Knowledge

Question 1

Describe a Capacitor

Answer 1

When a capacitor is connected to an e.m.f, electrons are drawn from away from a plate, making it positive, and into the other plate, making it negative. The build-up of charge slows as the charge on the plates increase. There is a limit to how positive the top plate can become and how negative the bottom plate can be. The charge will continue to build until the voltage drop across the capacitor is the same as the voltage of the battery.

Question 2

What is an RC Curcuit

Answer 2

These Circuits involve a resistor as well as a capacitor. When the circuit is on, a current flows through the circuit and the capacitor charges until it has the same voltage as the power source. As the charge builds on the capacitor, the current decreases exponentially.
The bigger the resistance of the resistor, the smaller the current and therefore the capacitor will charge slower.
The time constant T, is the time in seconds it takes the resistor to reach 63% of its final voltage. A capacitor is considered fully charged after 5 time constants.

Question 3

Describe resistance and Superconductors

Answer 3

Electrons collide with atoms as they flow through wires. As the temperature rises, the atoms move more and the collisions increase. This increases the resistance. Therefore the resistance of a wire increases as the temperature increases.
Superconductors have critical temperatures, that is, after the temperature drops below this critical level, the conductors have no resistance.

Question 4

What is an n-type semiconductor

Answer 4

This adds an impurity atom with five outer electrons. This means that four will join with the neighbouring silicone but there will be one free electron. This electron is free to move making it slightly more conducting. It is an n-type because it has a negative charge.

Question 5

What is a p-type semiconductor

Answer 5

P-type – This adds an impurity with three outer electrons. This leaves a hole in the lattice which other electrons can jump into. It is a p-type because it is positively charged.

Question 6

What is a junction diode (picture)

Answer 6

A junction diode is produced when an n-type semiconductor is joined with a p-type semiconductor.

Question 7

Forward Bias Diode

Answer 7

Forward biasing diodes happen when the negative terminal is connected to the n-type conductor. If the voltage is more than 0.6 then the electrons can cross over the depletion layer and create a working circuit.

Question 8

Reverse Bias Diode

Answer 8

Reverse bias diodes happen when the positive terminal is connected to the n-type conductor. This makes the positives go towards the negative terminal and negative to the positive terminal. This voltage will add to the barrier potential making current hardly possible. Current can be forced with a large enough voltage (the breakdown voltage) but this destroys the diode.

Question 9

Two types of rectification

Answer 9

Full wave, Half wave. See Picture

Question 10

What is Capacitor Smoothing

Answer 10

This occurs when a capacitor is placed in parallel with a diode. When the positive part of the voltage goes through the diode, it charges the capacitor, when the diode blocks the negative voltage, the capacitor discharges the load.

Question 11

Name examples of semiconductor devices

Answer 11

Photovoltaic Cell (solar cell) – this works similar to a diode except light is allowed to enter and produce electricity. The energy from the light knocks electrons out of bonds near the junctions. This forms holes and free electrons. Therefore current can flow.
LED – A diode that emits light when current flows through it in the forward direction. Current should be kept low in an LED therefore should be used with a resistor. 
LDR (light dependent resistor) – the resistance in this device varies depending on how much light falls on it. When light shines on an LDR it has low resistance and allows current to flow. When light does not shine it has high resistance and the current will not flow.

Question 12

What is the purpose of a voltage divider?

Answer 12

When two resistors are placed in series with a supply. Voltage dividers are used when it is necessary to provide a voltage to part of the circuit that is smaller than the supple voltage.

Question 13

What is a transducer?

Answer 13

Transducers are devices that transform energy from one form to another. An input transducer changes non-electrical energy into electrical energy and an output transducer transforms non-electrical energy into electrical energy. Input transducers include; light dependant resistor, thermistor and microphone. Output transistor includes LEDs, relays, metres and CROs.

Question 14

What is a relay?

Answer 14

These are electro mechanical switches used to turn other circuits on and off. These use the current from the original circuit to turn the electro magnet on or off. This forces the switch towards or away from it.

Question 15

What is a transistor?

Answer 15

An NPN transistor consists of two diodes back to back. The three zones of this transistor are called the emitter, base and collector. The emitter is heavily doped with n-type impurities. The collector is less heavily doped with n-type impurities. The base is a thin slice lightly doped with p-type impurities.

Question 16

How does a current amplifier work?

Answer 16

Current flows from the emitter to the base and flow out of the center electrode. However as the p-type material is very this, most of the electrons will rush through the base and go through to the collector. Once the electrons go through the collector they are attracted to the positive terminal of the voltage source. Therefore most of the electrons emitted by the emitter are collected by the collector with a very small number going to the base.

Question 17

How does a transistor switch work?

Answer 17

If the base voltage is less than 0.6V, then there is no base current. Where there is no base current, there is no collector-emitter current. Therefore the transistor is switched off.

Question 18

The And gate

Answer 18

Both inputs must be on to be on

Question 19

The Or gate

Answer 19

Or or Both the inputs must be on to be on

Question 20

The Not gate

Answer 20

Opposite of input

Question 21

The Nand gate

Answer 21

And gate followed by a not gate

Question 22

The Nor gate

Answer 22

Or gate followed by a not gate

Question 23

The Xor gate

Answer 23

Either but not both gates must be on

Question 24

The Nxor Gate

Answer 24

Xor gate followed by a not gate

Question 25

What is a flip flop?

Answer 25

These devices have the ability to remember the last input it received. A flip-flop is a logic circuit with two outputs, Q and Q ̅. Where Q=1 and Q ̅=0.

Question 26

What is a toggle flip flop

Answer 26

The output state will change (toggle) for each input pulse received. The output could change at the beginning or the end of the pulse (depending on what pulse is used). The behavior of a flip flop is best represented by a timing diagram.
This flip flop has the effect of dividing the number of pulses by 2. The period of Q = 2 x period of T.

Question 27

How to count with flip flops

Answer 27

Flip flops used together can be used to count pulses. The flip flop goes high only after one complete pulse has been received. This causes light A to go on. After the second pulse has been received, light A goes off and the second flip flop goes high, turning on light B. A third pulse turn light A on and light B remains on, and so on. As this counter has three flip flops it can count 3-bit numbers (up to 7).