Practical Circuit Applications

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.

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

1. A 60 W device transfers:

   • A. 60 J every second
   • B. 60 C every second
   • C. 60 V every second
   • D. 60 ohm every second

   Answer: A.

2. 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.

3. Excess current is dangerous because it can cause:

   • A. heating
   • B. lower energy transfer
   • C. lower resistance always
   • D. no change

   Answer: A.

A heater is rated at 1200 W and operates for 15 min. Calculate the energy transferred.

1. Convert time:

   $$15\ \text{min}=900\ \text{s}$$

2. Use:

   $$E = Pt = 1200\times900$$

3. Result:

   $$E = 1.08\times10^6\ \text{J}$$

Final answer: $1.08\ \text{MJ}$.

Prompt: explain why a fuse must be placed in series with the appliance and how it reduces risk during excessive current.