Electricity flashcards

Electricity at A-Level covers the behaviour of charge, current, potential difference and resistance in both direct current circuits. Students must understand how charge carriers move in conductors and how current, voltage and resistance relate through Ohm's law. Key ideas include resistivity and how it depends on material and temperature, the characteristics of ohmic and non-ohmic components, and how to analyse series and parallel circuits using Kirchhoff's laws. Power dissipation, electromotive force and internal resistance are essential, as is understanding how a potential divider works. Students should be able to apply these principles to solve circuit problems, interpret I-V characteristic graphs for components such as filament lamps, diodes and thermistors, and explain how resistance changes with physical conditions.

1. Card 1

   Define electric current in terms of charge flow.

   Electric current is the rate of flow of charge: I = Q / t, where I is in amperes, Q in coulombs and t in seconds.

2. Card 2

   What is the difference between EMF and terminal potential difference?

   EMF is the energy given per coulomb by the source; terminal potential difference is less than EMF when current flows, due to energy lost across internal resistance.

3. Card 3

   State Ohm's law and the condition under which it applies.

   Ohm's law states V = IR; it applies only to ohmic conductors where resistance stays constant at constant temperature.

4. Card 4

   How does resistance depend on the physical properties of a wire?

   R = (resistivity x length) / cross-sectional area. Resistance increases with length and decreases with greater cross-sectional area.

5. Card 5

   State Kirchhoff's first law and the principle it is based on.

   Kirchhoff's first law: the sum of currents entering a junction equals the sum leaving. It is based on conservation of charge.

6. Card 6

   State Kirchhoff's second law and the principle it is based on.

   Kirchhoff's second law: the sum of EMFs around a closed loop equals the sum of potential differences. It is based on conservation of energy.

7. Card 7

   How do you find the total resistance of resistors in series and in parallel?

   In series: R_total = R1 + R2 + ... In parallel: 1/R_total = 1/R1 + 1/R2 + ...

8. Card 8

   Describe the I-V characteristic of a filament lamp.

   The graph curves so that current increases less steeply as voltage rises, showing non-ohmic behaviour because resistance increases as the filament heats up.

9. Card 9

   How does a thermistor's resistance change with temperature?

   For a typical NTC thermistor, resistance decreases as temperature increases, because more charge carriers become available.

10. Card 10

    Write the equation linking power, current and resistance.

    Power P = I squared x R. This is derived by combining P = IV and V = IR, and shows power dissipated as heat in a resistor.

11. Card 11

    What is the equation for EMF including internal resistance?

    EMF (epsilon) = I(R + r), where R is external resistance and r is internal resistance. The term Ir is called the lost volts.

12. Card 12

    How does a potential divider circuit work?

    A potential divider splits a supply voltage across two resistors; V_out = V_in x R2 / (R1 + R2), allowing a chosen fraction of the voltage to be tapped off.

• Many students confuse EMF with terminal potential difference, forgetting that EMF is only equal to the terminal pd when no current is flowing.
• A common error is assuming that all components obey Ohm's law; only ohmic conductors at constant temperature have a constant resistance, so V = IR can always be used to find resistance but does not imply ohmic behaviour.
• When calculating parallel resistance, students often add the resistances directly instead of adding their reciprocals, giving an answer larger than the smallest individual resistance, which is physically impossible.

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