Orientation
Lesson goal: apply circuit concepts to power, energy transfer, safety devices, and household-style circuit reasoning.
This is not a lab manual yet. It is the conceptual and calculation layer that the later lab sheets should attach to.
Core Content
Electrical power is the rate of energy transfer:
$$P = VI$$
$$E = Pt$$
For resistive devices:
$$P = I^2R = \frac{V^2}{R}$$
Safety devices are designed around current, heating, and fault pathways.
| Device or feature | Physics role |
|---|---|
| fuse | melts when current is excessive |
| circuit breaker | opens circuit under fault condition |
| earth wire | provides low-resistance fault path |
| insulation | prevents unwanted current path |
| parallel household wiring | keeps appliances at supply voltage independently |
Concept Check
-
A
60 Wdevice transfers:- A.
60 Jevery second - B.
60 Cevery second - C.
60 Vevery second - D.
60 ohmevery second
Answer: A.
- A.
-
Household appliances are usually connected in parallel so that:
- A. each receives the supply voltage
- B. current is zero
- C. resistance disappears
- D. voltage is used up
Answer: A.
-
Excess current is dangerous because it can cause:
- A. heating
- B. lower energy transfer
- C. lower resistance always
- D. no change
Answer: A.
Applied Practice
A heater is rated at 1200 W and operates for 15 min. Calculate the energy
transferred.
-
Convert time:
$$15\ \text{min}=900\ \text{s}$$
-
Use:
$$E = Pt = 1200\times900$$
-
Result:
$$E = 1.08\times10^6\ \text{J}$$
Final answer: $1.08\ \text{MJ}$.
Deep Practice And Writing
Prompt: explain why a fuse must be placed in series with the appliance and how it reduces risk during excessive current.
Maintenance Loop
Retrieve $P=VI$, $E=Pt$, and one safety-device mechanism.