Module 6: Particles and Medical Physics Flashcards
Define capacitance.
The amount of charge a capacitor can store per unit voltage.
Define the Farad.
Coulomb per volt.
Describe a Q against V graph
Gradient - Capacitance
Area under the graph - Energy stored
What is the time constant, τ, of a capacitor
- The time taken for V, I, or Q to decrease to ≅37% when discharging
- τ = RC, where R is the equivalent resistance of the circuit and C is the equivalent capacitance of the circuit.
C ∝ A/d, define these letters and explain the constant of proportionality.
C - Capacitance
A - Area of one plate
d- Distance between plates
The constant of proportionality is ε.
ε = εᵣε₀, where εᵣ is the relative permittivity of the material between the plates, and ε₀ is the permittivity of free space.
What are some uses of capacitors?
- Camera flash
- Touchscreen
- Back up energy supplies
- When AC is converted to DC, it smoothes the current, ideally τ > > T, where T is the period of the alternating current
Define an electrical field.
An electric field is a region where a charged particle experiences a force due to its charge.
Define electrical field strength.
The force per unit positive charge.
What do the direction of electrical fields lines show?
The direction of the force on a positive test charge.
What is a uniform electrical field?
A region where electrical field strength is constant, all the field lines are equally spaced and parallel.
A source of this type of field are parallel plate capacitors.
What is a radial electrical field?
A region where electrical field strength varies with distance according to an inverse square law.
A source of this type of field is a point charge or spherically symmetrical charge.
State Coulomb’s law.
The magnitude of the electrostatic force, F, between two point charges, Q and q, is directly proportional to the product of the charges, and inversely proportional to the square of the distance, r, between them.
Define electrical potential.
Electric potential at a point is the work done per unit positive charge to move the charge from infinity to that point
Define electrical potential energy.
Electrical potential energy at a point is the energy required to bring a positive charged particle from infinity to that specific point in the field.
What is a magnetic field?
A region where a moving charge or magnetic material experiences a force.
What do magnetic field lines show?
The direction of the force on a free north pole in that field.
What is magnetic flux density?
The magnetic flux, Φ, per unit area, A, perpendicular to the field lines.
What happens to a charged particle moving perpendicular to a magnetic field?
It undergoes circular motion with radius:
r = mv/Bq
