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Science > Electrical Systems - Explanations - Level 3 > Flashcards

Electrical Systems - Explanations - Level 3 Flashcards

1
Q

How can the voltage of a resistor be changed?

A

The voltage of a resistor (Vr) can be changed by using a rheostat (variable resistor), which will change the total resistance of the circuit.

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2
Q

How can r and E be calculated?

A
  1. Take 5 readings for Vr and I
  2. Draw graph between Vr and I
  3. Use Vr = -rI + E
    (from form mx+c, with a negative gradient)
  4. Gradient of graph is equal to r
  5. y-intercept of the graph is equal to E
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3
Q

What is magnetic flux in the secondary coil (S) directly proportional to?

A

The magnetic flux in the secondary coil (S) is directly proportional to the changing current (I) in the primary coil (P).

(directions can be CW or ACW)

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4
Q

What happens when a coil rotates in a magnetic field?

A

When a coil rotates in a magnetic field, an A/C voltage and a current is induced at the two ends of the coil. When the coil rotates through 2π, the current will have two maximum and two minimum (0) values. Current will start at 0V then increase to a maximum value then back to 0V as it rotates through π. After rotating through π, the current will then increases to a maximum value (that is negative due to the reversed direction) then back to 0V as it rotates through 2π.

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5
Q

When is the induced current of a coil in a magnetic field maximum and minimum?

A

Current is maximum when the motion of the coil is perpendicular to the magnetic field and is minimum when the motion of the coil is parallel.

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6
Q

Why does current build up slowly?

A

V = -△Φ/△t
From the equation, back emf is created. The back emf opposes the supplied voltage and current (lenz law). This causes the current to slow down, taking more time to reach its maximum value.

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7
Q

How can the supply voltage or current of a generator be increased?

A

To increase the supply voltage or current of the generator, the magnetic field (B), number of turns (N) and the angular velocity (ω) of the coil can be increased.

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8
Q

What does the constant of mutual induction (M) depend on?

A
  1. The size of the coils
  2. The distance between the coils
  3. The material inside the coils
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9
Q

How can the magnetic flux be changed?

A

The magnetic flux can be changed by moving the coil in the magnetic field. Magnetic flux increases when the coil is moved into the magnetic field and decreased when it is moved out of the magnetic field.

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10
Q

How can power loss be reduced in a transformer?

A

Transformers are used in power transmission. Power is produced at a low voltage, converted to a higher voltage in power lines and then converted to a lower voltage before use. Power loss can be reduced in this process by…

  1. Lowering the current in the transmission lines (P = I^2R)
  2. Lowering the resistance in the power lines by using different metal conductors (Al, Cu)
  3. Lowering the resistance within the transformer
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11
Q

Why does bulb A in parallel connected directly to the power supply turn off suddenly, but bulb B in parallel connected directly to the inductor turn off gradually?

A

Self-inductance becomes effective when the switch is opened, therefore, when the lights are switched off, bulb A will go off suddenly, while bulb b will go off gradually.

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12
Q

When is self-inductance effective?

A

Self-inductance is only effective when…

  1. The switch in the circuit is opened
  2. The current in the coil is changing
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13
Q

Resistance in inductors

A

Inductors can have resistance since they are coils. When the current becomes steady in the circuit, only the resistance in the coil is effective.

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14
Q

What does the self-inductance of a coil depend on?

A

The self-inductance a the coil (L) is constant for the coil that depends on the number of turns (N) and the size of the coil (xN).

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15
Q

What happens when the DC current changes?

A

When the DC current changes, the inductor can absorb and store energy (E), which is then stored in the magnetic field it creates.

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16
Q

Inductor graph for powering up (current and voltage)

A
  1. Current
    At t = 0, current (I) = 0, it then increases until current (I) = maximum
  2. Voltage
    At t = 0, voltage (V) = maximum, it then decreases until voltage (V) = 0
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17
Q

What occurs when an inductor is powering up (switch is turned on)

A
  • When the inductor is powering up, there is a changing current which results in a changing magnetic field and changing magnetic flux. This causes a voltage to be induced (V = -L △I/△t), which is directed to oppose the changing current within itself.
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18
Q

Inductor graph for powering down (current and voltage)

A
  1. Current
    At t = 0, current (I) = maximum, it then decreases until current (I) = 0
  2. Voltage
    At t = 0, voltage (V) = maximum, it then decreases until voltage (V) = 0
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19
Q

What occurs when an inductor is powering down (switch is turned off)

A
  • When the inductor is powering down, there is a changing current which results in a changing magnetic field and changing magnetic flux. This causes a voltage to be induced (V = L △I/△t), which is directed to follow the changing current within itself.
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20
Q

How can an inductor be used to produce a larger maximum voltage?

A

Using an inductor with a high resistance will decrease the time constant (τ = L/R) and increase the rate of change of current, hence producing a larger maximum voltage.

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21
Q

Resistor graph for powering on (current and voltage)

A
  1. Current
    At t = 0, current (I) = 0, it then increases until current (I) = maximum
  2. Voltage
    At t = 0, voltage (v) = 0, it then increases until voltage (v) = maximum
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22
Q

Resistor graph for powering down (current and voltage)

A
  1. Current
    At t = 0, current (I) = maximum, it then decreases until current (I) = 0
  2. Voltage
    At t = 0, voltage (V) = maximum, it then decreases until voltage (V) = 0
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23
Q

What occurs when a capacitor is powering up (switch is turned on)

A
  • When a capacitor is powering up (being connected to a battery), charge builds up which creates an electric field between the plates (V = Ed). At first, the voltage increases quickly, then the increase in voltage slows as the voltage across the capacitor rises and opposes the voltage from the battery. As the charge continues to build up, it becomes more difficult to add a further charge into the capacitor. Eventually the equilibrium is established when the capacitor gains the maximum charge (Q) and the voltage (V).
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24
Q

Uses of a capacitor

A
  1. Tuning radio and TV circuits to receive signals
  2. Storing charges a supplying a quick current (car indicators, camera flashes)
  3. Smoothing the output voltage of a rectifier that is producing a DC current
  4. Converting AC to DC in a power supply
  5. Absorbing harmful currents to safeguard when appliances short circuit
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25
Q

What does capacitance depend on?

A

Capacitance depends on…

  1. The area of the parallel plates (C ∝ A)
    - Plates with a greater area are able to store more charge
  2. The distance between the plates (C ∝ 1/d)
    - Plates that are closer together are able to store more charge
  3. The properties of the dielectric material between the plates (C = (εo A)/d)
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26
Q

What happens to a capacitor when charge comes from the power supply?

A

When a capacitor is charged, charge (q) comes from the power supply at a voltage (V) and transfers energy to the capacitor. This energy is stored as potential energy (Ep = qV) by the charges on the plates. When the capacitor is discharged, the energy is dissipated in the resistance of the circuit as heat, light, etc.

27
Q

How can a capacitor be charged? - Resistor

A

A capacitor can be charged through a resistor. The voltage in the capacitor will build up faster at the beginning and then slow down over time (during the discharge, voltage in the capacitor will drop faster at the beginning and then slow down).

28
Q

How to tell the value of a graph when it is increasing/decreasing?

A
  • When a graph is increasing, the x-value at each time constant will be 63%
  • When a graph is decreasing, the x-value at each time constant will be 37%
29
Q

Capacitor graph for powering up (current and voltage)

A
  1. Current
    At t = 0, current (I) = maximum, it then decreases until current (I) = 0
  2. Voltage
    At t = 0, voltage (V) = 0, it then increases until voltage (V) = maximum
30
Q

Capacitor graph for powering down (current and voltage)

A
  1. Current
    At t = 0, current (I) = maximum, it then decreases until current (I) = 0
  2. Voltage
    At t = 0, voltage (V) = maximum, it then decreases until voltage (V) = 0
31
Q

What does a capacitor require to charge?

A

In order for a capacitor to charge, it must be placed in a completed circuit which includes a power source, a pathway, and a load (resistor, or light bulb of appropriate voltage).

32
Q

What occurs when a capacitor is fully charged?

A
  • The flow of electrons stops
  • Both plates have an equal and opposite amount of charge (+Q and -Q)
  • The potential different across the plates (V), equals the supply voltage
  • An electric field exists between the plates
33
Q

When are the charges stored on the plates of a capacitor released?

A
  • When a capacitor is charged, the charges can remain stored on the plates even after the battery is disconnected from the circuit.
  • When a capacitor is discharged, all of the charges will be released (provided there is no ‘leakage caused by damp air etc.).
34
Q

What will happen when a charged capacitor is connected to a lamp?

A

The lamp will glow briefly as electrons flow around the circuit and the capacitor discharges.

35
Q

Charge and voltage of capacitors in parallel

A
  • The voltage across each capacitor is the same as the voltage (V) of the cell
  • The total charge stored is the sum of the charge in each capacitor
    Q = Q1 + Q2 + …
36
Q

Charge and voltage of capacitors in series

A
  • The supply voltage is the sum of the voltages across each capacitor
    V = V1 + V2 + …
  • The charge on each capacitor is the same
37
Q

Why is the energy stored in a capacitor (1/2 QV) only half of the energy provided by the voltage source (QV)?

A

The energy stored in a capacitor (1/2 QV) is only half the energy provided by the voltage source (QV) because during charging, there is a current in the circuit and half of the energy provided by the cell is dissipated as heat in the resistance of the components, while the other half of the energy is stored in the charged capacitor as an electric field.

38
Q

When the terminals of a battery or power supply are connected to the ends of a conductor, what occurs?

A

When the terminals of a battery or power supply are connected to the ends of a conductor, an electric field (E) is formed in the conductor.

39
Q

When the terminals of a battery or power supply are connected to the ends of a conductor, what occurs?

A
  • When the terminals of a battery or power supply are connected to the ends of a conductor, an electric field (E) is formed in the conductor.
  • The direction of the electric field is the same as the direction of the force on a positive charge.
  • The electric field will cause the positive and negative charges in the conductor to experience a force.
  • As the conductor is a solid, only some of the electrons are free to move and thus they will travel along the conductor towards the positive terminal of the power supply (this is in the opposite direction to the electric field).
  • This flow of electrons is an electric current.
40
Q

Electric field inside a conductor

A
  • The direction of the electric field is the same as the direction of the force on a positive charge.
  • This will cause the positive and negative charges in the conductor to experience a force.
  • As the conductor is a solid, only some of the electrons are free to move and thus they will travel along the conductor towards the positive terminal of the power supply (this is in the opposite direction to the electric field).
  • This flow of electrons is an electric current.
  • As the electrons move along the conductor, work is done/change in energy occurs.
  • The voltage between the ends of the conductor is the work done/energy change per unit of charge.
41
Q

What happens to the energy lost when work is done on each coulomb of charge in a conductor?

A

This energy is dissipated as heat.

42
Q

Current, voltage and resistance of series connections

A
  • Current (I) is the same across each component
  • Voltages (V) of each component adds up to the total voltage of the supply
    VT = V1 + V2 + …
  • Resistance (R) of each component adds up to the total resistance
    RT = R1 + R2 + …
42
Q

Current, voltage and resistance of parallel connections

A
  • Currents (I) of each component adds up to the total current of the supply
    IT = I1 + I2 + …
  • Voltage (V) is the same across each component
  • Resistance (R) of each component inversed adds up to the total resistance
    1/RT = 1/R1 + 1/R2 + …
43
Q

How does terminal voltage change as current flows through an energy source?

A
  • When current flows through the source, some of the energy that is supplied to the charges is lost as heat to the internal resistance, before the charges pass into the external circuit.
  • The terminal voltage supplied to the external circuit is therefore a measure of the energy supplied to the source minus the energy used by each coulomb flowing through it.
44
Q

What voltage will be measured when the current in a circuit is equal to 0?

A

When no current flows through the circuit, the internal resistance does not change any energy to heat. Therefore, the voltmeter will measure the EMF.

45
Q

Why does the EMF of a cell remain constant?

A

The EMF of a cell remains constant even when the cell ‘gets flat’ because the EMF is caused by the chemical reactions occurring inside the cell. As long as a small amount of those chemicals remains, EMF is still produced. When the cell ‘gets flat’, the internal resistance increases due to changes in the chemicals of the cell.

46
Q

How does the component added to a circuit affect the terminal voltage?

A

The greater the resistance of a component, the smaller the current in the circuit. A smaller current will give a smaller voltage across the internal resistance, as less energy is changed to heat by the internal resistance. As the terminal voltage is the difference between the EMF and the voltage across the internal resistance, a smaller voltage drop across the internal resistance will give a greater terminal voltage.

47
Q

Why is the terminal voltage less than the EMF?

A

The terminal voltage is less than the EMF because of the internal resistance of the cell, which changes some of the energy per coulomb generated by the cell to heat. As the terminal voltage measures the energy per coulomb outside the terminals, it will not include the energy used by the internal resistance.

48
Q

Visualising Kirchhoff’s voltage law

A

The ‘high energy’ side of a component will be ‘with’ the current for energy suppliers (potential difference will be added) and ‘against’ the current for energy users (potential difference will be subtracted).

49
Q

Calculating the potential difference of a resistor

A

Use Ohms law V = IR to calculate the potential difference of a single resistor by deciding which of the several given currents is going through the resistor (that is in series with the resistor).
- Then visual use Kirchhoff’s voltage law to determine whether the voltage should be added or subtracted (the current direction must be known)

50
Q

Relationship between the voltage of the source, resistor and capacitor in a circuit for powering up

A

Vs = Vr + Vc
- During powering up, Vc increases, thus Vr must decrease (Vs remains constant). The voltage across the resistor Vr determines the current in the circuit according to Ohms law, therefore as Vr decreases, current will decrease also (Voltage is directly proportional to current)
I = Vr / R

51
Q

Relationship between the voltage of the source, resistor and capacitor in a circuit for powering down

A
  • During powering down, the voltage source is removed and so capacitor is the only source of voltage in the circuit. Therefore, the voltage across the resistor will be equal to the voltage across the capacitor. Therefore as Vc decreases, Vr must decrease also. The voltage across the resistor Vr determines the current in the circuit according to Ohms law, therefore as Vr decreases, current will decrease also (Voltage is directly proportional to current)
    I = Vr / R
52
Q

What happens when a magnet is moved in and out of a solenoid?

A

As the magnet is moved in and out of the solenoid, a current is induced. (There is no current when the magnet is not moving).

53
Q

What happens when a loop of wire is pushed into a magnetic field?

A

(Assuming magnetic field lines are into the page)
As the loop is pushed into the magnetic field, a current is induced in a direction given by the modified right hand slap rule, this is because only one side of the loop (give example) is crossing the magnetic field lines.

  • There is no voltage induced across the sides of the loop that do not cross the magnetic field lines (top and bottom)
54
Q

What happens when a loop of wire is pulled out of a magnetic field?

A

(Assuming magnetic field lines are into the page)
As the loop is pulled out of the magnetic field, a current is induced in a direction given by the modified right hand slap rule, this is because only one side of the loop (give example) is crossing the magnetic field lines.

55
Q

What happens when a loop of wire completely inside a magnetic field?

A

When the loop is completely inside the magnetic field, there is no induced current. The voltage induced across each of the two sides crossing the magnetic field lines balance out each other, as they are equal and opposite. Moreover, the area of the loop in the field is no longer changing, thus the rate of change of flux is zero.

56
Q

What happens when you increase magnetic flux?

A

If you increase the magnetic flux flowing through a coil by moving it into a magnetic field, it will generate an induced current to oppose the changing magnetic flux. The induced current will create a magnetic field that produces a force to oppose the motion of the wire in attempts to decrease the increasing flux.

57
Q

What happens when you decrease magnetic flux?

A

If you decrease the magnetic flux flowing through a coil by moving it out of a magnetic field, it will generate an induced current to oppose the changing magnetic flux. The induced current will create a magnetic field that produces a force to oppose the motion of the wire in attempts to increase the decreasing flux.

58
Q

What is required to keep a wire moving across a magnetic field?

A

The induced current generated by moving across the magnetic field, causes a force which opposes the movement. To keep the wire moving, work must be done against this opposing force.

59
Q

How does a transformer work?

A

When an AC supply is connected to the primary coil, the changing current in the circuit creates a changing magnetic flux in the iron core. This changing flux passes through the secondary coil, where it induces an alternating voltage.

60
Q

How can transformers be made more efficent?

A

Energy losses can be reduced by

  • Using an iron core to ensure a strong magnetic field between the primary and secondary coils
  • Making the core from many flat electrically insulated sheets or laminations, rather than solid irons to reduce induced currents in the core which would waste energy by causing heating
  • Using low-resistance copper wire for the primary and secondary coils
61
Q

How can the ends of a switch produce a spark when it is opened after being fully powered up?

A

An open switch has a very high resistance, making the time constant very small (since time constant = L/R and L is constant). A small time constant means the rate of change of current or the rate of change of flux) will be very high. The induced voltage is proportional to the rate of change of flux and so will also be very high. It is this induced voltage that provides the voltage necessary for the spark, (not the V from the battery). The voltage of the circuit is no longer limited by the battery and can reach very high values, as it is no longer the only source of voltage for the circuit. Kirchoff’s law is no longer obeyed, since it is no longer a closed circuit.

62
Q

What does current flowing in an inductor indicate?

A

Current in an inductor causes a magnetic field to form, in which energy is stored. An inductor in a circuit can cause a delay in a light reaching full brightness as energy from the power supply is being stored in the magnetic field of the inductor (rather than contributing to the brightness of the light).
- The stored energy will be released if the terminals of the inductor are connected together

If the current changes, the magnetic flux also changes, causing an induced voltage in the coil.

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