electricity Flashcards
What is a current? (amps) (A)
The rate of flow of charge round the circuit
What charge are electrons?
Negatively charged particles
What is the voltage (volts) (v)
What drives the current around
“electrical pressure”
-Amount of energy transferred per unit of charge
Resistance (ohm)
-Anything in the circuit which slows the flow down
-If you add more components, higher overall resistance
Another name for voltage:
Potential difference
If you increase voltage…
More current will flow
If you increase resistance…
Less current will flow
What does an ammeter measure:
The current flowing through the component
Rule for ammeter in series:
- Can be placed anywhere in the main circuit
-Current stays the same
Voltmeter
-Measures the voltage across the component
-Must be placed in parallel around the component not the variable resistor or the battery.
rules for series:
-The component, the ammeter, and the variable resistor are all in series, which means they can be put in any order
rule for parallel:
-the voltmeter can only be placed in parallel around the component under test.
voltage formula
voltage= current x resistance
Rules for gradient graphs:
- The gradient of an IV graph shows you how the resistance of the component behaves
-The steeper the graph the lower the resistance
-If the graph curves, it means the resistance is changing
Four important current-voltage graphs
-wire/fixed resistor
-Bulb/lamp
-LED/diode
-LDR
LED (light emitting diodes)
- emit light when a current flows through them
-used for numbers…traffic lights
-Dn’t have a filament that can burn out
Components that can change resistance
Light dependent resistor
-LDR- changes its resistance based on how much light falls
-In bright light, less resistance
-In dark light, more resistance
Thermistor:
Temperature dependent resistor
-In hot conditions, resistance drops
-In cool conditions, resistance goes up
Series vs Parallel
Series:
- The different components are connected in a line, end to end
-If you remove or disconnect one component, the circuit is broken and it stops working.
Series vs Parallel
Rules for series: 5 points
- Bigger supply p.d when more cells are in series
-Current is the same everywhere
-Size of current depends on total potential difference and resistance of circuit
-Voltage of the supply is shared between components
-Total resistance of the circuit depends on number of components and the type of components used.
Total R= R1+R2+R3
Series Vs Parallel
Parallel:
- each component is separately connected to the +ve -ve of supply, except for ammeters which are always connected
-If you remove one component, it will hardly affect the others
-The potential difference is the same across all branches V1=V2
-Current is shared between branches. Total current= total of all the current flowing through separate components
-The current depends on resistance. Higher resistance= harder for charge to flow
Charge
Current is the rate of flow of electrical charge
-Charged is carried by negatively charged electrons.
charge= current x time
When a charge drops through a voltage…
It transfers energy.
-Energy is supplied to charge at power source
-The bigger the change in voltage, the more energy is transferred for a given amount of charge.
-A battery with a bigger voltage will supply more energy to the circuit for every coulomb of charge which flows round it
-More charge, more energy transferred into circuit
What is equation for energy transferred?
energy transferred= charge x voltage
Energy transferred with a resistance:
energy transferred= charge x current x resistance
Electrical safety:
three wires in a plug
- Live
- Neutral
- Earth
Electrical safety:
Rule of wires
Live wire: alternates between high voltage and low voltage
Neutral: Always at 0V
Earth wire/ fuse: just for safety and work together
Electrical safety:
Earthing
Case must be attached to an earth wire. An earth conductor can never become live
Electrical safety:
Double insulated
If an appliance has a plastic casing and no metal parts showing
Earthing protection steps:
-If fault develops and the live touches metal case, because case is earthed, current flows through live, the case and earth wire
-The surge in current blows the fuse which cuts of live supply
-Impossible to get electric shock. Prevents risk of fire
Circuit breakers vs Fuses:
Circuit breakers
Electrical safety devices used in some circuits
- When circuit breakers detect a surge in current, they break circuit by opening a switch.
- Circuit breaker can easily be reset by flicking a switch on the device unlike fuses which need to be replaced when melted.
-safer: immediately break as soon as there is current surge
Energy and power:
Resistor
- Resistors get hot when there is an energy transfer
-Electrons collide with the ions in the lattice that make up resistor which gives ions energy ( vibrate and heat up)
-heating effect increases resistor’s resistance- so less current will flow
-Heating effect can cause other components to melt (eg fuses)
-
Electricity power and Fuse:
definition
-Electrical power is the rate at which appliance transfers energy
-An appliance with a high power rating transfers a lot of energy in a short time
Electric Power equation:
power= current x voltage
Electric power and fuse:
Fuse
To work out fuse needed… you need to work out current that the item will normally use.
-Fuse should be rated a little higher than the normal current.
Energy transferred:
- Power of appliance and how long it is on for (seconds)
Static electricity:
Rule of charge (3 points)
- Opposite charges attract
-Two things with same charge repel
-Forces get weaker the further apart two things are
Static charge meaning
-A charge which builds up in one place and is not free to move
-Common on insulators, where current cannot flow
Cause of static electricity
Friction
-When two insulating materials are rubbed together, electrons will be scraped off one and dumped on the other
-Leaves a positive electrostatic charge and a negative elec… charge.
both +ve and -ve charges are only produced by movemement of…
electrons
-positive charge does not move
-positive static charge is caused by electrons moving elsewhere
Static electricity:
Conductors
- Static charges can occur on conductors too
-Cars often get a static charge on the outside because they’ve gained /lost electrons
Static electricity:
Conductors
- A charged conductor can be discharged safely by connecting it to earth with a metal strap
-Electrons flow down the strap i=to the ground if charge is negative and up from the ground if charge is positive.
As charge builds up…
so does voltage
- An electric charge builds on an isolated object the voltage between the object and earth increases
- if the voltage is large enough, electrons can jump across gap between charged object and earth- spark.
Practical:
Investigating static electricity
(polythene and acetate)
- Rub on cloth
-When polythene rubs with duster
-Electrons move from duster to rod
-Duster is negative, rod is positive
