Fields flashcards

Fields in A-Level Physics covers gravitational, electric, and magnetic fields, exploring how objects exert forces on one another across space without direct contact. Students must understand field lines and equipotentials, Newton's law of gravitation, Coulomb's law, and how field strength is defined for each type of field. Key ideas include the inverse square law for both gravitational and electric fields, the concept of potential and potential energy, and how these quantities vary with distance. Students should be able to compare gravitational and electric fields, noting similarities in their mathematics but differences in that gravity is always attractive whilst electric forces can attract or repel. Magnetic fields, including the force on a current-carrying conductor and on moving charges, must also be understood, along with electromagnetic induction, Faraday's law, and Lenz's law.

1. Card 1

   How is gravitational field strength g defined at a point in a field?

   Gravitational field strength is the force per unit mass experienced by a small test mass placed at that point. g = F / m, measured in N kg-1.

2. Card 2

   State Newton's law of gravitation.

   The gravitational force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation: F = G m1 m2 / r^2.

3. Card 3

   What is gravitational potential at a point in a field?

   Gravitational potential is the work done per unit mass to move a small test mass from infinity to that point. It is always negative and measured in J kg-1.

4. Card 4

   How is electric field strength E defined?

   Electric field strength is the force per unit positive charge at a point: E = F / Q. It is measured in N C-1 or equivalently V m-1.

5. Card 5

   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 = 1 / (4 pi epsilon0). Force is attractive for opposite charges and repulsive for like charges.

6. Card 6

   What are equipotential surfaces and why are field lines always perpendicular to them?

   Equipotential surfaces are surfaces of equal potential. No work is done moving along them, so the force, and hence the field line, must always be perpendicular to the surface.

7. Card 7

   What is the key difference between gravitational and electric fields?

   Gravitational fields are always attractive, whereas electric fields can be either attractive or repulsive depending on the signs of the charges involved.

8. Card 8

   What does Faraday's law of electromagnetic induction state?

   Faraday's law states that the induced e.m.f. in a circuit is equal to the rate of change of magnetic flux linkage: e.m.f. = -d(N phi) / dt.

9. Card 9

   What does Lenz's law tell us about the direction of an induced current?

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

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 flux density, I is current, L is length in the field, and theta is the angle between the conductor and the field.

11. Card 11

    How does gravitational potential energy vary with distance from a planet?

    Gravitational potential energy becomes less negative as distance increases, approaching zero at infinity. Close to a planet it is a large negative value: E = -G M m / r.

12. Card 12

    What is magnetic flux linkage and what are its units?

    Magnetic flux linkage is the product of the number of turns N and the flux through each turn: N phi = N B A cos(theta). It is measured in Weber-turns (Wb) or equivalently V s.

• Students often forget that gravitational potential is always negative, and incorrectly treat it as a positive quantity when calculating potential energy or escape velocity.
• A common error is confusing field strength with potential, applying the inverse square law to potential when in fact potential follows a 1/r relationship rather than 1/r^2.
• When applying Lenz's law, students frequently state the induced current opposes the flux rather than correctly stating it opposes the change in flux, which leads to errors in determining current direction.

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