Physics Unit 6: Electricity for Gadgets

Resistance is the collision of electrons with the atoms of a metal and is measured in ohms. The higher the resistance, the more collisions occur in the wire and this slows the flow of electricity, lowering the current. When the electrons and atoms collide, the atoms vibrate more and this increases the number of collisions and the temperature of the wire. Therefore, the higher the resistance, the lower the current, the lighter the brightness of a bulb in a circuit. The equation is:

resistance = voltage/current

A rheostat (aka a variable resistor), is a resistor that varies its resistance to vary the current. This is achieved by changing the length of wire in a coil because the longer the length of a wire, the greater the resistance so the lower the current. This is how a dimmer switch works.

A fixed resistor (curcuit symbol is WITH an arrow through) maintains a constant resistance in a wire so the voltage-current graph (gradient hence showing resistance), would be directly proportional. 

When two resistors or more are arranged in a circuit they are called potential dividers. When the resistors are arranged in parrallel, their overall resistance decreases. Their overall resistance can be found using:

1/R(total) = 1/r1 + 1/r2 + 1/r3 (where r1 is resistor 1 etc) When the resistance of the 2nd resistor is larger than the 1st, the voltage in = voltage out but when the resistance of the 2nd resistor is much less than the 1st, Vout = 0. 

However, when in series, the total resistance can simply be found by adding each resistor.

A fixed resistor followed by a variable resistor can be used in order to produce a threshold voltage (the voltage required to turn on an appliance). An LDR (light dependent resistor) is one example of a variable resistor. When it is very bright, the resistance is low, so the voltage is also low and therefore the appliance (usually a light) doesn't turn on. Although, when it is dark, the resistance is high, so the output voltage is also high and therefore meets the threshold voltage to turn on the appliance.

Another example of a variable resistor is a thermistor (thermo-heat). When it is very cold, the resistance is low, so the voltage is also low and therefore the appliance doesn't turn on but when its dark, the resistance is high, so the output voltage is also high and the appliance turns on. The appliance is often a heater. 

A transistor is often used in many electronics as a tiny way to produce a larger current. A transistor looks like a 2 pronged fork. The handle being the base (where the intial current goes in), 1 prong being a collector (where the current builds up) and the 2nd prong being the emitter (where the larger current is emitted. The emitter current is: (when I is current)

Ie = Ib + Ic 

Two+ transistors can be combined to form a logic gate. This can produce a much larger overal current. There are 5 types of logic gates we need to know for the exam:

• AND gates: This means that a current is needed at the base of the first transistor (or handle of our fork) AND a current in the base of the second transistor to produce a current at the emitter. 
• OR gates: This means that a current is needed at the base of the first transistor (or handle of our fork) OR a current in the base of the second transistor to produce a current at the emitter. 
• NOT gate: This MUST be combined with either an AND gate or an OR gate. It produces the opposite of what is put in. E.g. if the current is on at output of the AND gate, then the NOT gate will turn it off and vice versa. (This is shown in truth tables)
• Combining a NOT gate with an AND gate makes a NAND gate
• Combining a NOT gate with an OR gate makes a NOR gate

Several logic gates can be combined to produce a logic circuit. 

A small base current is always needed to turn on the logic gate so as not to damage it. This is why a potential divider (2+ resistors) are used in the circuit. This can involve an LDR, thermistor or even a switch: when the switch is open, the input into the logic gate is low. 

Another way of isolating the current in a using a relay in the circuit. The iron arm or armature of the relay is attracted to the magnetic current produced by the current passing through the wire of the circuit. The iron armature then moves towards the coil and pushes 2 contacts together, allowing the current to pass through. A relay can also be used to switch on a larger current for appliances. 

First we need to understand that any movement of electrons is electricity. By this definition, magnetism, the attraction and repelling of electrons, is electricity. The direction of the magnetic field can be found using the right hand rule; the thumb points in the air when the current goes up and so your fingers coil towards your thumb (basically a thumbs up sign) and this is anticlockwise and therefore so is the magnetic field. This is how a generator produces electricity: a coil spinning in between the poles of a magnet (or vice versa) to induce a current on the coil. However, the current produced on each side of the coil moves in a different direction. Fleming’s left hand rule can be used here to show this (thuMb = Motion, First finger = Force, seCond finger = Current – all perpendicular to each other). The current can be increased by:

•  Increasing the number of coils
• Increasing the strength of the magnets
• Increasing the speed of the motor

 Some parts of the motor are:

• Commutator: keeps the coil spinning
•  Curved poles = radial field = constant force on wire
• The direction of the current in the coil is reversed every half turn to ensure the force is always in the same direction to make it a DC motor

Whenever a wire is moved through a magnet or vice versa, a voltage is induced upon it. The faster the wire is moved, the larger the induced voltage, and the larger the current that passes through it. When a wire is moved up through a magnet then down, the induced voltage is reversed and so is the direction of the current. 

In a power station generator, a turbine turns the coil of wire between the electromagnets and an alternating current is produced as the electrons are repelled and attracted by the south and north pole of the magnet. Slip rings are connected to the ends of the coil to stop them twisting and winding around themselves, with 'brushes' touching the slip rings and completing the circuit.

Transformers use an induced voltage to carry a voltage from one side of an iron core to another. They use the AC current from the mains on the primary coils to produce a continually changing direction of the magnetic field over the iron core of the transformer, inducing a changing voltage and an AC current on the secondary coils of the trasnformer. There are 3 types of transformers:

• Step-up transformers: more secondary coils than primary coils causing the voltage to increase, reducing the current and therefore thermal energy loss in electricity wires on the National Grid
• Step-down transformers: more primary coils than secondary coils causing the voltage to decrease, increasing the current for commercial use. 
• Isolating transformars: equal number of primary and secondary coils so the voltage stays the same. Used in bathrooms as there is no direct contact with mains due to the magnetic field inducing the voltage, so no danger if water is spilt.

The voltage can be calculated using:

voltage across primary / voltage across secondary = no. coils primary / no. coils secondary

A diode can convert an AC supply to a DC output, allowing most products to function. This is because a diode is made of an n-type silicon (with an excess of electrons) and a p-type silicon (with an excess of 'holes' that can be thought of as electrons), seperated by a gap. When a positive terminal of a supply, like a battery, is connected to the n-type of the silicon, the gap widens and no current passes but when the positive terminal is connected to p-type silicon, the gap narrows and eventually disappears allowing a current to pass. Thus produces a DC current as only a + current can produce a current when meeting a p-type silicon.

Although, a single diode only produces half-wave rectification and therefore more are required. This is called a bridge rectifier and involves 4 diodes. This then produces 0 voltage when the current would produce a negative voltage for example and then a voltage of 1 when the current would produce a positive voltage, a full wave recitification. 

This produces a very broken overall DC output that can be smoothed by a capacitor. A capacitor can be thought of as an elastic band: when the DC output from our bridge capacitator is positive, the charge is stored like when you stretch an elastic band. When the current changes direction and the voltage reaches 0 as the bridge rectifier stops the current in the direction, the energy stored is released, smoothing the circuit, like when you let the band go.