Definitions (AQA A-Level Physics): Revision Notes

Definitions

1. Back $emf$

• Definition: The electromotive force ($emf$) generated within a coil in a motor or AC generator that acts against the supplied potential difference.
• Explanation: Back emf opposes the initial current, which helps control the speed of a motor, as it increases with rotational speed and limits current to prevent overheating.

2. Capacitance ($C$)

• Definition: The amount of charge stored per unit potential difference in a capacitor, measured in farads ($F$).
• Formula: $C = \frac{Q}{V}$, where $Q$ is charge and $V$ is potential difference.

3. Capacitor

• Definition: An electrical component that stores charge. A parallel-plate capacitor is made of two conductive plates separated by an insulating material, known as a dielectric.
• Usage: Capacitors are used in circuits for energy storage, filtering signals, and timing applications.

4. Coulomb's Law

• Definition: The force between two point charges is proportional to the product of their charges and inversely proportional to the square of the distance between them.
• Formula: $F = \frac{k Q_1 Q_2}{r^2}$, where $k$ is Coulomb's constant.
• Explanation: Coulomb's Law explains attraction between opposite charges and repulsion between like charges.

5. Cyclotron

• Definition: A particle accelerator made of two $"D"$ shaped electrodes in which particles are accelerated by an alternating electric field.
• Application: Cyclotrons are used in physics research and medical treatments, such as proton therapy for cancer.

6. Dielectric

• Definition: An insulating material placed between the plates of a capacitor to increase its ability to store charge.
• Explanation: The dielectric increases the capacitance by allowing more charge to accumulate for a given voltage.

7. Electrical Conductor

• Definition: A material that contains free electrons which can move to carry charge.

8. Electrical Insulator

• Definition: A material with no free electrons; all electrons are bound to atoms, preventing charge flow.

9. Electric Field

• Definition: A region around a charged object that exerts a force on other charged objects.
• Explanation: The strength and direction of the field depend on the charge magnitude and distance from the source.

10. Electric Field Strength ($E$)

• Definition: The force per unit positive charge at a specific point in the field.
• Formula: $E = \frac{F}{q}$, where $F$ is the force on a small positive test charge $q$.
• Explanation: Field strength is a vector quantity, pointing in the direction that a positive charge would move.

11. Electric Potential ($V$)

• Definition: The work done per unit charge to bring a positive test charge from infinity to a point in the field.
• Explanation: Electric potential is measured in volts and represents potential energy per unit charge.

12. Electromagnetic Induction

• Definition: The process by which an emf is induced in a conductor moving relative to a magnetic field.

13. Equipotential

• Definition: A surface of constant potential, meaning no work is done by the field when moving along this surface.
• Explanation: Equipotential lines are perpendicular to electric field lines and show areas where a charge has the same potential energy.

14. Escape Velocity

• Definition: The minimum velocity an object requires to escape a gravitational field without further propulsion.
• Formula: $v = \sqrt{\frac{2GM}{r}}$

15. Faraday's Law

• Definition: The induced emf in a circuit is equal to the rate of change of magnetic flux linkage through it.
• Formula: $\text{emf} = -\frac{d(N\phi)}{dt}$
• Explanation: This law explains how changing magnetic fields generate electricity in transformers and generators.

16. Field Line / Line of Force

• Definition: A line that shows the path a positive charge or mass would take in a field.
• Explanation: Field lines indicate field direction and strength; closer lines represent stronger fields.

17. Geostationary Satellite

• Definition: A satellite with a 24-hour orbit around Earth's equator, remaining fixed above a specific point.
• Application: Used for communications, weather monitoring, and broadcasting.

18. Gravitational Field

• Definition: A region around a mass where another mass experiences an attractive force.
• Explanation: The Earth's gravitational field causes objects to fall towards it.

19. Gravitational Field Strength ($g$)

• Definition: The force per unit mass exerted on an object within a gravitational field.
• Formula: $g = \frac{F}{m}$

20. Gravitational Potential ($V$)

• Definition: The work done per unit mass to move a test mass from infinity to a point in a gravitational field.
• Explanation: Gravitational potential is negative, indicating attraction towards the mass creating the field.

21. Gravitational Potential Energy

• Definition: The energy due to an object's position in a gravitational field.
• Formula: $U = mV$, where $V$ is gravitational potential.

22. Kepler's Third Law

• Definition: The square of an object's orbital period is directly proportional to the cube of its orbital radius.
• Formula: $T^2 \propto r^3$
• Application: This law describes the motion of planets and satellites.

23. Lenz's Law

• Definition: The direction of an induced current is such that it opposes the change that caused it.
• Explanation: Lenz's Law is why a magnet falling through a coil creates a current in the opposite direction of the magnet's motion.

24. Magnetic Field

• Definition: A region around a magnet or current-carrying wire where a force acts on other magnets or current-carrying wires.
• Explanation: Magnetic fields are depicted by field lines; the closer the lines, the stronger the field.

25. Magnetic Flux (Φ)

• Definition: The product of magnetic flux density and the area it penetrates.
• Formula: $\Phi = B \times A$
• Explanation: Flux is measured in webers ($Wb$) and represents the amount of field lines passing through a given area.

26. Magnetic Flux Density ($B$)

• Definition: The force per unit current per unit length on a current-carrying wire at right angles to the field.
• Formula: $B = \frac{F}{I \times l}$
• Unit: Tesla ($T$)

27. Motor Effect

• Definition: When a current-carrying conductor within a magnetic field experiences a force perpendicular to both the field and the current.
• Explanation: The motor effect powers electric motors and is based on Fleming's left-hand rule.

28. Permittivity of Free Space (ε₀)

• Definition: A constant describing the ability of a vacuum to permit electric fields.
• Value: $\varepsilon_0 \approx$ 8.85 × 10⁻¹² F/m

29. Polarisation

• Definition: When an external field shifts electron clouds opposite to the positive nucleus, creating a dipole.

30. Potential Gradient

• Definition: The rate of change of potential per unit distance in a field.
• Explanation: A steeper potential gradient means a stronger field.

31. Radial Field

• Definition: A field where all field lines radiate from a central point.

32. Relative Permittivity

• Definition: The ratio of the capacitance with dielectric to the capacitance without it.
• Explanation: This property increases the effectiveness of capacitors.

33. Step-down Transformer

• Definition: A transformer that reduces output voltage by having fewer turns on the secondary coil than the primary coil.
• Application: Used in power supplies to lower voltage for safe household use.

34. Step-up Transformer

• Definition: A transformer that increases output voltage by having more turns on the secondary coil.
• Application: Used in power grids to transmit electricity over long distances.

35. Synchronous Orbit

• Definition: An orbit with a period matching the rotation of the body it orbits.

36. Time Constant (τ)

• Definition: The time for a capacitor to discharge to 37% of its initial charge.
• Formula: $\tau = RC$, where $R$ is resistance and $C$ is capacitance.

37. Uniform Field

• Definition: A field where field lines are parallel and equally spaced, indicating constant field strength throughout.