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Physics A Level Paper 1 > Topic 7: Electric And Magnetic Fields > Flashcards

Topic 7: Electric And Magnetic Fields Flashcards

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1
Q

What is a electric field?

A

Electric field is a force field in which charged particles experience a force.

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2
Q

What is a force field?

A
  • 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
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3
Q

What is the definition and formula for Electric field strength?

A

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

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4
Q

What does Coulomb’s law state and what is the formula?

A
  • 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
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5
Q

How will you know whether the force of an electric field will be repulsive or attractive?

A

If charges are the same, force is repulsive.
If charges are different, force is attractive.

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6
Q

What is the formula for electric field strength in a radial electric field?

A

E.f.sn= Q / 4πε₀r²

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7
Q

What is electric potential?

A

Electric potential is the amount of work done per unit charge at that point.
Greatest at surface of charge, zero at infinity

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8
Q

How could you tell if potential is positive or negative?

A

Positive charge - Positive potential - repulsive force

Negative charge - Negative potential - Attractive force

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9
Q

How can you form a uniform electric field?

A

Use a pair of parallel lates with a p.d across them. Charges move from positive to negative.

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10
Q

What is the formula for electric field strength formed between 2 parallel plates?

A

E.f.s = V/d

V - potential difference
d- distance

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11
Q

Formula for electric potential in a radial field

A

V = Q/4πε₀r

Q - charge
R - distance between charges

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12
Q

What is electric potential difference?

A

Electric potential difference is the energy required to move a unit charge between 2 points.

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13
Q

What is the difference between how electric force in a uniform and radial field exerted?

A
  • 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.
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14
Q

What does the distance between the field lines in a radial field represent?

A

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.

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15
Q

What does the formula of electric field between parallel plates show?

A
  • The greater the voltage, the stronger the e.f.s
  • The greater the distance between parallel plates, the weaker the e.f.s
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16
Q

What are equipotential surfaces?

A

Equipotential surfaces is a surface where potential is the same everywhere; when a charge moves along the surface, no work is done.

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17
Q

What is capacitance and formula?

A

Capacitance is the charge stored by a capacitor per unit p.d
C = Q/V
Capacitance measured in farad

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18
Q

What is the use of a capacitor?

A

Capacitors are electrical devices used to store energy in electronic circuits. Often used as backup release of energy.

19
Q

How do capacitors work?

A
  • 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
20
Q

How does the p.d change across a capacitor once the battery is on?

A

P.d increases exponentially from 0-ϵ.

  • 0V initially as plates begin as uncharged
  • As time goes on, charge increases and p.d increases
21
Q

How to work out energy stored by capacitor and formula?

A

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

22
Q

If energy given out by battery is QV but energy stored in capacitor is 0.5QV, where does the rest of the energy go?

A

Other half lost in resistor; if no resistor, then wires.
Half of energy is always lost as heat due to resistance.

23
Q

How does the current in a capacitor change as it charges?

A

Current decreases exponentially.
- Current starts off large
- Gradually falls to 0 as electrons stop flowing through circuit as capacitor stores max energy

24
Q

How do capacitors discharge?

A

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

25
Q

What does the rate at which the capacitor discharges depend on?

A

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

26
Q

How does the current change when discharging capacitor?

A

When discharging, current travels in opposite direction of charging direction.
- Starts large
- exponentially decreases to 0

27
Q

What is the time constant for a capacitor discharging?

A

Time taken for charge/current/p.d to decrease to 37% of original value.

28
Q

What is the time constant for a capacitor charging?

A

Time taken for charge/p.d capacitor to rise to 63% of max value.

29
Q

What is magnetic flux density (B)?

A

Magnetic flux density is the strength of the magnetic field.
More field lines = more strength
Measured in Tesla
Magnetic field strength

30
Q

Formula for magnetic flux density

A

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

31
Q

What is Flemming’s left hand rule?

A

Thumb represents force
Index represents field
Middle represents current`

32
Q

How to know if magnetic field is in/out of page?

A

Crosses represent into page
Dots represent out of page

33
Q

What is magnetic flux?

A

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.

34
Q

What is magnetic flux linkage?

A

Magnetic lux linkage is the measure of magnetic flux passing through a coil.
ΦN
Φ - magnetic flux
N - number of coils

35
Q

How to calculate magnetic force on charged particle?

A

F = BQv

F = magnetic force on particle
B = magnetic flux density
Q = charge of particle
v = speed of particle

36
Q

Why does a particle not experience magnetic field when parallel to magnetic field?

A

Maximum force on particle is experienced when particle is perpendicular to magnetic field.

37
Q

Why do charged particles move in a circle when near another magnetic field?

A

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.

38
Q

What does a wire with a current passing through it produce?

A

A current-carrying conductor produces its own magnetic field.

39
Q

How to work out force on a current-carrying conductor ?

A

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

40
Q

What is electromagnetic induction?

A

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.

41
Q

What is Faraday’s Law?

A

Faraday’s Law states the magnitude of induced emf is equal to the rate of change of flux linkage.

42
Q

What is Lenz’s law?

A

Lenz’s law states the direction of induced current is such as to oppose direction of magnetic flux that produces it.

43
Q

What are the factors affecting emf induced in a coil in electromagnetic induction?

A

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

Physics A Level Paper 1 (5 decks)

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