P6

Circuits, Voltage, Potential Dividers, LDRs and thermistors, Magnetic Fields, The motor effect, The simple electric motor, electromagnetic induction. generators, translformers, diodes and rectfication, capacitors, logic gates, using logic gates and LED's and Relays in Logic Circuits

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  • Created by: Amina
  • Created on: 24-04-12 17:45

Circuits

Current is the flow of electrons round a circuit. Measured in Amps.

Voltage is the driving force that pushes the current round. Measured in Volts.

Resistance is anything in the circuit which slows the flow down. Measuered in Ohms.

Increase in voltage = more current

Increase resistance = less current

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Voltage-Current Graphs and Resistance

Variable Resistor: A resistor whose resistance can be changed by moving a lever. They alter the current flowing through a circuit. More resistance=less current. 

The steeper the graph the lower the resistance. If the graph curves it means change in resistance. In a straight line graph the resistance is steady and is equal to the inverse of the gradient ofd the line: 1/gradient.

Resistance=potential diffferent/current r=v/i

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Potential Dividers

Potential dividers allow you to run a device that requires a certain voltage froma  battery of a different voltage. Vout = Vin x (r2/r1+r2)

A potential divider can be used to run a 6V device from a 9V battery. You could replace one of the resistors by a variable resistor so that you could change the vout to any value between 0 and 9 volts. 

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LDR's and Thermistors

Light Dependent Resistor (LDR):

In bright light, the resistance falls. In darkness the resistance is highest. This makes it a useful device for various electronic circuits such as automatic night lights and burglar detectors. 

Thermistor (Temperature Dependent Resistor):

In hot conditions resistance falls. In cool conditions the resistance goes up. Thermistors make useful temperature sensors such as in car engine temperature gauges and electronic thermostats.

Using a thermistor and a variable resistor in a potential divider you can make a temperature sensor that triggers an output device at a temperature you chose. You can make a temperature sensor that gives out a high voltage when its hot and low voltage when its cold.

Cold thermistor=resistance high, voltage drop across it is almost 5v so the voltage output is 0v. Hot thermistor=resistance low, voltage across it is almost 0v and the voltage output is nearly 5v.

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Magnetic Fields

The magnetic field inside a current carrying solenoid is strong and uniform. The ends of the solenoid act like the north and south poles of a bar magnet. If the direction of the current is reversed the N and S Poles will swap ends. You can increase the strength of the magnetic field around a solenoid by adding a magnetically soft iron core. It is then called an ELECTROMAGNET.


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The simple Electric Motor

4 Factors which speed it up:

More current    More coils   Stronger magnetic field    Soft iron core

The coil is on a  spindle so it rotates. A split ring commutator swaps the contacts every half turn to keep the motor rotating in the same direction. The direction of the motor can be reversed either by swapping the polarity of the DC supply or swapping the magnetic poles over.

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Electromagnetic Induction

Moving a magnet in a coil of wire induces a voltage. 

Electromagnetic induction means creating a voltage in a conductor. You can do this by moving a magnet in a coil of wire or moving a conductor in a magnetic field.

If you move the magnet in the opposite direction, then the voltage will be reversed to. If the polarity is reversed so will the voltage be reversed. If the magnet moves back and forwards constantly you produce AC current. 

Four factors affect the size of the induced voltage:

Strength of magnet   Area of Coil   Number of coils   Speed of movement

To reduce voltage you would reduce one of the four. Moving faster means higher voltage AND frequency. 

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Generators

Generators- magnetic field and movement induces a current. 

In an AC generator they rotate the coil in a magnetic field. As the coil spins, a current is induced in the coil. This current changes direction every half turn. Instead of a split ring commutator AC generators have slip rings and brushes so the contacts don't swap every half turn. They produce AC voltage.

Dynamos:

Dynamos are a different type of generator. They rotate a magnet instead of the coil. It causes a field through the coil to swap every half turn so the output is the same as a generator. 

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Transformers

Three types of Transformer:

1. Step up transformers: These step the voltage up. They have more turn son  the secondary coil than the primary coil. 

2. Step down transformers: These step the voltage down. They have more turns on the primary coil than the secondary. 

3. Isolating Transformers don't change the voltage at all. They have the same number of turns on the primary and secondary coils.

Transformers work by electromagnetic induction. The primary coil produces a magnetic field which stays within the iron core so nearly all of it passes through the secondary coil and hardly any is lost. Because there is AC current in the primary coil, the field in the iron core is constantly changing direction. The rapid change is felt by the secondary coil. The changing field induces an alternating voltage in the secondary coil. The number of turns on the two coils determines whether the voltage induced is greater or less than the voltage in the primary. If you supplied DC to the primary, you'd get nothing at the secondary. Transformers only work with AC(need changing current).

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Transformers

The iron core is purely for transferring the changing magnetic field from primary to secondary coils. No electricity flows around it.  Transformers are nearly 100% efficient. Power=Voltage x Current VpIp=VsIs   p primary s secondary V voltage I current

Transformer Equation:

Primary voltage/Secondary Voltage=Number of turns in primary/Number of turns on secondary.

Transformers are used on the national grid. To transmit a lot of power you need either high voltage or high current. High current makes loss (as heat) due to the resistance of the cables. It's much cheaper to boost the voltage up to 400 000 Volts and keep the current very low. This requires transformers as well as pylons with huge insulators. The transformers have to step up he voltage at one end for efficient transmission and then bring it back down to safe usable levels at the end.

Isolating Transformers are used in bathrooms because it is safe. Minimises the risk of getting electrocuted and of the live parts touching the earth lead.

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Diodes and Rectification

Diodes only let current flow in One Direction. They are made from semiconductors such as silicon. Silicon diodes are made from two different types of silicon joined together. One half of the diode is made from silicon that has an impurity added to provide extra free electrons. A different impurity is added to the other half of the diode so there are fewer free electrons than normal. There are lots of holes left in it. When there's no potential difference across the diode, electrons and holes recombine across the two parts of the diode. This creates a region where there are no holes or free electrons which acts as an electrical insulator. When there is a p.d across the diode the direction is important.

Applying a P.D in the RIGHT direction means the free holes and electrons have enough energy to get across the insulating region to the other side. This means that a current flows.  Applying a P.D in the WRONG direction means the free holes and electrons are being pulled the wrong way so they stay on the same side and no current flows.

A single diode only lets through current in half of the cycle. this is half wave rectification. To get full wave rectification you need a bridge circuit with four diodes. There is always a route to go through. 

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Capacitors

Capacitors store charge. You charge a capacitor by connecting it to a source of voltage e.g a battery. A current flows around the circuit and charge gets stored on the capacitor. The more charge that's stored on it, the larger the potential difference across it. When the voltage across the capacitor is equal to that of the battery, the current stops and the capacitor is fully charged. The voltage across the capacitor wont rise above the voltage of the battery. If the battery is removed, the capacitor discharges. Whilst discharging, the current flows in opposite direction until capacitor is fully discharged. The voltage falls as the capacitor discharges.

Capacitors are used in smoothing circuits. The output voltage from a AC power supply can be smoothed by adding a capacitor in parallel with the output device. A component gets current alternately from the power supply and the capacitor. 

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Logic Gates

AND and OR gates: An AND gate only gives an output of 1 if both the first input and second input are 1. An OR gate needs either the first OR the second input to be 1.

NOT Gate: A not gate just has one input and it can be either 1 or 0. The NOT gate is an inverter so when Input is 1 Output is 0 and Input is 0 Output is1.

NAND and NOR gates have the Opposite Output of AND and OR gates

If an AND gate would give an output of 0, a NAND gate would give 1 and vice versa.

If an OR gate would give an output of 0, a NOR gate would five an output of 1 and vice versa.

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Comments

Megan

Hi,

Slide 12 is incomplete, please try and only upload completed resources.

Thank you,

Megan (moderator)

Shama Nasinde

I believe this is P5?

joanne

No, this is P6

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