Electricity Flashcards
In terms of potential difference
The greater the resistance of the component the smaller the current for a given potential difference across the component
Required practical- resistance
1) set up a simple circuit. Attach a length of a wire along a metre ruler using pieces of tape. Attacha crocodile clip at one end
2) attach the second crocodile clip at x=10 cm along the wire and each time recording the current and voltage measured
3) repeat by moving the crocodile coop 10 cm along the wire and each time recording the current and voltage measured
4) calculate the resistance of the wire at each point using the equation V=IR where V is the potential difference and I is the current
5) plot a graph of the length of the wire (x axis) against the resistance of the wire (y axis)
What is zero error
Reading on a measuring instrument when the value should be zero
This is a systematic error we cannot reduce it by carrying out repeats
We need to subtract the zero error from all readings
What happens in a resistor graph
If the resistance is constant the current is directly proportional to the potential different so the graph is linear
What happens if the resistance of components is not constant
Lamps, diodes, thermistors and LDRS aren’t constant so the graph is non linear for example the resistance of a filament lamp increases as the temperature
What is the current through a diode
Flows in one direction only. Has a very high resistance in the reverse direction
How does the resistance change with current
As current increases electrons have more energy
When electrons flow through resistor they collide with the atoms in the resistor
Transfers energy to atoms causing them to vibrate more so makes it more difficult for electrons to flow through the resistor
Resistance increases current decreases
How does the resistance change with temperature and length
Temp- normal wires (same as current)
Thermistors- in hotter temps the resistance is lower these are often used in temperature detectors
Length- greater the length the more resistance and the lower the current
Electrons have to make way through more resistor atoms so it’s harder than using a shorter wire
How does resistance change with light and voltage
Light-LDR. The greater the intensity of light the lower the resistance
So the resistance is greatest when it is dark. These are used in automatic night lights
Voltage-Diodes- diode allows current to flow freely in one direction. In the opposite direction it has a very high resistance so no current can flow
What is the equation for resistance in series circuits
Resistor 1 +resistor 2 etc
What are series circuits
Closed circuits
Current only follows single path
Current same everywhere
Potential difference is shared across whole ciruit
PD of power supply=sum of PD across each component
Current at one point=current at any other point
Can only switch them off at once
What are parallel circuits
Branched circuits
Current splits into multiple parts
You can switch each component off individually
Potential difference is the same across all branches
PD of power supply= PD of each branch becuase charge can only pass through any one branch
Current is shared between each of the branches
Current through source= sum of current through each branch
Why does adding resistors in series increased the total resistance whilst adding resistors in parallel decreases
The resistance of two components is bigger than one of them because the charge has to push through both of them when flowing round the circuit
Two resistors in parallel will have a smaller overral resistance because charge has more than one branch to take so only some charge will flow along each beanch
What is the difference between direct and alternating current
In the uk the domestic energy supply has a frequency of 50 Hz and is about 230 V
In alternating current the mains supply repeatedly reverses in direction
In direct current the movement of charge in one direction inly
What is the live wire
Brown at 230 V
Carried alternating potential difference from the supply- this may be dangerous even if the mains circuit is off as current may be flowing through it still
What is the neutral wire
Blue at 0V
Completes the circuit
What is the earth wire
Green and yellow stripes at 0V.
Safety wire to stop appliance becoming live
If the live wire touches the metal casing of the appliance it will become live. You will get a serious electric shock if you touch it as current flows through you to the ground
What is power
Directly proportional to current and voltage
Power loss is proportional to resistance and the square of the current
Energy is transferred from chemical potential in batteries to electrical energy in wires to any form of useful energy in the devices they power
How are electrical energy transfers in everyday appliance
Kinetic energy for a motor or thermal energy in a kettle
Work is done when charge flows through a circuit and is also equal to energy transferred as all the electrical energy gets transferred to the appliance
Power rating of an appliance shows the power it uses in watts so greater power rating means more energy
What is the process of the national grid
The electricity from the power station passes through step up transformers which increase the pd to 400000 v
This is because less energy is lost in the power cables when the potential difference is high
Electricity passes through high voltage cables
The electricity passed through step down transformers which reduce the potential difference to 230 v to be passed into homes
What is the method of I-V characteristics
1) Construct the circuit shown in the diagram below
2)Set the variable power supply or variable resistor to the lowest setting for potential difference
3)record the current and voltage over the resistor
4)increase the current from the power supply by 2V and repeat your readings
5)Change the resistor to a filament lamp and repeat the experiment
Method of I-V characteristics (2)
6) Change the filament to a diode and protective resistor ensuring the diode is the correct direction to allow the flow of current. Change the ammeter to a millammeter since the current measure will be smaller than for other components
7) plot a graph if current against potential difference for each component
