Electrical Systems - Explanations - Level 3 Flashcards
How can the voltage of a resistor be changed?
The voltage of a resistor (Vr) can be changed by using a rheostat (variable resistor), which will change the total resistance of the circuit.
How can r and E be calculated?
- Take 5 readings for Vr and I
- Draw graph between Vr and I
- Use Vr = -rI + E
(from form mx+c, with a negative gradient) - Gradient of graph is equal to r
- y-intercept of the graph is equal to E
What is magnetic flux in the secondary coil (S) directly proportional to?
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)
What happens when a coil rotates in a magnetic field?
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.
When is the induced current of a coil in a magnetic field maximum and minimum?
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.
Why does current build up slowly?
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.
How can the supply voltage or current of a generator be increased?
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.
What does the constant of mutual induction (M) depend on?
- The size of the coils
- The distance between the coils
- The material inside the coils
How can the magnetic flux be changed?
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.
How can power loss be reduced in a transformer?
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…
- Lowering the current in the transmission lines (P = I^2R)
- Lowering the resistance in the power lines by using different metal conductors (Al, Cu)
- Lowering the resistance within the transformer
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?
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.
When is self-inductance effective?
Self-inductance is only effective when…
- The switch in the circuit is opened
- The current in the coil is changing
Resistance in inductors
Inductors can have resistance since they are coils. When the current becomes steady in the circuit, only the resistance in the coil is effective.
What does the self-inductance of a coil depend on?
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).
What happens when the DC current changes?
When the DC current changes, the inductor can absorb and store energy (E), which is then stored in the magnetic field it creates.
Inductor graph for powering up (current and voltage)
- Current
At t = 0, current (I) = 0, it then increases until current (I) = maximum - Voltage
At t = 0, voltage (V) = maximum, it then decreases until voltage (V) = 0
What occurs when an inductor is powering up (switch is turned on)
- 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.
Inductor graph for powering down (current and voltage)
- Current
At t = 0, current (I) = maximum, it then decreases until current (I) = 0 - Voltage
At t = 0, voltage (V) = maximum, it then decreases until voltage (V) = 0
What occurs when an inductor is powering down (switch is turned off)
- 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.
How can an inductor be used to produce a larger maximum voltage?
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.
Resistor graph for powering on (current and voltage)
- Current
At t = 0, current (I) = 0, it then increases until current (I) = maximum - Voltage
At t = 0, voltage (v) = 0, it then increases until voltage (v) = maximum
Resistor graph for powering down (current and voltage)
- Current
At t = 0, current (I) = maximum, it then decreases until current (I) = 0 - Voltage
At t = 0, voltage (V) = maximum, it then decreases until voltage (V) = 0
What occurs when a capacitor is powering up (switch is turned on)
- 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).
Uses of a capacitor
- Tuning radio and TV circuits to receive signals
- Storing charges a supplying a quick current (car indicators, camera flashes)
- Smoothing the output voltage of a rectifier that is producing a DC current
- Converting AC to DC in a power supply
- Absorbing harmful currents to safeguard when appliances short circuit
What does capacitance depend on?
Capacitance depends on…
- The area of the parallel plates (C ∝ A)
- Plates with a greater area are able to store more charge - The distance between the plates (C ∝ 1/d)
- Plates that are closer together are able to store more charge - The properties of the dielectric material between the plates (C = (εo A)/d)