Flashcards · 8463 · Topic 2

Electricity flashcards

33 cards on Topic 2, subtopics 4.2.1 to 4.2.5. Pick a subtopic or study them all.

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Electric current: definition and equation: The rate of flow of electrical charge. Q = It (Q in C, I in A, t in s).
Equation linking potential difference, current and resistance: V = IR (V in volts, I in amperes, R in ohms).
Ohmic conductor: A conductor in which current is directly proportional to potential difference at constant temperature, so its resistance is constant.
I–V graph of a filament lamp, and why it has that shape: An S-shaped curve that flattens at both ends. As the current rises the filament gets hotter and its resistance increases.
How does a diode behave?: Current flows in one direction only. Its resistance is very high in the reverse direction.
Thermistor vs LDR: Thermistor: resistance falls as temperature rises (used in thermostats). LDR: resistance falls as light intensity rises (used for lights that switch on in the dark).
How do you connect meters to measure the resistance of a component?: Ammeter in series with the component; voltmeter in parallel across it. Then R = V ÷ I.
Required practical 3: how does the resistance of a wire depend on its length?: Resistance is directly proportional to length. Keep the temperature constant (small current, switch off between readings).
A lamp has 3.0 V across it and 0.60 A through it. What is its resistance?: R = 3.0 ÷ 0.60 = 5.0 Ω

Current and potential difference in a series circuit: The current is the same through each component. The supply pd is shared between the components.
Current and potential difference in a parallel circuit: The pd is the same across each branch. The branch currents add up to the total current.
Total resistance of resistors in series: R_total = R₁ + R₂ (adding a resistor in series increases the total resistance).
Why does adding a resistor in parallel decrease the total resistance?: It gives the charge an extra path. The total resistance of two resistors in parallel is less than the smaller one.
A 10 Ω and a 15 Ω resistor are in series with a 5.0 V supply. Find the current.: R_total = 25 Ω, so I = 5.0 ÷ 25 = 0.20 A
In a thermistor and fixed-resistor circuit, what happens to the thermistor’s share of the pd when it gets colder?: Its resistance rises, so it takes a bigger share of the supply pd.

Direct vs alternating potential difference: Direct: one direction only (cells and batteries). Alternating: keeps reversing direction (the mains).
Frequency and potential difference of the UK mains supply: Alternating current at 50 Hz and about 230 V.
Live, neutral and earth wires: colours: Live: brown. Neutral: blue. Earth: green and yellow stripes.
What does each wire do, and what is its pd?: Live: carries the alternating pd, about 230 V. Neutral: completes the circuit, at or close to 0 V. Earth: safety wire at 0 V, carries current only if there is a fault.
Why is a live wire dangerous even when the switch is open?: It is still at about 230 V and your body is at 0 V, so touching it puts a large pd across you and a current flows through you: an electric shock.
What is the danger of a connection between the live wire and earth?: It is a low-resistance path, so a very large current flows, which can cause a fire (or a shock if the path is through a person).

Two equations for electrical power: P = VI P = I²R (P in W, V in V, I in A, R in Ω)
Two equations for energy transferred electrically: E = Pt (J, W, s) E = QV (J, C, V)
What does the energy transferred by an appliance depend on?: Its power and how long it is switched on. Work is done when charge flows; motors transfer energy to kinetic stores, heaters to thermal stores.
A 1.5 kW heater is on for 2 hours. How much energy does it transfer?: E = Pt = 1500 × 7200 = 10 800 000 J (10.8 MJ)
The National Grid: A system of cables and transformers linking power stations to consumers. Step-up transformers raise the pd for transmission; step-down transformers lower it for domestic use.
Why is electricity transmitted at a high potential difference?: For the same power (P = VI), a higher pd means a lower current. A lower current means much less heating of the cables (P = I²R), so less energy is wasted.

How do two insulators become charged when rubbed together?: Electrons are transferred from one to the other. The one that gains electrons becomes negative; the one that loses them becomes equally positive.
Forces between charged objects: Like charges repel; unlike charges attract. Both are non-contact forces.
Electric field: The region around a charged object where another charged object experiences a force.
Electric field lines round an isolated charged sphere: Radial: outward for a positive sphere, inward for a negative one. The field is strongest close to the sphere.
Why does sparking happen?: Charge builds up on an object, so the pd between it and a nearby earthed object becomes large enough for electrons to jump across the gap.
Exam trap: which charged particles move when an object is charged by friction?: Only electrons. Protons and “positive charge” do not move.