Skip to content
Marlbridge
Free trial

Flashcards · 9203 · Topic 5

Electricity and magnetism flashcards

23 cards on Topic 5, subtopics 3.5.1 to 3.5.2. Pick a subtopic or study them all.

Study mode needs JavaScript. Every card is listed below, so you can still cover the answers and test yourself.

All 23 cards

For a quick read-through or printing. Cover the right-hand side to test yourself.

3.5.1 Electrical circuits

Why do metals conduct, and which way does conventional current flow?
Metals contain many free electrons. Conventional current leaves the positive terminal; the electrons move the opposite way. A current needs a source and a complete loop.
Static electricity
Rubbing two insulators transfers electrons: the surface that gains electrons is negative, the one that loses them positive. The charge stays because there is no conducting route; given one, it discharges.
Current and charge. 1.8 A flows for 2.5 minutes: charge?
Current is the rate of flow of charge: I = Q/t. Q = It = 1.8 × 150 = 270 C. 1 A = 1 C/s.
Potential difference. A motor transfers 330 J from 55 C:
p.d. is the energy transferred per unit charge in a component: V = E/Q = 330 ÷ 55 = 6.0 V. 1 V = 1 J/C.
Voltage of a source vs potential difference
Voltage of a source: energy it gives to charges driving them round the whole circuit. Potential difference: energy transferred by charges in a component.
Resistance equation. A heater has 12 V across it and carries 0.50 A:
V = I × R, so R = 12 ÷ 0.50 = 24 Ω.
Ohmic conductor
A resistor at constant temperature: current is directly proportional to p.d., so the resistance is constant and the I–V graph is a straight line through the origin.
I–V graphs of a filament lamp and a diode
Filament lamp: a curve through the origin that flattens (resistance rises as it heats); symmetrical for negative p.d. Diode: almost no current in reverse and for small forward p.d., then a steep rise.
Explain why a filament lamp's resistance rises with current
The current heats the filament, so the positive ions vibrate with larger amplitude. Free electrons collide with the ions more often, so the flow of charge is hindered more.
Thermistor and LDR: how resistance changes, and a use
Thermistor: resistance decreases as temperature increases (thermostat, fire alarm). LDR: resistance decreases as light intensity increases (switching lights on when it gets dark).
Diode and LED
A diode conducts in one direction only (low forward, very high reverse resistance). An LED is a diode that emits light; LEDs use a much smaller current than other lighting.
A 9.0 V supply is across 8.0 Ω and 22 Ω in series. Current, and each p.d.?
Series: resistances add, same current, p.d. shared. R = 30 Ω, I = 9.0 ÷ 30 = 0.30 A. p.d.s: 0.30 × 8.0 = 2.4 V and 0.30 × 22 = 6.6 V.
A 24 V supply is across 40 Ω and 60 Ω in parallel. Branch currents and combined resistance?
Parallel: same p.d. across each branch, currents add. 24 ÷ 40 = 0.60 A and 24 ÷ 60 = 0.40 A, total 1.0 A. Combined R = 24 ÷ 1.0 = 24 Ω (less than either branch).
Cells in series
The combined voltage is the sum of their voltages (cells facing the same way). Four 2.0 V cells give 8.0 V.
Heating effect of a current
Moving electrons collide with the stationary atoms of the wire, transferring energy so the atoms vibrate more and the wire heats up. A filament lamp wastes a lot of energy this way; a CFL wastes less.

3.5.2 Magnetism and electromagnetism

Forces between poles, and the test for a magnet
Like poles repel; unlike poles attract. Induced magnetism always attracts, so only repulsion proves an object is a magnet.
Magnetic field, and the magnetic materials
The space around a magnet where another magnet or a magnetic material experiences a force. Magnetic materials: iron, steel, cobalt, nickel.
Field line rules, and a uniform field
Lines go out of north and into south, never cross, and are closer where the field is stronger. A uniform field has parallel, equally spaced lines.
Induced magnet
A material that becomes a magnet when placed in a magnetic field (its near end becomes the opposite pole, so it is attracted) and loses most or all of its magnetism quickly when removed.
The Earth's field and a plotting compass
The Earth has a magnetic field. A compass is a small, freely turning magnet that lines up with the field; its north-seeking end points north.
Magnetic field around a straight wire
Concentric circles in a plane perpendicular to the wire. Stronger closer to the wire and with a larger current; reversing the current reverses the field.
Solenoid and electromagnet: field, and how to make it stronger
A solenoid's field is strong and uniform inside and like a bar magnet outside. An electromagnet is a solenoid with an iron core. Stronger with a larger current, more turns, an iron core.
Three uses of electromagnets
Scrapyard crane (lifts then releases scrap), electric bell (the coil pulls an armature that breaks the circuit, then a spring resets it), relay (a small current closes a switch in a second circuit).

Revise Electricity and magnetism further