10 GCSE Physics questions about the smartphone, from Foundation to Higher, each with a full worked solution. Your phone stores energy in a lithium-ion battery in its chemical energy store. It sends and receives calls and data using electromagnetic waves (microwaves and radio waves), which travel at the speed of light.
Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Topics: Electricity · Waves
📌 KEY DEFINITIONS – learn them word for word
Current: the rate of flow of electrical charge.
Potential difference: the energy transferred per unit charge passed between two points.
Frequency: the number of waves passing a point each second.
Electromagnetic waves: transverse waves that transfer energy from the source of the waves to an absorber. All travel at the same speed in a vacuum.
Equations you’ll use
Q = I t | E = Q V | P = V I | E = P t | v = f λ | efficiency = useful output ÷ total input
| Quantity | Symbol | Unit |
|---|---|---|
| charge | Q | coulombs (C) |
| current | I | amps (A) |
| time | t | seconds (s) |
| energy | E | joules (J) |
| potential difference | V | volts (V) |
| power | P | watts (W) |
| wave speed | v | metres per second (m/s) |
| frequency | f | hertz (Hz) |
| wavelength | λ | metres (m) |
Use FIFA for every answer
- F – Formula: write the equation as it appears on the sheet.
- I – Insert: put in the numbers (convert to standard units first).
- F – Fix: rearrange to make the unknown the subject.
- A – Answer: calculate and always give the unit.
Questions
Q1 (F) A charger supplies a current of 2.0 A to a phone for 30 minutes. Calculate the charge that flows.
Show solution
Convert first: 30 minutes = 1800 s
F: Q = I t
I: Q = 2.0 × 1800
F: Q is already the subject
A: 3600 C
Q2 (F) The charger has an output of 5.0 V and 2.0 A. Calculate its power output.
Show solution
F: P = V I
I: P = 5.0 × 2.0
F: P is already the subject
A: 10 W
Q3 (F) Calculate the energy transferred when 3600 C of charge passes through the 5.0 V charger.
Show solution
F: E = Q V
I: E = 3600 × 5.0
F: E is already the subject
A: 18 000 J
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Q4 (F) State the speed of all electromagnetic waves in a vacuum, and name two types of electromagnetic wave a phone uses.
Show answer
• speed = 3 × 108 m/s (300 000 000 m/s)
• microwaves (mobile signals, Wi-Fi) and radio waves; also accept visible light from the screen or infrared (face recognition)
Q5 (F/H) A phone battery has a potential difference of 3.85 V. When fully charged, it can supply a current of 4.0 A for 1.0 hour. Calculate the energy stored in the battery.
Show solution
Convert first: 1.0 hour = 60 × 60 = 3600 s
Step 1 – charge: Q = I t = 4.0 × 3600 = 14 400 C
Step 2 – energy: E = Q V = 14 400 × 3.85 = 55 440 J
A: about 55 000 J (55 kJ)
Q6 (F/H) 4G mobile signals in the UK can use microwaves with a frequency of 1.8 GHz. Calculate their wavelength. (speed = 3.0 × 108 m/s)
Show solution
Convert first: 1.8 GHz = 1.8 × 109 Hz
F: v = f λ
I: 3.0 × 108 = 1.8 × 109 × λ
F: λ = 3.0 × 108 ÷ 1.8 × 109
A: 0.17 m (2 s.f.)
Q7 (F/H) The screen uses 0.80 W. Calculate the energy it transfers in 2 hours.
Show solution
Convert first: 2 hours = 7200 s
F: E = P t
I: E = 0.80 × 7200
F: E is already the subject
A: 5760 J
Q8 (H) The phone uses an average power of 1.5 W. Use your answer to Q5 to estimate how many hours the full battery lasts.
Show solution
F: E = P t
I: 55 440 = 1.5 × t
F: t = 55 440 ÷ 1.5 = 36 960 s
A: 36 960 ÷ 3600 = about 10 hours
Q9 (H) A fast charger supplies 9.0 V and 2.0 A. Charging is 85% efficient. Calculate the time to put 55 440 J into the empty battery. Give your answer in minutes.
Show solution
Step 1 – input power: P = V I = 9.0 × 2.0 = 18 W
Step 2 – useful power: 0.85 × 18 = 15.3 W
Step 3 – time: t = E ÷ P = 55 440 ÷ 15.3 = 3624 s
A: 3624 ÷ 60 = about 60 minutes
Q10 (H) A phone gets warm when it charges. Explain why, and why this is a problem. [4 marks]
Show answer
Marking points:
• there is a current in the phone’s circuits and battery
• the circuits have resistance, so work is done and energy is transferred to the thermal store (heating effect)
• this energy is dissipated to the surroundings and is wasted
• so charging is less than 100% efficient (it takes longer and costs more), and too much heating can damage the battery
⚠️ Where students lose marks
- Leaving time in hours or minutes in Q = I t. Change it to seconds first, so the charge comes out in coulombs.
- Forgetting to convert GHz to Hz (× 109) in v = f λ.
- Writing the speed of light as 3 × 106 m/s. It is 3 × 108 m/s.
- Using total input power instead of useful power in efficiency questions.
More devices: all 40 everyday devices · Electricity · Waves
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