Topic 7: Electric And Magnetic Fields Flashcards
What is a electric field?
Electric field is a force field in which charged particles experience a force.
What is a force field?
- A force field is an area where an object experiences a non-contact force.
- Represented as vectors to show directions.
- Represented in diagrams by field lines
What is the definition and formula for Electric field strength?
Electric field strength is the force per unit charge. Describes how strong/weak an electric charge is at a certain point.
E = F/Q
Constant in uniform fields; varies in radial fields
What does Coulomb’s law state and what is the formula?
- Coulomb’s Law states that the magnitude of the force between 2 charges is inversely proportional to the square distance between them. The charges electric fields interact causing a force.
- F=Q₁Q / 4πε₀r²
How will you know whether the force of an electric field will be repulsive or attractive?
If charges are the same, force is repulsive.
If charges are different, force is attractive.
What is the formula for electric field strength in a radial electric field?
E.f.sn= Q / 4πε₀r²
What is electric potential?
Electric potential is the amount of work done per unit charge at that point.
Greatest at surface of charge, zero at infinity
How could you tell if potential is positive or negative?
Positive charge - Positive potential - repulsive force
Negative charge - Negative potential - Attractive force
How can you form a uniform electric field?
Use a pair of parallel lates with a p.d across them. Charges move from positive to negative.
What is the formula for electric field strength formed between 2 parallel plates?
E.f.s = V/d
V - potential difference
d- distance
Formula for electric potential in a radial field
V = Q/4πε₀r
Q - charge
R - distance between charges
What is electric potential difference?
Electric potential difference is the energy required to move a unit charge between 2 points.
What is the difference between how electric force in a uniform and radial field exerted?
- Electric force exerted in a uniform field the same everywhere in the field; shown by parallel and equally spaced field lines.
- Electric force exerted in a radial field depends on the distance between 2 charges.
What does the distance between the field lines in a radial field represent?
The distance between the field lines signify the magnitude of the force;
E.g in the radial field, as a charge moves further away from the centre, the distance between the field line increases; force decreases.
What does the formula of electric field between parallel plates show?
- The greater the voltage, the stronger the e.f.s
- The greater the distance between parallel plates, the weaker the e.f.s
What are equipotential surfaces?
Equipotential surfaces is a surface where potential is the same everywhere; when a charge moves along the surface, no work is done.
What is capacitance and formula?
Capacitance is the charge stored by a capacitor per unit p.d
C = Q/V
Capacitance measured in farad
What is the use of a capacitor?
Capacitors are electrical devices used to store energy in electronic circuits. Often used as backup release of energy.
How do capacitors work?
- When the capacitor is connected to power supply, current starts to flow and negative charge builds up on plate connected to negative terminal
- On opposite plate, electrons repelled by negative charge building up on initial plate
- These electrons move to positive terminal
- Equal and opposite charge formed on each plate, creating potential difference
How does the p.d change across a capacitor once the battery is on?
P.d increases exponentially from 0-ϵ.
- 0V initially as plates begin as uncharged
- As time goes on, charge increases and p.d increases
How to work out energy stored by capacitor and formula?
Energy stored by capacitor is area under a graph of charge against p.d.
W= 0.5x QV
0.5 as graph is right-angle triangle
If energy given out by battery is QV but energy stored in capacitor is 0.5QV, where does the rest of the energy go?
Other half lost in resistor; if no resistor, then wires.
Half of energy is always lost as heat due to resistance.
How does the current in a capacitor change as it charges?
Current decreases exponentially.
- Current starts off large
- Gradually falls to 0 as electrons stop flowing through circuit as capacitor stores max energy
How do capacitors discharge?
When the power supply is disconnected, capacitor starts to discharge through the resistor.
- Electrons flow back from negative plate to positive terminal
- until p.d across plates =0
What does the rate at which the capacitor discharges depend on?
The time taken for capacitor discharges depends on resistance of circuit.
- If resistance is high, current will decrease **slowly
- charge will flow from capacitor plates more slowly
- capacitor takes longer to discharge
How does the current change when discharging capacitor?
When discharging, current travels in opposite direction of charging direction.
- Starts large
- exponentially decreases to 0
What is the time constant for a capacitor discharging?
Time taken for charge/current/p.d to decrease to 37% of original value.
What is the time constant for a capacitor charging?
Time taken for charge/p.d capacitor to rise to 63% of max value.
What is magnetic flux density (B)?
Magnetic flux density is the strength of the magnetic field.
More field lines = more strength
Measured in Tesla
Magnetic field strength
Formula for magnetic flux density
F = BIL
B = F/IL
B = magnetic flux density (T)
F = magnetic force on current-carrying wire
I = current (direction of positive charge)
L = length of wire
What is Flemming’s left hand rule?
Thumb represents force
Index represents field
Middle represents current`
How to know if magnetic field is in/out of page?
Crosses represent into page
Dots represent out of page
What is magnetic flux?
Magnetic flux is a value describing the magnetic field lines passing through a given area.
When field is perpendicular to area.
How a conducting material is affected by a magnetic field
Φ = BA
measurement of the total magnetic field which passes through a given area.
What is magnetic flux linkage?
Magnetic lux linkage is the measure of magnetic flux passing through a coil.
ΦN
Φ - magnetic flux
N - number of coils
How to calculate magnetic force on charged particle?
F = BQv
F = magnetic force on particle
B = magnetic flux density
Q = charge of particle
v = speed of particle
Why does a particle not experience magnetic field when parallel to magnetic field?
Maximum force on particle is experienced when particle is perpendicular to magnetic field.
Why do charged particles move in a circle when near another magnetic field?
Force exerted is always perpendicular to the motion of travel
- causes charged particles to follow a circular path when in a magnetic field, because the force induced by the magnetic field acts as a centripetal force.
What does a wire with a current passing through it produce?
A current-carrying conductor produces its own magnetic field.
How to work out force on a current-carrying conductor ?
F = BIL sinΘ
F - magnetic force on current-carrying conductor
B - magnetic flux density of external magnetic field
I = current in conductor
L - length of conductor in field
Θ - angle between conductor and external flux lines
What is electromagnetic induction?
Electromagnetic induction is when an emf is induced when a conductor moves through a magnetic field.
Occurs when there is a change in magnetic flux.
What is Faraday’s Law?
Faraday’s Law states the magnitude of induced emf is equal to the rate of change of flux linkage.
What is Lenz’s law?
Lenz’s law states the direction of induced current is such as to oppose direction of magnetic flux that produces it.
What are the factors affecting emf induced in a coil in electromagnetic induction?
Number of turns
- directly proportional to induced emf
Magnetic flux density
- directly proportional to induced emf
Area of cross section of coil
- directly proportional to induced emf
Time taken for motion
- inversely proportional to induced emf
