AC and DC oscilloscope traces
A DC trace stays on one side of the centre line. An AC trace goes above and below the centre line.
DC
AC
S3 Physics – Electricity 3: Power and Concepts
Key links
Start with the Knowledge Organiser, then use the booklet and extra revision files, then try mixed practice.
Knowledge Organiser start here
Quick rules, equations, fuse guidance, AC/DC, oscilloscope traces, and electric fields.
Electricity 3 Booklet practice
Use this for step-by-step examples and revision questions.
Physics Past Paper Finder practise these first
Go to Electrical Charge Carriers and Electrical Power & Energy first.
Block 3 Practice Questions Ms Woodhouse
Extra revision questions across L1–L6 by Ms Woodhouse.
Practice Questions – Answers Ms Woodhouse
Self-check answers and completed field diagrams by Ms Woodhouse.
Completed B3 Booklet Ms Woodhouse
Completed support booklet with worked examples and notes by Ms Woodhouse.
Past Paper Finder: start with Electrical Charge Carriers and Electrical Power & Energy.
Visual quick recap
Use these two reminders before trying the questions.
Electric field direction
Field lines go from positive to negative. A positive particle moves with the arrows. A negative particle moves against them.
Remember: arrows show the direction a positive test charge would move.
Worked examples
Open one example at a time.
Worked example 1 — Q = Itcharge
Question: A current of 2.5 A flows for 6 minutes. Calculate the charge transferred.
Step 1: Convert time to seconds: 6 × 60 = 360 s
Step 2: Use Q = It
Step 3: Q = 2.5 × 360 = 900
Answer: 900 C
Worked example 2 — P = E / tpower
Question: A heater transfers 1800 J of energy in 45 s. Calculate the power.
Step 1: Use P = E / t
Step 2: P = 1800 / 45
Step 3: P = 40
Answer: 40 W
Worked example 3 — P = IVpower
Question: A component has a voltage of 12 V and a current of 0.35 A. Calculate the power.
Step 1: Use P = IV
Step 2: P = 12 × 0.35
Step 3: P = 4.2
Answer: 4.2 W
Worked example 4 — P = I²Rpower
Question: A resistor carries a current of 250 mA and has a resistance of 120 Ω. Calculate the power.
Step 1: Convert current: 250 mA = 0.25 A
Step 2: Use P = I²R
Step 3: P = (0.25)² × 120
Step 4: P = 0.0625 × 120 = 7.5
Answer: 7.5 W
Worked example 5 — P = V² / Rpower
Question: A 100 Ω resistor is connected to a 9.0 V supply. Calculate the power.
Step 1: Use P = V² / R
Step 2: P = 9.0² / 100
Step 3: P = 81 / 100 = 0.81
Answer: 0.81 W
Worked example 6 — Fuse choice using I = P / Vfuses
Question: A kettle is rated at 1840 W and uses UK mains. Calculate the current and choose a fuse.
Step 1: Use V = 230 V
Step 2: Use I = P / V
Step 3: I = 1840 / 230 = 8.0 A
Step 4: 3 A is too small, so choose the next suitable standard fuse.
Answer: Current = 8.0 A, so choose a 13 A fuse.
Worked example 7 — Charged particles in a fieldfields
Question: Two parallel plates are set up with the left plate positive and the right plate negative. Which way does a positive particle move? Which way does a negative particle move?
Step 1: Electric field arrows go from + to −, so the field points left to right.
Step 2: A positive particle moves in the same direction as the arrows.
Step 3: A negative particle moves in the opposite direction to the arrows.
Answer: Positive particle moves right. Negative particle moves left.
Quick practice
Try the questions first, then open the answers.
Set 1: Charge and currentQ = It
1. A current of 3 A flows for 8 s. Calculate the charge transferred.
2. 48 C of charge passes through a lamp in 12 s. Calculate the current.
3. A device carries a current of 0.4 A. How long does it take for 120 C of charge to flow?
4. Explain in one sentence what 1 A means.
Check answers
1. 24 C
2. 4 A
3. 300 s
4. 1 coulomb of charge flows each second.
Set 2: Power and energyP = E / t
1. A 60 W lamp is on for 20 s. Calculate the energy transferred.
2. 3600 J of energy is transferred in 60 s. Calculate the power.
3. A 100 W heater works for 5 minutes. Calculate the energy transferred. Hint: convert the time first.
4. Why does a higher power appliance transfer more energy each second?
Check answers
1. 1200 J
2. 60 W
3. 5 min = 300 s, so E = Pt = 100 × 300 = 30 000 J
4. Because power is the amount of energy transferred each second.
Set 3: Choosing the correct power equationmixed
1. A component has V = 6.0 V and I = 0.5 A. Calculate the power.
2. A resistor has I = 2.0 A and R = 5.0 Ω. Calculate the power.
3. A resistor has V = 12 V and R = 24 Ω. Calculate the power.
4. A resistor has I = 300 mA and R = 40 Ω. Calculate the power. Hint: convert mA to A first.
5. State which equation you would choose if the question gives V and R.
Check answers
1. 3.0 W
2. 20 W
3. P = V² / R = 12² / 24 = 144 / 24 = 6 W
4. 300 mA = 0.3 A, so P = I²R = 0.3² × 40 = 0.09 × 40 = 3.6 W
5. P = V² / R
Set 4: Fuses, AC/DC and oscilloscope tracesconcepts
1. A mains appliance is rated at 460 W. Calculate the current taken from a 230 V supply.
2. Which fuse should be used for the 460 W appliance: 3 A or 13 A?
3. State one difference between AC and DC.
4. A straight horizontal oscilloscope trace shows what type of supply?
5. A wave going above and below the centre line shows what type of supply?
Check answers
1. I = P / V = 460 / 230 = 2 A
2. 3 A fuse
3. AC changes direction; DC goes in one direction only.
4. DC
5. AC
Set 5: Electric fields and charged particlesfields
1. In an electric field, field lines go from which type of charge to which type of charge?
2. What do the arrows on field lines show?
3. Why must field lines never cross?
4. A positive particle is placed in a field. Does it move with the arrows or against them?
5. A negative particle is placed in a field. Does it move with the arrows or against them?
Check answers
1. From positive to negative
2. The direction of force on a positive test charge
3. Because the field cannot point in two directions at the same place
4. With the arrows
5. Against the arrows