Topic 2: Electricity Flashcards
Equation linking P.d. current and resistance?
Potential difference = Current x Resistance
V = IR
V = A x Ohms (Omega)
Electric current is the…?
Flow of electrical charge
Potential difference is the…
The difference in electrical potential energy between two points
The ammeter…
Measures the current (in amps) flowing through the wire
Must always be placed in series
The voltmeter
Measures the potential difference (pd) across the wire (in volts)
Practical to test the resistance of a length of wire?
Attach a crocodile clip to the wire, measuring 0cm.
Attach another crocodile clip, starting at 10cm
Record the current and the pd
Repeat this in increments of 5
Use your measurements to calculate resistance using V = IR (But rearranged to R = V/I)
plot a graph
The resistance is ______ _______ to the ______ of the wire
The resistance is directly proportional to the length of the wire
The longer the wire…?
The greater the resistance
The resistance of an Ohmic Conductor? (e.g. a resistor at a constant temperature)
The current through an Ohmic conductor (at a constant temperature) is directly proportional to pd, so you get a straight line going from bottom right to top left when you graph it.
The resistance of a Filament Lamp?
As the current increases, the temp of the filament increases, so the resistance increases. Meaning less current can flow per unit pd, so the graph gets shallower, and a curve is formed.
The resistance of a Diode?
Current will only flow through a diode in one direction so in a graph, a steep upwards curve will only be in the top right quadrant. The diode has a very high resistance in the reverse direction.
In bright light, the resistance of an LDR…?
Decreases
In darkness, the resistance of an LDR…?
Increases
In heat, the resistance of a thermistor…?
Decreases
In cool conditions, the resistance of a thermistor…?
Increases
Example of a use for LDRs
Automatic night lights/ street lamps
Example of a use for thermistors
Car engine temperature sensors and electronic thermostats
In series circuits, the total pd of the supply is…?
Shared amongst the various components. So the pd around a series circuit adds up to the source pd:
Vtotal = V1 + V2 + …
In a series circuit, the….. (current)
Same current flows through all components:
I1 = I2 = …
In a series circuit, the total resistance of two components is…
Just the sum of their resistances:
Rtotal = R1 + R2
In parallel circuits, all components… (pd)
Get the full source pd:
V1 = V2 = …
In parallel circuits, the total current flowing around the circuit is…
Equal to the total of all the currents through the separate components:
Itotal = I1 + I2 + …
If you have two resistors in parallel…
Their total resistance is less than the resistance of the smallest resistance of the smallest two resistors.
RP for resistance in series and parallel?
Make a series and parallel circuit with two resistors each.
Measure the pd and the current and use V = IR to find the resistance (R = V/I)
Add another resistor to each circuit and repeat
You should find that adding resistors in a series circuit in series increases the total resistance of the circuit (and decreases the total current)
You should also find that adding resistors in a parallel circuit in parallel increases the total CURRENT of the circuit (and decreases the total RESISTANCE)
The two types of energy supplies?
Alternating current (ac) and Direct current (dc)
In ac supplies, the current is constantly…?
Changing direction. Alternating currents are produced by alternating voltages in which the positive and negative ends keep alternating.
The UK mains supply is…?
An ac supply at around 230Hz
The frequency of the ac mains supply is…?
50 cycles per second or 50Hz
Cells and batteries supply…?
Direct current
Direct current is a…?
Current that’s always flowing in the same direction. Created by direct voltage.
The neutral wire?
Blue
Completes the circuit and carries away current
Around 0V
The Live Wire?
Brown
Provides the alternating pd (at about 230Hz) from the mains supply
Earth Wire?
Green and yellow
Protecting the wiring and for safety.
It stops the appliance casing from becoming live.
It usually doesn’t carry a current - only when there’s a fault
Also at 0V
Equation for power, energy transferred and time?
Energy transferred (J) = Power (W) x Time (s) E = Pt
The equation for charge flow, energy transferred and pd?
Energy transferred (J) = Charge flow (C) x pd (V) E = QV
Power equation?
Power (W) = Pd (V) x Current (A)
P = VI
Power equation if pd is unknown?
Power (W) = Current^2 x Resistance (Ohms)
P = I^2R
What is the national grid?
The national grid is a series of cables and transformers that covers the UK and connects power stations to consumers.
To transmit the huge amount of power needed, you either need…?
A high potential difference or a high current.
The problem with a high current is that…?
you lose loads of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings
Along the national grid, the pd is…?
400,000V
The step-up transformer…?
increases the pd to 400,000V
The step-down transformer…?
decreases the pd to make it safe for domestic use.
When certain INSULATING materials are rubbed together…?
NEGATIVELY charged electrons will be scraped off of one and deposited onto the other, leaving the materials electrically charged, with a positive static charge on one and an equal negative static charge on the other.
When a polythene rod is rubbed with a cloth duster…?
POSITIVELY charged electrons will be deposited onto the duster.
When an acetate rod is rubbed with a cloth duster…?
NEGATIVELY charged electrons will be deposited onto the duster.
Both +ve and -ve electrostatic charges are only ever produced by…?
The movement of electrons. The positive charges DO NOT move.
An electric field is created around…?
ANY electrically charged object
Electric field lines…?
Go from positive to negative
Are always perpendicular to the surface
The closer together the lines are, the stronger the field is
When a charged object is placed in the electric field of another object…?
It feels a force
Which either causes repulsion or attraction
The force is caused by the electric fields of each charged object interlacing with each other
The force on an object is linked to the strength of the electric field it is in.
As you increase the distance between the charged objects, the strength of the field decreases and the force between them gets smaller.
Sparks are caused when…?
There is a high enough pd between a charged object and the earth (or an earthed object)
The high pd causes a strong electric field between the charged object and the earthed object.
The strong electric field causes electrons in the air particles to be removed (known as ionisation)
Air is normally an insulator, but when it is ionised it is much more conductive, so a current can flow through it, causing a spark.
Ohm’s law?
The current through a conductor between two points is directly proportional to the voltage across the two points
V = IR
Potential difference = Current x Resistance
