Question 1

How is power related to voltage and current in a circuit?

Accepted Answer

Power is the rate at which energy is supplied or absorbed, measured in watts. It equals the product of instantaneous voltage and current across an element: P = V × I. A higher voltage or current increases power delivery. For example, a 100-watt light bulb delivers more power than a 60-watt bulb, producing more light.

Question 2

What is the passive sign convention and why does it matter?

Accepted Answer

The passive sign convention determines whether an element absorbs or supplies power based on current direction and voltage polarity. When current enters at the positive terminal and exits the negative terminal, the element absorbs power. Conversely, current entering the negative terminal and exiting the positive terminal means the element supplies power. This convention is essential for correctly calculating power direction.

Question 3

How do you calculate the energy absorbed or supplied by a circuit element?

Accepted Answer

Energy is calculated as the integral of instantaneous power over time: W = ∫P dt, where W is energy in joules and t is time in seconds. For steady-state conditions, energy equals power multiplied by duration. This relationship allows engineers to determine battery runtime or energy consumption in devices like cassette players.

Question 4

Why must total power supplied equal total power absorbed in a circuit?

Accepted Answer

Total power supplied must equal total power absorbed because energy is conserved in electrical circuits. Sources like batteries supply power, while resistors and other elements absorb it. This balance validates the law of energy conservation and ensures circuit analysis accuracy. Any imbalance indicates an error in calculations or circuit configuration.

Question 5

Can power be negative, and what does that indicate?

Accepted Answer

Yes, power can be negative depending on voltage and current values. Negative power indicates that an element is supplying energy rather than absorbing it, which occurs when the passive sign convention is violated. The sign of power reveals the direction of energy flow, helping engineers identify sources and loads within electric circuit elements.

Question 6

How does voltage increase affect energy transfer in a circuit?

Accepted Answer

A rise in voltage results in more work done on the circuit, passing more current and increasing energy transfer. Higher voltage drives more charge through the circuit per unit time, amplifying power delivery. This relationship explains why higher-voltage power supplies deliver more energy to devices, enabling faster charging and greater performance.

Question 7

How can you determine a battery-powered device's runtime using power and energy?

Accepted Answer

Device runtime is calculated using the battery's energy capacity and the device's power consumption. If a cassette player draws steady current, multiply the battery voltage by current to find power, then divide total battery energy by power to get runtime in hours. This calculation helps predict how long a device operates before battery depletion.

Puissance et énergie

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