Please enable JavaScript.
Coggle requires JavaScript to display documents.
Current electricity - Coggle Diagram
Current electricity
Resistance
Ohms law The resistance (R) of a conductor is the ratio of p.d across it to the current flowing through it: R=V/I. Resistance is a scalar quantity and it's unit is the ohm. We can measure R by an ohmmeter.
Metal The resistance of a metallic conductor increases as the temperature increases. The resistance changes linearly with temperature.
Semiconductor: The resistance of an insulator or semi-conductor decreases as temperature increases example a thermistor decreases rapidly with increasing temperature.
-
-
Resistors in series: Each resistor is on the same wire. The voltage is different for each resistor. V total= V1+V2. The current is the same for each resistor I=I1=I2 and the resistance increases R=R1+R2
Resistors in parallel: Each resistor on a different wire. Voltage is same for each resistor. The current is different for each resistor and the resistance decreases 1/R=1/R1+1/R2
Experiments
Joules law states that the rate at which heat is produced in a conductor is directly proportional to current squared its resistance is constant/ P=I SQUARED R
Resistivity We found the resistivity of nichrome to be 1.29 multiply 10 to -6. The accepted value is 1.0. No graph in this experiment.
Current voltage
-
filament Bulb: As current increases the temperature of the filament increases causing an increase in resistance. This increasing resistance limits the current o that the graph levels off.
-
Diode in forward biases: As v increases I does not flow readily because of the depletion layer acting as an insulator. Once the junction voltage is overcome, there is a huge increase in current for a small increase in voltage.
Semiconductor is a substance whose resistivty is between that of a good conductor and a good insulator. The resistivity of SC decreases with increasuring temp. Two types of charge carriers: negative electrons and positive holes.
Temp vs resistance
Semiconductor diode: used a thermistor. We found that as temp increaed there was a rapid decrease in resistance of the thermistor. The relationship between the two layers is non-linear.
Metal: Used a metallic conductor. We found that for the temp range of 20-80 degrees Celsius, we had a linear proportional graph
Semiconductors
-
-
p-n junction is a piece of semiconductor that has one half doped n-type and the other half is doped p-type. When a p-type and n-type semiconductor is put together a depletion layer is formed.
Electromagnetism
-
Magnets+magnetic fields
A magnetic field is any region of space where magnetic forces can be felt. The direction of the MF at any point is the direction of the force on a test pole ( taking as being a free north pole) if it were placed at any point within that field.
A line drawn in a MF so that the tangent to it at any point shows the direction of the MF at that point is called a magnetic field line. example lines formed in iron fillings around a magnet.
-
-
Potential difference/EMF
The potential difference between 2 points in a circuit is the energy lost by 1 coulomb as it moves from 1 point to another. Unit is joule per coulomb which is also called volts (V) Power in a circuit is P=VI We use a voltmeter to measure potential difference. It is always connected in paralell with the part of the circuit across which the p.d. is to be measured..
A voltage when applied to a circuit (like we get from a battery) is called an electromotive force (emf).
Domestic circuits
Safety
Fuses is a piece of wire within a ceramic casing that is placed in series with the live wire of an electrical appliance, normally in the plug. If a fault develops within the appliance where it is drawing too much current the wire within the fuse would be unable to maintain this extra current and melts, which breaks the circuit
RCD'S operate by detecting a difference between the current in the live and the neutral-which could arise if somebody came contact with a live wire. When the difference between the two reaches a preset value( normally 30mA) the RCD trips very quickly and cuts off electricity whereas the fuse and MCB would take longer.
MCB'S are used sometimes instead of fuses in the distribution box. They contain a bimetal strip and an electromagnet. When the current is larger than the preset value, 2 contact are separates and thus breaking the flow of electricity. Bimetal strip is used to trip small currents while the electromagnet is used for larger currents
Types
Radial circuit is one where appliances that take a large current, like an electric cooker or electric shower have a separate live and neutral wire coming from the distribution box
Ring circuit is where the live terminals of each socket are connected together. Power is thus fed along both sides of the ring to each socket. The neutrals are also connected together and connected back to the neutral at the distribution box.