Flashcards · 0625 · Topic 4

Electricity and magnetism flashcards

58 cards on Topic 4, subtopics 4.1 to 4.5. Pick a subtopic or study them all.

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4.1 Simple phenomena of magnetism

Forces between magnetic poles, and the test for a magnet: Like poles repel; unlike poles attract. Only repulsion proves an object is a magnet, because a magnet attracts unmagnetised iron too.
Induced magnetism: A magnet attracts an unmagnetised magnetic material by making it a magnet, with the opposite pole facing the magnet.
Soft iron vs steel: Soft iron: easy to magnetise, loses it easily; temporary magnets, electromagnet cores. Steel: keeps its magnetism; permanent magnets.
Magnetic and non-magnetic materials: Magnetic: iron, steel, nickel, cobalt. Non-magnetic: e.g. copper, aluminium, plastic, wood.
Magnetic field, and the direction of a field line: A region in which a magnetic pole experiences a force. The direction is the force on an N pole, so lines run N → S outside a magnet.
How do you plot the field of a bar magnet with a compass?: Mark the tip of the needle, move the compass so its tail is on that dot, mark again, and repeat; join the dots and add the arrow. Iron filings show the pattern but not the direction.
What causes magnetic forces, and how is field strength shown on a diagram?Extended: Magnetic forces are due to interactions between magnetic fields. Closer field lines mean a stronger field.

4.2 Electrical quantities

Charging by friction: what moves, and which way?: Only electrons move. The object that gains electrons becomes negative; the one that loses them becomes positive. Rubbed polythene becomes negative; rubbed acetate becomes positive.
Conductors vs insulators (electron model): Conductors (metals) have free electrons that can move through them. In insulators the electrons are held in their atoms.
Direct current vs alternating current: d.c. flows in one direction only (cells, batteries). a.c. keeps reversing direction (the mains supply).
Electromotive force (e.m.f.): The electrical work done by a source in moving a unit charge around a complete circuit. Measured in volts.
Potential difference (p.d.): The work done by a unit charge passing through a component. Measured in volts.
How are an ammeter and a voltmeter connected?: Ammeter in series. Voltmeter in parallel, across the component.
Resistance equation. A lamp has 6.0 V across it and 0.25 A through it: resistance?: R = V/I = 6.0 ÷ 0.25 = 24 Ω.
How do the length and thickness of a wire affect its resistance?: Longer wire: more resistance. Thicker wire: less resistance.
Resistance, length and cross-sectional area. Length doubled, area halved: effect on R?Extended: R is proportional to length and inversely proportional to area. ×2 for length and ×2 for halving the area: 4 times larger.
Electrical power and energy equations. A 12 V lamp takes 2.0 A: power?: P = IV and E = IVt. P = 2.0 × 12 = 24 W.
Kilowatt-hour (kW h): The energy transferred by a 1 kW appliance in 1 hour. 1 kW h = 3.6 × 10⁶ J.
An 800 W microwave runs 15 min a day for 30 days at 25 cents per kW h. Cost?: 0.80 kW × 0.25 h × 30 = 6.0 kW h. 6.0 × 25 = 150 cents.
Current in terms of charge. 0.50 A flows for 2.0 minutes: charge?Extended: I = Q/t, so Q = It = 0.50 × 120 = 60 C.
e.m.f. and p.d. in terms of work and charge. A battery does 54 J moving 6.0 C: e.m.f.?Extended: E = W/Q (and V = W/Q for p.d.). E = 54 ÷ 6.0 = 9.0 V.
Electric field: definition and directionExtended: A region in which a charge experiences a force. Its direction is the force on a positive charge: radially outward from a positive point charge; parallel and evenly spaced from + to − between plates.
I–V graphs: fixed resistor, filament lamp and diodeExtended: Resistor: straight line through the origin. Filament lamp: curve getting less steep (filament heats, R increases). Diode: no current in reverse; current rises steeply after a small forward p.d.

4.3 Electric circuits

Series circuit: current and combined resistance: The current is the same everywhere. Combined resistance R = R₁ + R₂ + …
Two resistors in parallel: combined resistance and currents: The combined resistance is less than either resistor alone. The current from the source is larger than the current in each branch.
Why are lighting circuits wired in parallel?: Each lamp gets the full p.d., can be switched on and off separately, and stays lit if another lamp fails.
Three 1.5 V cells in series, all facing the same way. Total e.m.f.?: The e.m.f.s add: 3 × 1.5 = 4.5 V.
NTC thermistor and LDR: NTC thermistor: resistance falls as temperature rises. LDR: resistance falls as light intensity increases.
Potential divider: which resistor gets the bigger p.d.?: The larger resistance takes the larger share of the supply p.d.
Combined resistance of 20 Ω and 30 Ω in parallelExtended: 1/R = 1/20 + 1/30 = 5/60, so R = 12 Ω.
Currents at a junction, and whyExtended: Current into a junction = current out, because charge is not used up or stored at a junction.
p.d. rules for series and parallelExtended: Series: the p.d.s across the components add up to the total. Parallel: each branch has the same p.d.
3.0 kΩ and 1.0 kΩ in series across 12 V. p.d. across the 1.0 kΩ?Extended: R₁/R₂ = V₁/V₂. Total 4.0 kΩ; the 1.0 kΩ takes ¼ of 12 V = 3.0 V (and 9.0 V across the 3.0 kΩ).
Diode and LEDExtended: A diode lets current through in one direction only. An LED also conducts one way only, and emits light when it conducts.
An LDR is the lower resistor of a potential divider. It gets darker. What happens to the p.d. across it?Extended: Its resistance rises, so its share of the p.d. rises. This can switch on a lamp at dusk.

4.4 Electrical safety

Four electrical hazards: Damaged insulation, overheating cables, damp conditions, and overloaded plugs, extension leads and sockets.
The three wires in mains wiring, and where the switch and fuse go: Live (line), neutral and earth. The switch and the fuse go in the live wire.
Why must the switch be in the live wire?: So that when it is off, the appliance is disconnected from the high voltage. In the neutral, the appliance would stay live.
Fuse vs trip switch: A fuse melts if the current exceeds its rating. A trip switch (circuit breaker) opens automatically and can be reset.
A 1.2 kW hairdryer runs on 230 V. Choose a fuse: 3 A, 5 A or 13 A.: I = P/V = 1200 ÷ 230 = 5.2 A, so 13 A (the rating must be just above the normal current; 5 A would melt).
Why is a metal casing earthed?: If the live wire touches the casing, a large current flows to earth and the fuse melts, so the casing does not stay live.
Double-insulated appliance: It has a non-conducting (plastic) casing, so it needs no earth wire. Its fuse still protects the circuit and cabling.

4.5 Electromagnetic effects

When is an e.m.f. induced in a conductor?: When it moves across a magnetic field (cuts field lines), or when the magnetic field linking it changes.
Three ways to increase an induced e.m.f.: Faster movement, a stronger magnet (field), more turns on the coil.
A magnet is pushed into a coil joined to a meter, held still, then pulled out. What does the meter show?: A deflection while it moves in, zero while it is still, and a deflection the other way as it is pulled out.
Direction of an induced e.m.f.: the rule and the hand ruleExtended: The induced e.m.f. opposes the change causing it. Fleming's right-hand rule: thumb = motion, first finger = field, second finger = induced current.
a.c. generator: connection, and coil positions for maximum and zero e.m.f.Extended: Slip rings and brushes connect the rotating coil. e.m.f. is maximum when the plane of the coil is parallel to the field, zero when it is perpendicular.
Magnetic field of a straight wire and of a solenoid: Wire: concentric circles (right-hand grip rule). Solenoid: like a bar magnet outside; strong and nearly uniform inside.
Relay: A small current in a coil magnetises it and closes a switch in a second circuit, so a small current can switch a large one (e.g. a car ignition switching the starter motor).
How does a loudspeaker work?: A changing current in a coil in a permanent magnet's field makes the coil and cone vibrate, producing sound.
Three ways to increase the turning effect on a current-carrying coil in a magnetic field: More turns on the coil, a larger current, a stronger magnetic field.
Fleming's left-hand ruleExtended: First finger = field, second finger = current, thumb = force (motion). For an electron beam, the current is opposite to the electrons' motion.
What does the split-ring commutator in a d.c. motor do?Extended: It reverses the current in the coil every half turn, so the forces keep turning the coil the same way.
Transformer equation. 400 primary turns, 230 V in, 11.5 V out: secondary turns?: Vp/Vs = Np/Ns, so Ns = 400 × 11.5 ÷ 230 = 20 turns.
Step-up vs step-down transformer: Step-up: more secondary turns than primary, so Vs > Vp. Step-down: fewer secondary turns, so Vs < Vp. Both have a soft-iron core.
Why is electricity transmitted at high voltage?: For the same power the current is smaller, so less energy is wasted heating the cables, and thinner, cheaper cables can be used.
How does a transformer work, and why does it need a.c.?Extended: Alternating current in the primary makes a changing magnetic field in the core, which induces an alternating e.m.f. in the secondary. A steady d.c. gives no changing field.
Ideal transformer power equation, and power lost in cablesExtended: IpVp = IsVs (100% efficient). Cable loss P = I²R, so ten times the current gives one hundred times the loss.