P2 - Circuits Flashcards
What is voltage?
The amount of energy transferred (joules) per coulomb of charge that passes between two points in a circuit
What is current?
The rate of flow of charge
Charge / time
What is the symbol for a cell?
Horizontal line connecting to long vertical line, then a gap, then a short vertical line connecting to a horizontal line
What is the symbol of a battery?
Symbol for a cell then a horizontal dotted line then another cell symbol
What is the symbol for a switch?
Horizontal line to small circle to diagonal line
Then another small circle and a horizontal line
This is an open switch
What is the symbol for a lamp?
Circle with an X touching the sides inside it
What is the symbol for a fuse?
Horizontal line goes through a long rectangle
What is the symbol for a diode?
Horizontal line connecting to hollow triangle then a vertical line connecting to horizontal
Whole thing inside circle
What is the symbol for an LED?
Diode with 2 diagonal arrows coming off it
What is the symbol for a resistor?
Horizontal line then long rectangle then horizontal line
What is the symbol for a variable resistor?
Resistor with diagonal arrow through it
What is the symbol for an LDR?
Resistor with circle round it and 2 diagonal arrows pointing at it
What is the symbol for a thermistor?
Start with a resistor
Start a horizontal line below it and quite soon, turn it into a diagonal line that goes through the resistor
Describe the basic test circuit
The order of the circuit is:
Cell
Variable resistor
Ammeter
Component to be tested with voltmeter in parallel around it
Back to cell
What is the test circuit used for?
You vary the resistance of the variable resistor and look at the voltage and current
You can plot all of the data in a V/I graph and find the resistance of the component
What is a V/I graph?
A graph where voltage (V) is on the x axis and current (I) is on the y axis
It can be used to calculate resistance of the component
What is the V/I graph like for different resistors?
A straight line with a positive gradient (gradient varies for different resistors)
The straight line means that the resistance is constant
What is the V/I graph like for filament lamps?
The line approaches the x axis with an increasing gradient (curve)
When the line is above the x axis, it has a decreasing gradient
This is because as the temperature increases, the resistance increases
What is the V/I graph like for a diode?
The current remains at 0 until a certain point where the line goes up at an increasing gradient
This is because the current will only flow through a diode in one direction
The resistance in the opposite direction is very high
Why does temperature increase resistance?
When electrical charge flows through a resistor, some electrical energy is transferred to hear energy that makes the resistor hot
The heat energy causes ions in the conductor to vibrate more making it difficult for the charge carrying electrons to get through so the resistance is higher
Why do resistors have a limit to the current that can pass through?
More current means an increase in temperature which means an increase in resistance which means the current decreases again
This is why the graph for the filament lamp levels off at high currents
How do you calculate resistance from a V/I graph?
Use V = IR to calculate the resistance from the current and voltage deltas (draw triangle under line)
The steeper the graph, the lower the resistance
Describe diodes
Made from semi conductor (like silicon)
Used to let current through it in only one direction because there is a high resistance in the other direction
Describe LEDs
Emits light when a current flows through it in the forward direction
Better than filament lamps because they need a smaller current to produce the same light intensity
Describe LDRs
Resistor that changes resistance depending on light intensity
Bright light gives lower resistance and vice versa
Describe thermistors
Temperature dependent resistor
In hot conditions, resistance drops and vice versa
What happens to voltage in series circuits?
Voltage is shared between the components
The voltage in a component is directly proportional to the resistance
The voltages of cells add up
What happens to current in a series circuit?
The current is same in all parts of the circuit
What happens to the voltage in a parallel circuit?
It is the same across all components
What happens to the current in a parallel circuit?
Current is shared between branches
The amount of current going down one path is inversely proportional to the total resistance of that path
