Electricity and magnetism flashcards

Electricity and magnetism covers the behaviour of electric charges, fields, and circuits, as well as magnetic fields and their interaction with moving charges and currents. Students must understand electric charge, Coulomb's law, and electric field strength, then apply Ohm's law and Kirchhoff's laws to analyse series and parallel circuits involving resistance, EMF, and internal resistance. Power dissipation, potential dividers, and the relationship between current and drift velocity are also essential. On the magnetism side, students need to understand magnetic flux density, the force on a current-carrying conductor and on moving charges (using the left-hand rule), and electromagnetic induction including Faraday's and Lenz's laws. Connecting these ideas, students should appreciate how changing magnetic flux induces EMF and how this underpins generators and transformers.

1. Card 1

   What is electric field strength E at a point?

   Electric field strength is the force per unit positive charge at that point: E = F / q, measured in N C-1.

2. Card 2

   State Coulomb's law for the force between two point charges.

   The electrostatic force between two point charges is F = k q1 q2 / r^2, where k = 8.99 x 10^9 N m^2 C-2.

3. Card 3

   What is the difference between EMF and terminal voltage?

   EMF is the total energy per unit charge supplied by a source; terminal voltage is less than EMF when current flows, due to the voltage drop across internal resistance.

4. Card 4

   State Kirchhoff's current law.

   Kirchhoff's current law states that the sum of currents entering a junction equals the sum leaving it, reflecting conservation of charge.

5. Card 5

   State Kirchhoff's voltage law.

   Kirchhoff's voltage law states that the sum of all EMFs around a closed loop equals the sum of all potential drops, reflecting conservation of energy.

6. Card 6

   How does resistance change with temperature for a metallic conductor?

   For a metallic conductor, resistance increases with temperature because greater thermal vibration of ions causes more frequent collisions with conduction electrons.

7. Card 7

   What is the formula for electrical power dissipated in a resistor?

   Electrical power P = I V = I^2 R = V^2 / R, where I is current, V is voltage, and R is resistance, measured in watts.

8. Card 8

   How do you find the combined resistance of two resistors in parallel?

   For parallel resistors, 1/R_total = 1/R1 + 1/R2. The combined resistance is always less than the smallest individual resistance.

9. Card 9

   What is drift velocity and how does it relate to current?

   Drift velocity v is the mean velocity of charge carriers; current I = n A v q, where n is carrier density, A is cross-sectional area, and q is charge per carrier.

10. Card 10

    What is the force on a current-carrying conductor in a magnetic field?

    The magnetic force is F = B I L sin(theta), where B is magnetic flux density, I is current, L is length, and theta is the angle between the conductor and the field.

11. Card 11

    State Faraday's law of electromagnetic induction.

    Faraday's law states that the induced EMF is equal in magnitude to the rate of change of magnetic flux linkage: EMF = -N x (delta flux / delta t).

12. Card 12

    What does Lenz's law tell us about induced currents?

    Lenz's law states that the induced current flows in a direction that opposes the change in magnetic flux that caused it, consistent with conservation of energy.

• Students often confuse EMF with terminal voltage, forgetting to subtract the voltage drop across internal resistance when current is flowing.
• A common error is applying the left-hand rule incorrectly by using the wrong fingers for current direction, field direction, and force, leading to the wrong force direction.
• Many students incorrectly believe that charge carriers travel at the speed of light in a circuit, when in fact the drift velocity is very slow (often millimetres per second) and it is the electric field that propagates rapidly.

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