Electricity Flashcards
Define capacitance
The charge stored on one plate per unit of potential difference between the plates. (C = Q/V)
Describe the process by which energy is stored on a capacitor
Work is done to push additional negative electrons towards the negative plate of the capacitor, against the repulsion of electrons already present, to increase the charge on the negative plate.
Energy stored in a capacitor (E) =
Area under a voltage-charge graph
E = 1/2 VQ
E = 1/2 V^2 C
E = 1/2 (Q^2 / C)
Total capacitance for capacitors in parallel =
Capacitance in parallel is added (C total = C1 + C2).
Total increases.
Total capacitance for capacitors in series =
1/C total = 1/C1 + 1/C2
Decreases total.
Define an electric field
A field of force created by an electric charge.
State Coulomb’s law
Any 2 point forces exert a force on each other which is directly proportional to the product of their charges, and inversely proportional to the square of their separation. (F = (Q1Q2)/(4piE0 r^2) )
Electrical field strength of a point charge in a radial field (E) =
E = F/Q = V/d E = Q / (4piE0 r^2)
State and explain the relationship between potential energy and distance moved in an electric field
Potential energy and distance moved against the field lines are directly proportional.
Work is done to move the charge against the field lines.
Therefore when a charged particle moves against the field lines it gains electrostatic potential energy.
Define the electric potential at a point
The work done in bringing unit positive charge from infinity to that point.
Potential gradient (m) =
Gradient on an electric potential - distance graph
m = - V/r
Potential gradient = -field strength (m = - E)
Define root-mean-square current (Irms = )
The steady current which delivers the same average power as the a.c. to the resistive load. (rms = I0/rt2)
Explain the advantage of using alternating current
The voltage can be changed (e.g. Stepped up or down)
Explain the advantage of using higher voltages
The flow of current is reduced
So less power is lost as heat
State the turns-ratio equation
Np/Ns = Vp/Vs
Explain the source of energy loss in a transformer
1) Resistance of windings (small amount of energy lost as heat)
2) flux leakage (but only if designed badly)
3) Eddy currents (induced in the iron core by the magnetic field)
- cause energy to be lost as heat
- can be reduced by having core laminated
Define rectification
The process of changing AC to DC using a diode.
Explain the disadvantage of half-wave vs full-wave rectification
In half-wave rectification, the input power is reduced, because for half of the time voltage = 0.
Explain the process of ‘smoothing’ (alternating current)
A capacitor is incorporated in parallel to the resistor R
The capacitor charges while voltage is present, and discharges when voltage = 0.
Produces an output voltage with a ripple.
Define a sensing device
An electronic component with a property which changes when a physical quantity is changed.
Define the piezo electric effect
When a force is applied to piezo-electric crystals, causing the crystals to change shape, an electric field is produced.
Explain the source of the piezo electric effect
Piezo electric crystals consist pf a regular arrangement of positive and negative ions.
When the crystal is stressed, the movement of ions will produce a small voltage.
Describe the function of a piezo electric transducer in a microphone
The sound wave hits the side of the sheet
Compressions and rarefactions cause the pressure to increase and decrease.
The crystal changes shape and a small voltage is produced.
Describe the way in which LDRs operate
Made of a high resistance semi-conductor
If the light falling on the LDR is of a high enough frequency, photons will be absorbed.
These will cause electrons to be released from atoms in the LDR.
These free electrons can conduct electricity, so resistance is decreased.
Describe the operation of an NTC
(Negative temperature coefficient)
Made from a semi-conductor material
As temperature rises, number of free electrons increases, so resistance decreases.
Describe a metal-wire strain gauge (🔺R = )
Measures changes in resistance due to changes in length and cross-sectional area. (🔺R = pl/A)
Describe how LDRs and thermistors can be incorporated in an electric circuit, to function as sensing devices
The LDR/thermistor is placed in series with a fixed resistor (forming a potential divider).
The voltage is shared in proportion to the resistance.
Vout = Vin (R2 /(R1+R2))
Define the gain of an op-amp
The magnitude by which the input voltage is amplified (G0 = output voltage / input voltage).
State the properties of an ideal op-amp (6)
- Infinite open-loop voltage gain
- Infinite input resistance / impedance
- Zero output resistance / impedance
- Infinite bandwidth
- Infinite slew rate
- Zero noise contribution
State an equation for the op-amp as a comparator
Vout = G0 (V+ ➖ V-)
Define negative feedback
When the output of an op-amp is connected to the inverting (-) input
So as to reach a point of equilibrium.
Closed loop gain (G) =
G = Vout/Vin = G0/1+G0
State the benefits of negative feedback
1) Less distortion
2) increased bandwidth
3) A more stable gain,
4) A low output impedance and high input impedance
Describe an inverting amplifier
The non-inverting input is connected to the 0V line.
Vin is connected to the inverting input.
Part of the output voltage is also connected to the inverting input.
Explain the virtual earth approximation
Point connecting the output voltage with Vin is referred to as a virtual earth.
Because the gain is very large (so difference between V(-) and V(+) is around 0)
V(+) is earthed, so both points must have almost 0 voltage.
State an equation for the gain of an inverting amplifier
G = - Rf/Rin
State an equation for the gain of a non-inverting amplifier
G = 1 + (R1/R2)
Define a relay
An electromagnetic switch used to switch on a large current circuit using a small current.
Explain the purpose of the reverse-biased diode (D1) in an electric circuit
Switching off a relay can damage an op-amp, because there will be a rapid fall in the magnetic flux within the coil, inducing a large e.m.f.
D1 can pass current around the coil without damaging the op-amp
Explain the purpose of resistors in a circuit containing LEDs
Resistors added in series to limit current reaching diode,
Because large currents can damage LEDs
