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All 12 Flashcards — Electrical power and energy
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Question
Define electrical power.
Answer
The **rate at which electrical energy is transferred** (turned into heat, light, motion). Unit: the **watt (W)** = 1 joule per second.
Question
What is the unit of power, and what is 1 watt?
Answer
The **watt (W)**. 1 W = **1 joule of energy every second** (1 J s⁻¹).
Question
Three forms of the electrical power equation?
Answer
$P = IV = I^{2}R = \dfrac{V^{2}}{R}$ — all **given** in the data booklet.
Question
Know I and V — which power form?
Answer
**P = IV** — current × voltage, the simplest form.
Question
Know I and R but not V — which power form?
Answer
**P = I²R** — avoids having to find V first.
Question
Know V and R but not I — which power form?
Answer
**P = V²/R** — avoids having to find I first.
Question
At a FIXED voltage, how does power depend on resistance?
Answer
**P = V²/R**, so P ∝ 1/R — **more** resistance means **less** power (e.g. double R → half the power).
Question
At a FIXED current, how does power depend on resistance?
Answer
**P = I²R**, so P ∝ R — **more** resistance means **more** power.
Question
Formula linking energy, power and time?
Answer
$E = Pt$ — energy = power × time. **Given** in the data booklet.
Question
What is a kilowatt-hour (kWh)?
Answer
The energy a **1 kW** appliance uses in **1 hour** (= 3.6 × 10⁶ J). Energy bills are charged per kWh.
Question
How do you find the cost of running an appliance?
Answer
Energy in **kWh** (power in kW × time in hours), then **× the price per kWh**.
Question
A wire is made twice as long (same metal and thickness) — what happens to its resistance?
Answer
It **doubles** — for a uniform wire R ∝ length (L).
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Topic 2.5 hub
Current and circuits
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