P1- Electricity Flashcards
Current is?
The flow of electrical charge
What is potential difference?
Voltage
The driving force that pushes charge round the circuit
What is resistances unit?
Ohms
Ω
The greater the ____ across a component, the smaller the ______ that flows (for a given ____ ______ across the component.
Resistance
Current
Potential Difference
What two things does the total charge of a circuit depend on?
Current and time
Size of current =
= rate of flow of charge
Formula for charge passed
Q = It
Charge flow = Current x time
Coulombs (C) = A x s
Circuit diagrams
In book
Potential difference and current formula
V = I x R
PD = Current x Resistance
V = A x Ω
What are ohmic conductors?
Resistance doesn’t change with the current , it flows at a constant temperature and is directly proportional to the potential difference.
Which 2 components do change resistance?
Diodes and filament lamps
Why do filament lamps resistance change with current?
When electrical charge flows through them, some energy is transferred to the thermal energy store which is designed to heat up. Resistance increases with temperature, so increasing current causes more heat and therefore more resistance.
Why do diodes resistance change with current?
It depends on the direction of the current. They let current flow in one direction but have a very high resistance if it is reversed.
What axis are current and voltage on (in a I-V graph)
Y (vertical) = Current (I)
X (horizontal) = Voltage (V)
What does an ohmic conductors current against voltage (I-V) graph look like?
Straight, diagonal line through the entire cross, from bottom left corner to top right corner.
What does a filament lamps I-V graph look like?
Line starts bottom left cand curves towards Y axis, passes through the cross section of the graph and curves away from the Y axis till it reaches the top right
What does a diodes I-V graph look like?
Line starts on the far left of the X axis and goes straight along it till it reaches 1/3 of the way across the right of the X axis, where it curves up towards the top middle of the right corner.
What does LDR stand for?
Light dependent resistor
What is an LDR?
A resistor that is dependent on the intesity of light.
How does an LDR work?
In bright light, the resistance falls. In darkness, the resistance is highest.
What are LDRs used for?
Automatic night lights
Outdoor lighting
Burglar detectors
What is a thermistor ?
A resistor dependent on temperature
How does a thermistor work?
In hot conditions, the resistance drops. In cool conditions, the resistance goes up.
What are thermistors used for?
Temperature detectors such as car engine temperature sensors and electronic thermostats.
What are sensing circuits?
Can be used to turn on or increase the power to components depending on the conditions they are in.
What is a series circuit?
The different components are connected in series, in a line end to end between the +ve and -ve of the power supply. (except for voltmeters which are always connected in parallel)
What is the downside of a series circuit?
If one component breaks or is removed, the circuit is broken and they all stop.
How does potential difference work in a series circuit?
It is shared between the components
How does current work in a series circuit?
The same current flows through all components
The current is determined by I = V/R
How does resistance work in a series circuit?
The total resistance of two components is the sum of their resistances as by adding a resistor in series, the two resistors have to share the total PD.
How does cell potential difference work in a series circuit?
There is a bigger PD when more cells are in series if they’re connected in the same way. So when 2 cells with a PD of 1.5 V are connected in series they supply 3V between them.
What is a parallel circuit?
Each component is separately connected to the +ve and -ve of the supply (except ammeters which are always connected in series)
What is the upside of using a parallel circuit?
If you disconnect or break one component, it will hardly affect the others.
How does potential difference work in a parallel circuit?
It is the same across all components, they all get the full source PD.
How does current work in a parallel circuit?
It is shared between branches, the total current going into a junction has to equal the total current leaving.
How do resistors work in a parallel circuit?
If you have 2 resistors in parallel, their total resistance is less than the resistance of the smallest of the two resistors.
What are the two types of electricity supply?
Alternating currents and direct currents
Main supply uses ____ currents and cells and battery supply uses _____ currents.
Alternating (ac)
Direct (dc)
Which current (ac or dc) is constantly changing direction?
ac
Why do ac currents change direction?
They are produced by alternating voltages in which the positive and negative ends keep alternating.
The UKs mains supply (home electricity) is an ac supply at around _____V
230 V
The frequency of the ac mains supply is _____Hz
50 Hz
Why does dc always flow in the same direction?
It is created by a direct voltage.
Most cables have three separate wires, what are they made of?
A core of copper and a coloured plastic coating
What are the colours of insulation of the 3 wires and what do they show?
Brown = Live wire
Blue = Neutral wire
Green & Yellow= Earth wire
What does the live wire do?
Provides the alternating potential difference at about 230 V from the mains supply
What does the neutral wire do?
Completes the circuit, current flows through the live and neutral wire. It is around 0 V.
What does the earth wire do?
Protects the wiring and for safety it stops the appliance casing from becoming live. It doesn’t usually carry current unless there is a fault. It is at around 0 V.
Dangers of the live wire are?
Could give an electric shock that could injure or kill you as our bodies are at 0 V, so touching the wire produces a large PD across your body and current flows through you.
Dangers of the live and earth connecting?
If the link creates a low resistance path to earth, a huge current will flow which could result in a fire.
The power of an appliance =
= the energy that it transfers per second
Energy transferred by electrical work formula
E = Pt
ET= Power x Time
J = W x S
What does the power rating of an appliance mean?
It is their maximum (safe) operating power which is the maximum amount of energy transferred between stores per second (when the appliance is in use)
PD is ____ transferred per ____ passed.
Energy
Charge
Formula linking energy transferred and charge flow
E = QV
ET = Charge flow x PD
J = C x V
Formula linking power, PD and current
P = VI
Power = PD x Current
W = V x A
Formula linking power, current and resistance
P = I^2 x R
P = Current^2 x Resistance
W = A x Ω
What is the National Grid?
A giant system that distributes energy across the UK
What are the components of the National Grid?
Cables, transformers, power stations, consumers
The National Grid uses a high ___ ___ and a low ___
Potential Difference
Current
What is the PD of the National Grid in volts?
25 000 V original
400 000 V step up
230 V step down
Diagram of National Grid
In book
PD across secondary coil (V) x current in secondary coil (A) =
PD across primary coil (V) x current in primary coil (A)
What is static electricity?
Charges that are not free to move, they build up in one place causing a spark or shock when they finally move.
What causes a build up of static?
Friction
How does friction cause static build up?
When certain insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped on the other. This leaves the materials electrically charges with a positive static charge on one and an equal negative static charge on the other.
Positive charges do not?
Why?
Move
Only electrons move, they cause a positive static charge by moving elsewhere. The material that loses the electrons loses some negative charge and is left with an equal positive charge.
What electric charges attract and repel?
Opposites attract
Same repel
What are electric fields?
Created around any electrically charged object
Drawing an electric field
In book
Charged objects in an electric field feel a force
In book
Why does sparking occur in electric fields?
- Sparks are cause when there is a high enough PD between a charged object and the earth or and earthed object
- A high PD causes a strong electrical field between the charged and earthed object
- The strong electric field causes electrons in the air particles to be removed, this is 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. This is the spark.
