Ohm's law and power worksheet: electricity at home and in the car
Twelve problems in three levels: circuits to apply V = I · R, real appliance labels to find current and power (a 12 V, 55 W car headlight bulb, a coffee maker, a phone charger) and problems from home: how much a power strip can handle, whether a resistor burns out and how much it costs to run a TV.
The answers are scrambled at the bottom, with hints for the most common mistakes, like mixing up mA and A.
What's included
- 2 student pages: 12 problems in 3 levels, with circuits, labels and a grid to work on.
- Scrambled answers at the bottom, with hints for the most common mistakes.
- 1 teacher page: answer key, common mistakes and an exit question.
Check your answers
The problems
7 cm given? unknownformula
V = I · R
I = VR R = VI
I = VR R = VI
P = V · I
I = PV V = PI
I = PV V = PI
Also:
P = I² · R P = V²R
P = I² · R P = V²R
Cover the letter you are looking for in the triangle: what is left is the formula. Units: V (volt), A (ampere), Ω (ohm), W (watt).
●○○Level 1 · Circuits
1
What current flows?
2
What does the voltmeter read?
3
What is the resistance?
4
A 2.2 kΩ resistor is connected to 11 V. What current flows, in mA?
●●○Level 2 · Labels
Read the label and choose the values you need.
5
How many amps does it draw?
CAR HEADLIGHT BULB
12 V · 55 W
6
What current does it draw?
COFFEE MAKER
120 V ~ 60 Hz · 900 W
7
How much power does it deliver at the output?
PHONE CHARGER
Input: 100–240 V ~ 0.5 A
Output: 5 V ⎓ 3 A
8
What current does it draw?
MICROWAVE OVEN
120 V ~ 60 Hz · 1200 W
●●●Level 3 · Problems
9
A 1,500 W space heater runs on 120 V. What current does it draw? What is its resistance?
10
A power strip is rated for 15 A at 120 V. What is the most power it can handle? Can you plug in a 1,500 W kettle and a 1,000 W toaster at the same time?
11
A 100 Ω resistor is rated for 0.25 W at most. Can you connect it to 9 V?
12
A 100 W TV is on 5 hours a day. How many kWh does it use in 30 days? If electricity costs $0.16 per kWh, how much does that cost?
Answers (scrambled): 15 W · 48 Ω · 7.5 A · 0.81 W (no) · 5 mA · 10 A · 4.58 A · 15 kWh ($2.40) · 0.3 A · 1,800 W (no) · 9.4 V · 12.5 A and 9.6 Ω
Can't find your answer? In #4, if you got 0.005: that is in amps; multiply by 1,000 for mA. In #5, if you got 660: you multiplied instead of dividing. In #11, if you got 0.09: you found the current, not the power.
Can't find your answer? In #4, if you got 0.005: that is in amps; multiply by 1,000 for mA. In #5, if you got 660: you multiplied instead of dividing. In #11, if you got 0.09: you found the current, not the power.
Safety: never experiment with wall outlets (120 V or 230 V). The circuits on this sheet use batteries or low-voltage power supplies.
For the teacher: answer key and common mistakes
Answer key
- I = 4.5 ÷ 15 = 0.3 A
- V = 0.02 × 470 = 9.4 V
- R = 24 ÷ 0.5 = 48 Ω
- I = 11 ÷ 2,200 = 0.005 A = 5 mA
- I = 55 ÷ 12 ≈ 4.58 A
- I = 900 ÷ 120 = 7.5 A
- P = 5 × 3 = 15 W
- I = 1,200 ÷ 120 = 10 A
- I = 1,500 ÷ 120 = 12.5 A; R = 120 ÷ 12.5 = 9.6 Ω (or V² ÷ P = 14,400 ÷ 1,500)
- P = 15 × 120 = 1,800 W; 1,500 + 1,000 = 2,500 W > 1,800 W: no
- P = V² ÷ R = 81 ÷ 100 = 0.81 W > 0.25 W: no (5 V at most)
- 100 W × 5 h × 30 = 15,000 Wh = 15 kWh; 15 × $0.16 = $2.40
Mistakes to watch for
- Using the triangle from memory and solving the wrong way (multiplying instead of dividing).
- Mixing up mA and A, kΩ and Ω, kW and kWh.
- Thinking that a more powerful appliance has more resistance (at the same voltage, it's the other way around).