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Flashcards · J249 · Topic 3

Electricity flashcards

21 cards on Topic 3, subtopics P3.1 to P3.2. Pick a subtopic or study them all.

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P3.1 Static and charge

Why does an object usually have no overall charge?
It contains equal amounts of positive charge (protons) and negative charge (electrons), so its net charge is zero.
How do two insulators become charged by rubbing?
Electrons are transferred from one to the other. The one that gains electrons becomes negative; the one that loses them becomes positive, by an equal amount.
Why does static charge build up only on insulators?
Charge cannot flow through an insulator, so it stays where it is. A conductor held in the hand loses its charge through the body to the ground.
Why is a charged balloon attracted to a neutral wall?
Electrons in the wall’s surface shift slightly, so the nearest surface gets the opposite charge. The attraction to these nearer charges is stronger than the repulsion from the farther ones.
Electric field
The space surrounding a charged object in which any other charge experiences a force.
Electric field lines: direction and spacing
They show the direction of the force on a small positive charge: away from positive, towards negative. Closer lines mean a stronger field. Lines never cross.
Two conditions needed for a current to flow
A source of potential difference and a closed (complete) circuit.
Current, and the charge equation
Current is the rate of flow of charge. Q = I × t (C, A, s).
72 C of charge flows past a point in 4.0 minutes. What is the current?
t = 240 s, so I = 72 ÷ 240 = 0.30 A
Conventional current vs electron flow
Conventional current is drawn from positive to negative. In a metal, the electrons actually move from negative to positive.

P3.2 Simple circuits

Series vs parallel: current and potential difference
Series: same current everywhere; the pd is shared. Parallel: the same pd across each branch; branch currents add up to the supply current.
Potential difference
The energy transferred per unit charge between two points. 1 V = 1 J per coulomb. E = Q × V.
How are an ammeter and a voltmeter connected?
Ammeter: in series (very low resistance). Voltmeter: in parallel across the component (very high resistance).
Equation linking pd, current and resistance
V = I × R (V, A, Ω)
I–V graph of a filament lamp, and why its resistance rises
An S-shaped curve. More current heats the filament; its ions vibrate more, so electrons collide with them more often and resistance rises.
How do a diode, an NTC thermistor and an LDR behave?
Diode: current in one direction only (huge resistance in reverse). NTC thermistor: resistance falls as temperature rises. LDR: resistance falls as light intensity rises.
Why does adding a resistor in parallel decrease the net resistance?
It gives the charge another path, so the total current from the supply increases for the same pd.
A 6.0 V supply has 4.0 Ω and 12 Ω in parallel. Supply current and net resistance?
Currents 1.5 A and 0.50 A, so 2.0 A in total. Net R = 6.0 ÷ 2.0 = 3.0 Ω
Two equations for electrical power
P = V × I P = I² × R
Kilowatt-hour
A unit of energy: the energy transferred by a 1 kW device in 1 hour. 1 kW h = 3 600 000 J. E = P × t with P in kW and t in h.
A 3.0 kW oven runs for 90 minutes. Energy in kW h?
E = 3.0 × 1.5 = 4.5 kW h

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