Flashcards · 5054 · Topic 4

Electricity and magnetism flashcards

27 cards on Topic 4, subtopics 4.1 to 4.6. Pick a subtopic or study them all.

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Magnets: the test for a magnet, soft iron vs steel, and field direction: Only repulsion proves a magnet. Soft iron: temporary magnet (easily magnetised and demagnetised); steel: permanent. Field direction = force on an N pole; lines run N to S outside the magnet.
Induced magnetism: A magnetic material (iron, steel, nickel, cobalt) placed near a magnet becomes a magnet itself, with the end nearest the magnet an opposite pole, so it is attracted.
How are magnetic field lines plotted, and what does their spacing show?: With a plotting compass (its N end points along the field direction) or with iron filings. Lines closer together show a stronger field, so the field is strongest near the poles.
Uses of permanent magnets and electromagnets: Permanent: compasses, loudspeakers, motors, fridge door seals. Electromagnets (can be switched off and their strength changed): relays, electric bells, scrapyard cranes.

Charging by friction: Only electrons move. The object that gains electrons becomes negative; the one that loses them becomes positive. Like charges repel; unlike attract.
Current and charge. 540 C passes a point in 30 minutes: current?: I = Q/t. t = 1800 s, so I = 540 ÷ 1800 = 0.30 A.
e.m.f. vs p.d.: e.m.f.: work done by a source per unit charge, E = W/Q. p.d.: work done by unit charge passing through a component, V = W/Q. Both in volts (J/C).
Ohm's law and the I–V graphs of a resistor, a filament lamp and a diode: Current is proportional to p.d. at constant temperature (resistor: straight line through origin). Filament lamp: curve flattening (hot filament, higher R). Diode: conducts one way only.

Combined resistance of 6.0 Ω and 12 Ω in parallel, and in series: Parallel: R = (R₁ × R₂) ÷ (R₁ + R₂) = 72 ÷ 18 = 4.0 Ω. Series: 6.0 + 12 = 18 Ω.
A thermistor in series with a fixed resistor warms up. What happens to the p.d.s?: The thermistor's resistance falls, so its p.d. falls and the fixed resistor's p.d. rises. (The larger resistance takes the larger share.)
Current and p.d. rules for series and parallel circuits: Series: the current is the same everywhere; the p.d.s across the components add up to the supply p.d. Parallel: the currents into a junction equal the currents out; each branch has the same p.d.
A 2.0 kΩ and a 3.0 kΩ resistor form a potential divider across 10 V. Find the p.d. across each.: The p.d. divides in the ratio of the resistances (R₁/R₂ = V₁/V₂): 2.0 kΩ gets 10 × 2/5 = 4.0 V and 3.0 kΩ gets 6.0 V.
How does a light-dependent resistor (LDR) act as a sensor?: Its resistance falls as the light gets brighter. In a potential divider its share of the p.d. changes with light level, which can switch a circuit, such as a lamp that comes on when it gets dark.

Where do the fuse and switch go, and why is a metal case earthed?: Both in the live wire, so the appliance is disconnected from the live supply when off or after a fault. If live touches an earthed case, a large current flows to earth and the fuse melts.
A 2.4 kW heater runs for 3.0 h at 15 cents per kW h. Cost?: Energy = 2.4 kW × 3.0 h = 7.2 kW h. Cost = 7.2 × 15 = 108 cents.
A kettle takes 10 A from a 230 V supply for 2.0 minutes. Power and energy?: P = IV = 10 × 230 = 2300 W. E = IVt = 10 × 230 × 120 = 276 000 J.
Hazards of using mains electricity: Damaged insulation, overheating cables, damp conditions, and excess current from overloading plugs, extension leads and sockets.
Advantages of connecting lamps in parallel in a lighting circuit: Each lamp gets the full supply p.d., each can be switched on and off independently, and if one fails the others stay lit.

Electromagnetic induction and Lenz's law: An e.m.f. is induced when a magnet and coil move relative to each other; larger with faster cutting of field lines and more turns. Lenz's law: the induced current opposes the change that causes it.
Fleming's left-hand rule, and how to increase a motor's turning effect: First finger = field, second finger = current, thumb = force. More turns, a larger current or a stronger field increase the turning effect.
Transformer: 2300 primary turns on 230 V, 150 secondary turns. Output p.d.?: Vp/Vs = Np/Ns, so Vs = 230 × 150 ÷ 2300 = 15 V (step-down). Transformers need a.c.
The a.c. generator: slip rings, and coil position against e.m.f.: A coil rotates in a magnetic field; slip rings and brushes connect it to the circuit. The e.m.f. is a peak or trough when the coil is parallel to the field and zero when it is perpendicular. Faster rotation gives higher peaks and a shorter period.
How do a relay and a loudspeaker use the magnetic effect of a current?: Relay: a small current in an electromagnet closes a switch in a separate high-current circuit. Loudspeaker: a varying current in a coil in a permanent magnet's field makes the coil and cone vibrate.
Why is electricity transmitted at high voltage?: For the same power, a higher voltage means a smaller current, so less power is lost heating the cables (and thinner cables can be used).

What does an oscilloscope show, and what do the Y-gain and timebase set?: It displays a waveform as a trace. The Y-gain sets the p.d. per vertical division; the timebase sets the time per horizontal division.
Y-gain 5.0 V/div, timebase 2.0 ms/div. The peak is 2.4 divisions above the centre and one cycle spans 5.0 divisions. Find the peak p.d., period and frequency.: Peak p.d. = 2.4 × 5.0 = 12 V. Period = 5.0 × 2.0 = 10 ms = 0.010 s. Frequency = 1 ÷ 0.010 = 100 Hz.
On an oscilloscope trace, what shows a louder note and what shows a higher note?: Louder: a taller trace (larger amplitude). Higher pitch: more complete waves across the screen (shorter period, higher frequency).