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Feuille d’exercices : loi d’Ohm

Twelve problems on Ohm’s law and the power law: find the current, the resistance, the voltage or the watts from the two you are given. The answer key on the second page shows the working for every one, so a wrong answer tells you which step went wrong. Print it, work it with a pencil, then mark it.

Ohm's law worksheet · sheet #1

NomDateScore/ 12

V = I × RI = V ÷ RR = V ÷ IP = V × IP = I² × RP = V² ÷ R

  1. A 2 Ω heater element is on 16 V. What current does it draw?

  2. A load draws 6 A on 600 V. What is its resistance?

  3. 20 A flows through 8 Ω. What voltage is across it?

  4. A load draws 8 A at 240 V. How many watts?

  5. A 2,500 W load runs on 500 V. What current does it draw?

  6. A heater is rated 375 W at 75 V. What is its hot resistance?

  7. 2 A through 12 Ω. How much power turns into heat?

  8. A 5 Ω heater element is on 12.5 V. What current does it draw?

  9. A load draws 6 A on 24 V. What is its resistance?

  10. 4 A flows through 40 Ω. What voltage is across it?

  11. A load draws 3 A at 18 V. How many watts?

  12. A 3 W load runs on 6 V. What current does it draw?

sparkytools.net/worksheets/ohms-law — free practice sheet. Practice numbers only; nothing here is a code-book value.

Answer key · Ohm's law worksheet · sheet #1

Show the answers and the working
  1. 8 A I = V ÷ R = 16 ÷ 2 = 8 A
  2. 100 Ω R = V ÷ I = 600 ÷ 6 = 100 Ω
  3. 160 V V = I × R = 20 × 8 = 160 V
  4. 1 920 W P = V × I = 240 × 8 = 1 920 W
  5. 5 A I = P ÷ V = 2 500 ÷ 500 = 5 A
  6. 15 Ω R = V² ÷ P = 75² ÷ 375 = 5 625 ÷ 375 = 15 Ω
  7. 48 W P = I² × R = 2² × 12 = 4 × 12 = 48 W
  8. 2,5 A I = V ÷ R = 12,5 ÷ 5 = 2,5 A
  9. 4 Ω R = V ÷ I = 24 ÷ 6 = 4 Ω
  10. 160 V V = I × R = 4 × 40 = 160 V
  11. 54 W P = V × I = 18 × 3 = 54 W
  12. 0,5 A I = P ÷ V = 3 ÷ 6 = 0,5 A

How to use it

  1. Print the sheet (or save it as a PDF from the print dialog). The answer key prints on its own page, so you can hand the sheet out and keep the key.
  2. Work every problem on paper, with the formula strip at the top, before you look at anything.
  3. Mark it from the key. For each miss, read the working line and find the exact step you did differently.
  4. Press New sheet for another twelve; the sheet number is in the link, so the same sheet can be printed again.

Worked example

A heater’s resistance from its rating

  1. A heater is rated 1,500 W at 120 V. You want its resistance.
  2. You have P and V, and you want R. Of the three power forms, P = V² ÷ R has exactly those, so turn it round: R = V² ÷ P.
  3. 120² = 14,400. Then 14,400 ÷ 1,500 = 9.6 Ω.
  4. Check it the long way: I = P ÷ V = 1,500 ÷ 120 = 12.5 A, and R = V ÷ I = 120 ÷ 12.5 = 9.6 Ω. Same answer.

That is the element hot. Measured cold with a meter it reads lower — the wire’s resistance climbs as it heats.

Questions fréquentes

Which formula do I use?

Look at what the problem gives you and what it asks for, then pick the form that has exactly those three letters in it. Given P and V, asked for I: P = V × I has all three, so I = P ÷ V. The power wheel on the formula sheet lays out every one.

How exact should my answers be?

The key rounds to two decimals. If yours differs only in the last digit because you rounded an in-between step, that is fine — carry more digits through the middle next time.

Can I print a different sheet?

Yes. Press New sheet and you get another twelve problems with their own answer key; the sheet number goes into the link, so you can print it again later or send it to someone and they get the same sheet. Sheet #1 is always the one this page opens on.

Does it cost anything or need an account?

No. Every worksheet is free, prints from your own browser (or saves as a PDF from the print dialog), and needs no sign-up.

Voir aussi

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