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Physics A2 > Term 2 > Flashcards

Term 2 Flashcards

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1
Q

Define capacitance and farad

A

The charge stored on one plate per unit of potential difference between the plates.

The unit of capacitance is the farad. Equivalent to Charge per Volt.

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2
Q

State the formula for capacitance

A

C = Q/V

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3
Q

Describe how the formulas for capacitance is derived for capacitors in parallel and series

A

Parallel:

  • Total charge is the sum of charge going through each capacitor
  • As voltage is constant the capacitance becomes C = C1+ C2…

Series:

  • Total voltage is the sum of the potential differences of the capacitors.
  • As charge is constant capacitance becomes C = 1/C1 + 1/C2
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4
Q

State the formulas required to calculate the work done in charging up a capacitor

A
  • area under graph of p.d against charge is equal to work done
    W = 1/2 QV
    W = 1/2 CV^2
    W = 1/2 (Q^2/C)
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5
Q

Describe what happens if a capacitor is charged and then connected to a second capacitor

A
  • Charge stored is equivalent to charge when the initial capacitor is charged.
  • This means the charge is shared between the capacitors in proportion to their capacitances.
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6
Q

Describe how light dependent resistors work

A
  • made of high resistance semiconductor
  • as light intensity increases resistance decreases, non-linear relationship
  • if light falling on LDR is of high enough frequency then electrons are released from the semiconductor.
  • these free electrons conduct electricity and resistance is decreased.
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7
Q

Describe how thermistors work

A
  • negative temperature coefficient thermistors mean that as temp rises, resistance falls
  • made of semiconductor material
  • relationship is not linear
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8
Q

Describe how piezo electric transducers work and explain their use in microphones

A
  • contains crystals that when force is applied the shape changes and an electric field is produced
  • consists of positive and negative ions in regular arrangement so when stressed a small voltage is produced.
  • microphones contain the crystal which is made into a thin sheet with metal connections on opposite ends.
  • Sound wave causes compressions and rarefactions causing pressure to increase and decrease and small voltage is created across connections.
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9
Q

Describe how a metal wire strain gauge works

A
  • As wire is stretched metal wire becomes longer and narrower increasing electrical resistance
  • opposite occurs if wire is compressed
  • change in length of wire is directly proportional to change in resistance.
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10
Q

Describe the properties of an ideal op-amp

A

Infinite open loop voltage gain: With no feedback loop then a small input signal will be amplified to an ‘infinite’ output signal resulting in output being saturated.

Infinite input resistance: no current is drawn from the supply meaning there are no lost volts and input voltage is max.

Zero output resistance: no lost volts in current supplied by op amp

Infinite bandwidth: Range of frequencies that are amplified by the same amount. Ideally all signals of all frequencies are amplified the same.

Infinite slew rate: Means output is produced instantaneously as input is changed. No time delay.

Zero noise contribution: Does not produce any noise itself

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11
Q

State the formula for open loop gain

A

G = Vout/ (V+ - V-)

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12
Q

Describe the use of op amps as a comparator

A
  • compares V+ and V-
  • if V+ is slightly greater than V- then Vout will be equal to positive power supply voltage and vice versa.
  • Tells us which input is larger
  • Can also be used to compare light levels or temperatures if LDR or thermistor is used in place of fixed resistor.
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13
Q

State the formula for closed loop gain

A

G = Go/ (1+Go)

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14
Q

State the benefits of using negative feedback

A
  • less distortion
  • increased bandwidth
  • gain is more stable
  • output impedance is low and input impedance is high.
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15
Q

Describe the virtual earth approximation

A
  • Vin must be very close to 0 so that the op-amp is not saturated
  • this is required in inverting amplfiers
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16
Q

State the formula for gain in an inverting amplifier

A

G = -Rf/Rin

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17
Q

State the formula for gain in a non-inverting amplifier

A

G = 1 + (Rf/Rg)

18
Q

Describe the use of relays as output devices and explain how the op amp is protected from it

A
  • electromagnetic switch operated by small current
  • magnetic field attracts magnetic field causing contacts to be connected completing a second circuit that contain larger voltages or currents
  • when current is turned off a large change in magnetic flux causes a large emf to be produced by relay causing damage to op-amp
  • reverse biased diode is placed across relay coil allowing current to flow without damaging op-amp.
  • diode is also used to ensure current only flows through one end.
19
Q

Describe the use of light emitting diodes as output devices

A
  • Show whether state of output is positive, negative or zero
  • must be placed in series with resistor if diode is between zero-volt line and output of op-amp so that voltage entering diode is decreased and diode is not damaged.
20
Q

Describe the use of calibrated meters as output devices

A
  • used to measure physical quantities
  • voltmeter must be calibrated in terms of physical quantity
  • calibration curve is used to change the voltmeter reading into value for physical quantity
21
Q

State the formula for the force on a conductor

A

F = BILsin()

22
Q

Define magnetic flux density

A

Force experienced per unit length by a long straight conductor carrying unit current and placed at right angles to the field at that point. Unit is tesla which is equivalent to 1 N/ Am

23
Q

State the formula for the magnetic force on a moving charge

A

F = BQv sin ()

angle is angle the charged particle is moving to the magnetic field.

24
Q

Describe how a hall voltage is formed

A
  • a small current flows through the probe from one end to the other meaning when a magnetic field is applied electrons are pushed sideways and so accumulate along one side of the probe
  • This is the hall effect and a hall voltage is generated.
  • Greater flux density means greater Hall voltage.
25
Q

State the formula for the Hall voltage

A

VH = BI/nte where t is the thickness of the slice

26
Q

State the formula for the radius of the movement of a charged particle in a uniform magnetic field

A

r = mv/Be

27
Q

State the formula for the charge to mass ratio of an electron

A

e/me = v/Br

28
Q

Describe how a velocity selector works

A
  • two parallel plates are placed in an evacuated chamber providing a uniform electric field
  • region between plates also contains uniform magnetic field at right angles to electric field
  • due to F = Bev, electrons that move faster will have greater force.
  • if electrons have specific v then electric and magnetic field will be balanced and particle is undeflected.
29
Q

Describe how magnetic resonance imaging works

A
  • nuclei of certain atoms have spin that causes them to act as tiny magnets in a magnetic field.
  • hydrogen is used in MRI
  • with strong magnetic field the protons line up in the field
  • magnetic axis of proton rotates around direction of external field, spinning is known as precession.
  • pulse of RF waves causes resonance of proton’s precession.
  • the time taken for the protons to return to lower energy state. Rate of relaxation can be used to distinguish different tissues.
30
Q

Describe the Larmor frequency and state its formula

A

Angular frequency of precession
w = γBo
- γ is gyromagnetic ratio and is measure of magnetism

31
Q

Define magnetic flux and magnetic flux linkage

A

Product of magnetic flux density normal to an area and this area A

Product of magnetic flux and number of turns in the coil

32
Q

State the formula for magnetic flux and magnetic flux linkage

A

Φ = BA, unit is Weber

Magnetic flux linkage = NΦ or BAN cos()

33
Q

State Faraday’s Law of EM induction and state its formula

A

Magnitude of induced emf is proportional to rate of change of magnetic flux linkage

E = Δ(N)Φ/Δt

34
Q

State Lenz’s law

A

Any induced current or emf will be established in a direction so as to produce effects which oppose the change that is producing it

35
Q

State the formulas for alternating current and voltage

A

I = Io sin wt

V = Vo sin wt

36
Q

Describe the root mean square value of alternating current

A
  • steady current which delivers same average power as a.c to resistive load.
  • I(rms) = Io/root 2
37
Q

State the formulas involved in transformers

A

ns/np = Vs/Vp

Ps = Pp so VsIs = VpIp

Ip/Is = Vs/Vp

38
Q

Describe how transformers work

A
  • AC is supplied to primary coil and produces varying magnetic field in soft iron core
  • magnetic flux linkage in secondary coil is constantly changing meaning varying emf is induced
39
Q

State how transformers can be made more efficient

A
  • use of low resistance wire coils to reduce heating
  • use of laminated iron core to reduce currents induced in core itself (eddy currents)
  • use of soft iron core that magnetises/demagnetises easily.
40
Q

State why AC is used for electricity supply

A
  • can be transformed to high voltages so current flowing is reduced leading to less power losses
  • reduces costs as more power can be generated with less fuel
41
Q

Define rectification

A

The process of converting a.c to d.c

42
Q

Describe smoothing

A
  • capacitor placed in parallel with load resistor
  • capacitor discharges gradually allowing voltage to rise again
  • this means a more steady d.c is formed.

Physics A2 (3 decks)

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