charged when rubbed with a cloth.
36. Two charged balloons repel each other. What does this indicate about their charges?
37. Describe the energy transfers during a lightning strike.
38. Sketch the electric field lines around a negatively charged sphere.
39. Why does a person receive a shock after walking on a carpet and touching a metal door handle?

Section I: Experimental Analysis

40. In an experiment, a wire’s resistance is measured at different lengths. The graph of resistance vs. length is a straight line through the origin. What conclusion can be drawn?
41. A student investigates how the resistance of a thermistor changes with temperature. Identify the independent and dependent variables.
42. A voltmeter connected to a thermistor circuit is calibrated to measure temperature. Explain how extrapolation is used in this context.
43. When testing a diode, the current is zero for reverse p.d. Why?

Section J: Problem-Solving & Application

44. A 9 V battery is connected to a 20 Ω resistor and a lamp in series. The current is 0.3 A. Calculate the lamp’s resistance.
45. Two resistors (6 Ω and 12 Ω) are connected in parallel to a 12 V battery. Calculate the total current.
46. A 230 V heater transfers 690 kJ of energy. How long was it switched on?
47. A lightning flash transfers 500 C of charge in 0.002 s. Calculate the average current.
48. A 115 V, 60 Hz supply powers a lamp in the USA. Compare this to the UK’s 230 V, 50 Hz supply.
49. A circuit contains a 750 Ω resistor and a thermistor in series. At 20°C, the thermistor’s resistance is 250 Ω. Calculate the total current.
50. A student’s hair stands up when touching a Van de Graaff generator. Explain this using electric fields.

Answers

1. Electric current is the flow of charge, measured in amperes (A).
2. Potential difference is the work done per unit charge, measured in volts (V).
3. Conductors allow charge to flow (e.g., copper, aluminum, iron). Insulators resist charge flow (e.g., plastic, rubber, glass).
4. Equation: Q=I×tQ=I×t; rearranged for time: t=QIt=IQ​.
5. A resistor limits current or controls voltage in a circuit.
6. LDR symbol:  (curved arrow inside a resistor).
7. Diode symbol: . An LED emits light and has arrows pointing outward.
8. Circuit: Cell → Ammeter (in series) → Variable resistor → Lamp.
9. A fuse melts to break the circuit if current exceeds a safe value.
10. Voltmeters measure p.d. across components without altering the current.
11. Parallel. If one lamp fails, others stay on.
12. Total resistance in series: Rtotal=10+10=20 ΩRtotal​=10+10=20Ω.
13. Total resistance in parallel: 1Rtotal=15+115=415Rtotal​1​=51​+151​=154​ → Rtotal=3.75 ΩRtotal​=3.75Ω.
14. V2=12−4=8 VV2​=12−4=8V.
15. More paths for current reduce total resistance.
16. R=VI=60.5=12 ΩR=IV​=0.56​=12Ω.
17. I=VR=230240≈0.96 AI=RV​=240230​≈0.96A.
18. Resistor: R=90.3=30 ΩR=0.39​=30Ω; Lamp: V=0.2×60=12 VV=0.2×60=12V; LED: I=250=0.04 AI=502​=0.04A.
19. E=V×Q=12×300=3600 JE=V×Q=12×300=3600J.
20. E=P×t=60×(2×3600)=432,000 JE=P×t=60×(2×3600)=432,000J.
21. Ohmic conductor: Straight line through origin.
22. Filament heats up, increasing resistance (atoms vibrate more).
23. Diode graph: Current only flows above ~0.7 V forward bias.
24. Thermistor: Fire alarms, thermostats.
25. LDR circuit: High resistance in darkness → switches light on.
26. AC alternates direction (UK: 230 V, 50 Hz); DC flows one way (batteries).
27. Live (brown), neutral (blue), earth (green/yellow).
28. Earthing prevents electric shock by diverting fault current.
29. P=V×I=230×4=920 WP=V×I=230×4=920W.
30. Current: I=100230≈0.43 AI=230100​≈0.43A. A 3 A fuse won’t blow → unsafe.
31. High voltage reduces current, minimizing power loss (P=I2RP=I2R).
32. V1V2=I2I1V2​V1​​=I1​I2​​ → I2=25,000×1000400,000=62.5 AI2​=400,00025,000×1000​=62.5A.
33. P=I2R=502×200=500,000 WP=I2R=502×200=500,000W.
34. Step-down transformers reduce voltage to 230 V for safe use.
35. Electrons transfer from cloth to comb, giving comb a negative charge.
36. Like charges repel (both negative/positive).
37. Energy transfers: Electrostatic → thermal + light + sound.
38. Field lines point toward the negative sphere.
39. Static charge builds up on person, discharging via metal.
40. Resistance ∝ length (directly proportional).
41. Independent: Temperature; Dependent: Resistance.
42. Extrapolation estimates temperatures beyond measured data.
43. Diode has high resistance in reverse bias.
44. Total resistance: Rtotal=90.3=30 ΩRtotal​=0.39​=30Ω. Lamp: 30−20=10 Ω30−20=10Ω.
45. Current through 6 Ω: I1=126=2 AI1​=612​=2A; 12 Ω: I2=1212=1 AI2​=1212​=1A. Total: 2+1=3 A2+1=3A.
46. t=EP=690,000230×It=PE​=230×I690,000​ (need current).
47. I=Qt=5000.002=250,000 AI=tQ​=0.002500​=250,000A.
48. USA: Lower voltage, higher frequency → devices may underperform.
49. I=12750+250=0.012 AI=750+25012​=0.012A.
50. Like charges on hairs repel, causing them to stand.