Electricity Flashcards
What happens in direct current
The flow of electrons are consistently in one direction around the circuit
What happens in alternating current
The direction of electron flow continually reverses
What is electrical current
A flow of electrons
How do you measure current
With an ammeter that must be placed in series with that component
How to measure p.d
With a voltmeter that must be placed in parallel with that component
What do conductors and insulators have
- conductors has a low resistance
- insulators have a high resistance
What is the iv characteristics for components
The relationship between current through and voltage across a component
What is the voltage in a fixed resistor (resistor at a constant temp)
The voltage is directly proportional to the current
What is ohms law
The rule that states that the current flowing through a resistor is directly proportional to the voltage across the resistor, provided the temp remains constant
What does directly proportional mean
If one quantity goes up by a certain percentage/ number, the other quantity goes up by the same percentage/ quantity as well
What would doubling the amount of energy into the resistor result in
A current twice as fast as the resistor
How do you know if the relationship is ohms law
Because the resistance of the resistor is fixed (because the temp doesn’t change)
What is a resistor…
An ohmic conductor
What would a graph plotting p.d against current for a fixed resistor look like
Diagonal straight across (because it is directly proportional)
What happens to the current in a filament bulb
- The current doesn’t increase at the same rate as the voltage
- doubling the amount of energy doesn’t cause a current twice as fast
What happens if you put more energy into the bulb of a filament lamp
The harder it is for the current to flow (resistance of the bulb increases)
What happens when the voltage increases in a filament lamp
The temp of the thin wires inside the bulb also increase
What happens in result of the temp being increased in a filament bulb
- the increased vibrations of the ions in the filament make it harder for the electrons to get past
What does a graph plotting p.d against current for a filament bulb look like
- Looks like an f
- The line is an upward curve that levels out and starts to dip as p.d increases
What does a semiconductor diode do
Only allows current to flow in one direction
What is the resistance of the diode like
Very large meaning no current would flow if the p.d was arranged to try and push current the wrong way
What does a graph plotting p.d against diode look like
- line is horizontal on the x axis then curves upwards sharply
What is the relationship in the resistance in a thermistor
- the resistance of a thermistor varies w temp
- when the temp increases the resistance decreases
What is the relationship of the resistance of an LDR
- the resistance of an LDR varies with light intensity
- the more light on the LDR the smaller the resistance of it
What happens in series circuits
Electrical components are connected one after another in a single loop
What are the rules in a series circuit
- electrons with pass through every component on its way round the circuit
- if one bulb is broken then the current won’t flow round
- if one bulb goes out they all go out
Current in series
- a series circuit is one loop
- all electrons in the loop form one current
Explain how the resistance of a filament lamp changes as the p.d across it increases
- the pd increases which increases current flow
- this heats wire
- through heat, the current gains more energy causing it to vibrate more
- this lets fewer electrons through and causes a greater resistance
If light intensity decreases what happens to the pd and current
- pd increases
- current decreases
What is pd
A measure of the energy given to the charge carries in a circuit
What happens to pd in series circuits
- the pd difference supplied is equal to the total of the pd across other components
Resistance in series
If resistors are connected in series, the current flows through both of them meaning the resistances are added together
Key points in series circuits
- the current is the same through each component
- the total pd difference of the power supply is shared between the components
- the total resistance of the circuit is the sum of individual resistors
What is the energy transferred each second equation
P = I x V or P = I squared R
What do fuses do
- breaks the circuit if an appliance causes too much current to flow
- this protects the wiring and the appliance if something goes wrong
How does a fuse protect electrical circuits and appliances
- fuse contains a piece of wire that melts easily
- I’d current going through fuse is too great, wire heats up until it melts and breaks the circuit
What are the standard fuse ratings
- 3A
- 5A
- 13A
- if the device works at 3A, use a 5A fuse
- if the device works at 10A, use a 13A fuse
Why are copper wires used in a plug
They are good conductors
Why is a plastic coating used in wires
Plastic is a good insulator
What does the live wire do
Carries the current that enters the device
What does the neutral wire do
0V, completely safe, connects to cable in wall and completes the circuit
What does earth wire do
- if there is a fault, the earth wire carries away dangerous currents
- low resistance path to ground
What is the equation that links joules watts and power
E= p x t
What does the national grid do
- It distributes electricity across the country
- the electricity is produced by a power station turning a generator
What are transformers used for
- to change voltages and currents in transmission lines
How do you determine whether the transformers will step up or down the voltage
The number of coils
What does increasing voltage do in terms of transformers
Decreases current
What does decreasing voltage do in terms of transformers
Increases current
What does a step up transformer do and why
Increases voltage and reduces current
- less current so that less energy is lost through heating the wire
What does a step down transformer do
Reduces the voltage to a safer voltage (230V) for home use
What is the equation that links power current and resistance
P = I squared x R
How to ensure minimum amount of power is lost from cables (national grid topic)
- they are thick so resistance is low
- high voltages used to reduce current through the transmission lines
Advantages and disadvantages of transformers
