Topic 7 - Electric and Magnetic Fields Flashcards
What is a force field?
A region in which a non-contact force is experienced by a corresponding interacting particle
What is an electric field?
A region in which a charged particle will experience a non-contact force
What is electric field strength?
The force per unit charge experienced by a charged particle when placed at that point in the field
Force per unit charge acting on a small positive charge
State the equation used to calculate the force a charge experiences in an electric field.
Force = electric field strength x charge
F = EQ
What law determines the magnitude of the electric force between two charges?
Coulomb’s Law
State Coulomb’s law in words.
The force between two charges is directly proportional to the product of their charges and inversely proportional to the square of their separation
Give Coulomb’s law in equation form.
F = (kQq) / r^2
Where k = 1 / 4πε0
Describe the electric field pattern around a positive point charge.
A radial field, acting outwards
Describe the electric field pattern around a negative point charge
A radial field, acting inwards
Describe the uniform electric field between parallel plates.
- straight lines of equal spacing
- from positive to negative
What equation is used to calculate the electric field between parallel plates?
E = V/d
What is electric potential?
The amount of energy required to move a positive test charge from infinity to a given point in an electric field
How do you calculate electric potential for a radial field?
F = kQ / r
Where k = 1 / 4πε0
What are equipotentials?
Equipotentials are lines along which the electric potential remains the same
How do you calculate the work done in a moving charge along an equipotential?
No work is done when moving a charge along an equipotential since the electric potential doesn’t change
What does a capacitor do?
Capacitors are used to store charge in a circuit
What two factors determine how much charge can be stored by a capacitor?
- The potential difference across it
- The capacitance
What is capacitance?
Capacitance is a measure of how much charge can be stored by a capacitor per unit potential difference across it
State the equation used to calculate the capacitance from the charge and potential difference.
Capacitance = charge / potential difference
C = Q/V
What is the unit of capacitance?
CV^-1
F (farad)
What is represented by the area under a capacitor’s charge-potential graph?
The energy stored by a capacitor
What is represented by the gradient of a capacitor’s charge-potential graph?
Capacitance
Give three equations to calculate the energy stored by a capacitor.
W = 1/2 QV
W = 1/2 CV^2
W = (1/2 Q^2) / C
Describe a charge-time graph for charging a capacitor. What does the gradient represent?
- positive curve (like top left corner of circle)
- plateaus off
- gradient = current
Describe a current-time graph for charging a capacitor. What does the area represent?
- negative curve (like bottom left corner of a circle)
- exponential
- area = charge
Describe a voltage-time graph for charging a capacitor.
- positive curve (like top left corner of a circle)
- plateaus off
What value is given by the product of resistance and capacitance?
The time constant of the circuit
What does the time constant of a circuit tell you?
The time the capacitor will take to reach 63% of its total charge, and the time taken for it to discharge to 37% of its full charge
What is a magnetic field?
A magnetic field is a region in which a magnetic pole will experience a non-contact force
In which direction do magnetic field lines point?
From north to south
What is magnetic flux density?
Magnetic flux density is a measure of the strength of a magnetic field. It can be viewed as the number of magnetic field lines that pass through a given area
What three factors determine the force exerted on a charge moving through a magnetic field?
- the magnetic flux density
- the charge of the particle
- the velocity of the particle perpendicular to the field
State the equation used to calculate the force exerted on a charge moving through a magnetic field.
F = Bqvsinθ
What is the relationship between the direction of a charge’s motion and the direction of the magnetic force it experiences?
The force is always perpendicular to the charge’s motion
Describe and explain the path taken by a charge in a magnetic field.
The charge will move in a circular path. This is because the magnetic force always acts perpendicular to the charge’s motion and so acts as a centripetal force
What is produced by a current-carrying wire?
A magnetic field is produced in concentric circles around a wire when a current passes through it
What occurs when a current-carrying wire is placed in a magnetic field?
The wire will experience a force due to the permanent magnetic field interacting with a the wire’s magnetic field
What three factors affect the force experienced by a current-carrying wire placed in a magnetic field?
- the magnetic flux density of the wire
- the current passing through the wire
- the length of the wire
State the equation used to calculate the force experienced by a current-carrying wire.
F = BILsinθ
What is Fleming’s left-hand rule used for?
To determine the direction of the force experienced by a current-carrying wire or moving charge in a magnetic field
What does the thumb represent when using Fleming’s left-hand rule?
The thumb represents the direction of the force
What does the first finger represent when using Fleming’s left-hand rule?
The direction of the field
What does the second finger represent when using Fleming’s left-hand rule for a moving charge?
The direction that a positive charge would move. This means that if it is a negative charge, you must point your second finger on the opposite direction to its motion
What does the second finger represent when using Fleming’s left-hand rule for a current-carrying wire?
The direction of conventional current flow
What is electromagnetic induction?
When a conducting rod moves relative to a magnetic field, the electrons in the rod will experience a force (as they are charged particles), and build up on one side of the rod, causing an emf to be induced in the rod
What is Faraday’s law?
The magnitude of induced emf is equal/directly proportional to the rate of change of flux linkage
ε = N (ΔΦ) / (Δt)
What is Lenz’s law?
The induced emf is always in such a direction as to oppose the change/motion that caused it
ε = -d(NΦ) / dt
