Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Physics A Level CIE > Chapter 10 Alternating Currents > Flashcards

Chapter 10 Alternating Currents Flashcards

1
Q

An alternating current (a.c) is defined as

A
  • A current which periodically varies from positive to negative and changes its magnitude continuously with time
  • This means the direction of an alternating current varies every half cycle
  • The variation of current, or p.d., with time can be described as a sine curve ie. sinusoidal
  • Therefore, the electrons in a wire carrying a.c. move back and forth with simple harmonic motion
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

As with SHM, the relationship between time period T and frequency f of an alternating current is given by:

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Peak current (I0), or peak voltage (V0), is defined as:

A

The maximum value of the alternating current or voltage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Peak current, or voltage, can be determined from the amplitude of the graph

A
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Mains electricity is supplied as alternating current

A
  • Power stations produce alternating current
  • This is the type of current supplied when devices are plugged into sockets
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

The equation representing alternating current which gives the value of the current I at any time t is:

A
  • I = I0 sin(⍵t)
  • Where:
    • I = current (A)
    • I0 = peak current (A)
    • ⍵ = angular frequency of the supply (rad s-1)
    • t = time (s)
    • Note: this a sine function since the alternative current graph is sinusoidal
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

A similar equation can be used for representing alternating voltage:

A

V = V0 sin(⍵t)

Where:
V = voltage (V)
V0 = peak voltage (V)
Recall the relation the equation for angular frequency ⍵:

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Root-Mean-Square Current

A
  • Root-mean-square (r.m.s) values of current, or voltage, are a useful way of comparing a.c current, or voltage, to its equivalent direct current, or voltage
  • The r.m.s values represent the d.c current, or voltage, values that will produce the same heating effect, or power dissipation, as the alternating current, or voltage
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q
  • The r.m.s value of an alternating current is defined as:
A

The value of a constant current that produces the same power in a resistor as the alternating current

  • The r.m.s current Ir.m.s is defined by the equation:
  • So, r.m.s current is equal to 0.707 × I0, which is about 70% of the peak current I0
  • The r.m.s value of an alternating voltage is defined as:

The value of a constant voltage that produces the same power in a resistor as the alternating voltage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q
  • The r.m.s voltage Vr.m.s is defined by the equation:
A
  • Where:
    • I0 = peak current (A)
    • V0 = peak voltage (V)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q
  • The r.m.s value is therefore defined as:
A

The steady direct current, or voltage, that delivers the same average power in a resistor as the alternating current, or voltage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q
  • A resistive load is any electrical component with resistance eg. a lamp
A

Vr.m.s and peak voltage. The r.m.s voltage is about 70% of the peak voltage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Mean Power

A
  • In mains electricity, current and voltage are varying all the time
  • This also means the power varies constantly, recall the equations for power:
  • Where:
    • I = direct current (A)
    • V = direct voltage (A)
    • R = resistance (Ω)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

The r.m.s values means equations used for direct current and voltage can now be applied to

A
  • alternating current and voltage
  • These are also used to determine an average current or voltage for alternating supplies
  • Recall the equation for peak current:

l0= √2 Ir.m.s

  • Therefore, the peak (maximum) power is related to the mean (average) power by:

Pmean= Ir.m.sR

Pmean= P/2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

The mean power in a resistive load

A

is half the maximum power for a sinusoidal alternating current or voltage

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q
  • Rectification is defined as:
A

The process of converting alternating current and voltage into direct current and voltage

17
Q

Rectification is used in electronic equipment which requires a

A

direct current

  • For example, mains voltage must be rectified from the alternating voltage produced at power stations
18
Q

There are two types of rectification

A
  • Half-wave rectification
  • Full-wave rectification
19
Q

For half-wave rectification

A
  • The graph of the output voltage Vout against time is a sine curve with the positive cycles and a flat line (Vout = 0) on the negative cycle
  • This is because the diode only conducts in the positive direction
20
Q

For full-wave rectification:

A
  • The graph of the output voltage Vout against time is a sine curve where the positive cycles and the negative cycles are both curved ‘bumps’
21
Q

The difference between the graphs of full-wave and half-wave rectification

A
22
Q

Half-wave rectification consists of a

A
  • single diode
    • An alternating input voltage is connected to a circuit with a load resistor and diode in series
  • The diode will only conduct during the positive cycles of the input alternating voltage,
    • Hence there is only current in the load resistor during these positive cycles
  • The output voltage Vout across the resistor will fluctuate against against time in the same way as the input alternating voltage except there are no negative cycles
  • This type of rectification means half of the time the voltage is zero
  • So, the power available from a half-wave rectified supply is reduced
23
Q

Full-wave rectification requires a

A
  • bridge rectifier circuit
    • This consists of four diodes connected across an input alternating voltage supply
  • The output voltage Vout is taken across a load resistor
  • During the positive cycles of the input voltage, one terminal if the voltage supply is positive and the other negative
    • Two diodes opposite each other that are in forward bias will conduct
    • The other two in reverse bias will not conduct
    • A current will flow in the load resistor with the positive terminal at the top of the resistor
  • During the negative cycles of the input voltage, the positive and negative terminals of the input alternating voltage supply will swap
    • The two diodes that were forward bias will now be in reverse bias and not conduct
    • The other two in reverse bias will now be in forward bias and will conduct
    • The current in the load resistor will still flow in the same direction as before
24
Q

In full-wave both the positive and negative cycles s, the current in the load resistor is the…

A
  • same
  • Each diode pair is the same as in half-wave rectification
    • Since there are two pairs, this equates to full-wave rectification overall
  • The main advantage of full-wave rectification compared to half-wave rectification is that there is more power available
    • Therefore, a greater power is supplied on every half cycle
25
Q

When A is positive and B is negative, diodes 2 and 3 will conduct and 1 and 4 will not. When A is negative and B is positive, diodes 1 and 4 will conduct and diodes 2 and 3 will not. The current in the load resistor R will flow downwards

A
26
Q

Smoothing

A
  • In rectification, to produce a steady direct current or voltage from an alternating current or voltage, a smoothing capacitor is necessary
  • Smoothing is defined as:

The reduction in the variation of the output voltage or current

27
Q

Smoothing works in the following ways:

A
  • A single capacitor with capacitance C is connected in parallel with a load resistor of resistance R
  • The capacitor charges up from the input voltage and maintains the voltage at a high level
  • As it discharges gradually through the resistor when the rectified voltage drops but the voltage then rises again and the capacitor charges up again
  • The resulting graph of a smoothed output voltage Vout and output current against time is a ‘ripple’ shape
28
Q

A smoothing capacitor connected in parallel with the load resistor. The capacitor charges as the output voltage increases and discharges as it decreases

A
29
Q

A smooth, rectified current graph creates a ‘rippling’ shape against time

A
30
Q

The amount of smoothing is controlled by the

A
  • capacitance C of the capacitor and the resistance R of the load resistor
    • The less the rippling effect, the smoother the rectified current and voltage output
  • The slower the capacitor discharges, the more the smoothing that occurs ie. smaller ripples
  • This can be achieved by using:
    • A capacitor with greater capacitance C
    • A resistance with larger resistor R
  • Recall that the product RC is the time constant τ of a resistor
  • This means that the time constant of the capacitor must be greater than the time interval between the adjacent peaks of the output signal

Physics A Level CIE (14 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions