A-Level Physics Flashcards: Electricity and Fields

A-Level Physics — Electricity and Fields Flashcards

20 flashcards with spaced repetition. Press Space to flip, then rate your recall (1—4).

What These Flashcards Cover

These flashcards test your knowledge of the core electricity and fields topics in A-Level Physics. You should be able to define key quantities, state formulas, explain circuit behaviour, and describe how fields exert forces on charges and currents.

Key areas:

• Current and Resistance: Define current as the rate of flow of charge ($I = \frac{\Delta Q}{\Delta t}$). Know Ohm’s law ($V = IR$) and when it is valid (ohmic conductors at constant temperature). Distinguish between ohmic and non-ohmic components from an I-V characteristic.
• CKirchhoff’s Laws: The junction rule ($\sum I_{\text{in}} = \sum I_{\text{out}}$) and the loop rule ($\sum V = 0$ around a closed loop). These are consequences of conservation of charge and energy.
• Electric Fields: The field strength $E = \frac{F}{Q}$ for a uniform field, or $E = \frac{kQ}{r^{2}}$ for a point charge. Field lines point from positive to negative. The force on a charge in a field is $F = QE$.
• Capacitance: $Q = CV$. Energy stored: $E = \frac{1}{2}CV^{2} = \frac{1}{2}QV = \frac{Q^{2}}{2C}$. In a charging RC circuit, charge builds as $Q = Q_{0}(1 - e^{-t/RC})$.
• Magnetic Fields and Force on a Current: $F = BIL\sin\theta$ for a straight conductor. $F = Bqv\sin\theta$ for a moving charge. Fleming’s left-hand rule gives the direction of the force.
• Electromagnetic Induction: Faraday’s law: $\mathcal{E} = -N\frac{\Delta\Phi}{\Delta t}$. Lenz’s law: the induced EMF opposes the change in flux that caused it. The flux linkage is $\Phi = BA\cos\theta$.

Intuition

Think of electric field lines like the contour lines on a map of terrain. Where lines are close together the “slope” (field strength) is steep, and a charge placed there feels a strong push. Voltage is like height on the terrain: current flows “downhill” from high to low potential, just as water flows downhill under gravity.

For capacitors, imagine filling a water tank: the more water (charge) you put in, the higher the water level (voltage). The size of the tank is the capacitance — a bigger tank needs more water to reach the same level.

Common Pitfalls

1. Confusing EMF and PD. EMF is the energy per unit charge supplied by a source; PD is the energy per unit charge dropped across a component. They have the same units (volts) but opposite roles.
2. Forgetting the negative sign in Faraday’s law. The minus sign encodes Lenz’s law. If you omit it you lose the direction information and may get the induced current wrong.
3. Assuming Ohm’s law applies to everything. Ohm’s law ($V = IR$ with constant $R$) only holds for ohmic conductors at constant temperature. A filament lamp, diode, or thermistor does not obey it.
4. Mixing up series and parallel capacitor rules. Capacitors in series add as $\frac{1}{C} = \frac{1}{C_1} + \frac{1}{C_2}$ (opposite to resistors), while capacitors in parallel add directly: $C = C_1 + C_2$.

Cross-References

• Mechanics: Electricity connects to mechanics
• Nuclear: Nuclear physics involves electrical phenomena
• Waves: Electromagnetic waves are electricity-related