P2.3 Currents in Electrical Circuits NOT FINISHED Flashcards
What are electrical conductors? Can you give an example of one?
A material which allows an electrical charge to easily move through it - metals are good conductors of
What are electrical insulators? Can you give an example of two?
A material which electrical charge struggles to move through - plastics and rubbers are good insulators
Why are wires made up of both electrical conductors and insulators?
The core is made up of an electrical conductor so electrical charge can easily flow through it, and they have an electrical insulator casing to stop you getting an electric shock when in contact with the wire
What is a static charge?
An electric charge which cannot move
Where is static charge most commonly found?
In electrical insulators
What happens when you rub certain electrical insulators together?
Negatively charged electrons will be scraped off one and dumped on the other - this leaves one material which has a positive static charge and one with a negative static charge
How do insulators become positively or negatively charged?
Through the movement of electrons
How is a positive static charge achieved?
From losing electrons
How is a negative static charge achieved?
From gaining electrons
What do two things with opposite electric charges do?
They are attracted to each other
What do two things with the same electric charge do?
They will repel each other
How is a voltmeter connected to a circuit?
It is always connected ‘across’ a component - or ‘in parallel’
How is an ammeter connected to a circuit?
It is always connected ‘in line’ with a component - or ‘in series’
What is an electric current?
A flow of electric charge
What do the different sized lines represent in batteries and cells?
The longer line represents the positive terminal; and the negative line represents the negative terminal
How does currently flow through a circuit?
From positive to negative
What is the formula for finding current?
I = Q/t ==> Current in amperes (A) = charge in coulombs (C) / time in seconds (s)
When does more charge pass around a circuit?
When a bigger current flows
What must there be in a component to allow current to flow?
Potential difference (p.d.)
What is potential difference?
The driving force that pushes the current round the circuit - or in other words, it’s the work done per coulomb of charge that passes between two points in an electrical circuit
What is the formula for potential difference?
V = W/Q ==> Potential difference in volts (V) = work done in joules (J) / charge in coulombs (C)
What is the potential difference of a component?
The amount of energy transferred by that component per unit of charge supplied
What is resistance?
Anything in a circuit which reduces the flow of current
What is resistance measured in?
Ohms
What does a greater resistance mean?
The smaller current that flows (for a given p.d. across the component)
What is the formula for Ohm’s law?
V = I x R ==> P.d. in volts (V) = current in amperes (A) x resistance in ohms (Ω)
What do current-potential difference (I-v) graphs show?
How the current varies as you change the p.d.
What does a straight-line graph suggest on an I-V graph?
Because the gradient of the line is constant, the resistance of the component is steady
What does a curved graph suggest on an I-V graph?
That the resistance is constantly changing
What is on the x-axis on an I-V graph?
V - potential difference
What is on the y-axis on an I-V graph?
I - current
How do you work out the resistance at a point from an I-V graph?
The p.d. of the point, divided by the current of the point (a straight line graph will give you the same value for each point)
Why do different resistors have different gradients on I-V graphs?
Different resistors have different resistances and the current through a resistor (at constant temp) is directly proportional to p.d.
What do filament lamps contain? What are they designed to do?
They contain a filament, which is designed to get hot and glow
What happens to a filament lamp as more current flows through it? What does this do to an I-V graph?
As more current flows through the lamp, the temperature of the filament increases causing the resistance to increase, making the graph curve on an I-V graph
What happens when an electrical charge flows through a component with resistance?
Some of the electrical energy is transferred to heat energy and the components get hot
Why does resistance increase when there is an increase in temperature?
The heat energy causes the ions in the conductor to vibrate more, these ions make it difficult for charge-carrying electrons to get through the component - as the current can’t flow easily, the resistance increases
What do most circuits limit?
The amount of current that can flow
Why do many components level off at high currents in I-V graphs?
As the more current which flows, increases the temperature, and this causing an increase in resistance thus decreasing the current…
What does a diode allow current to do?
It lets current flow freely through it in one direction, but not in the other
Why doesn’t current flow through the reverse direction of a diode?
Because it has a very high resistance in the reverse direction
What does LED stand for?
Light-emitting diode
What does an LED do?
Emits light when a current flows through it in the forward direction
What can LEDs be used for?
To indicate the presence of a current in a circuit
What advantages do LEDs have over filament bulbs? (3)
They have a much longer lifetime, they are more energy efficient, and they use much less current to operate so are more cost efficient and cheaper
What advantages do filament bulbs have over LEDs? (2)
They are cheaper to buy and you need multiple LEDs to provide the same amount of light as a filament bulb
What other uses do LEDs have? (4
In appliances to show if they are switched on, remote controls when you press a button, LED TVs, traffic lights etc
What does LDR stand for?
Light-dependent resistor
What is a LDR?
A resistor that is dependent on the intensity of light
How do LDRs react to light?
In bright light, the resistance falls and in darkness, the resistance is highest
When are LDRs used?
When you only want a component to work in the dark
What are thermistors? What do they depend on?
They are resistors which (their resistance) depend on their temperature
What does NTC stand for?
A type of thermistor - Negative Temperature Coefficient
How do NTCs react to temperature?
In hot conditions, the resistance drops and in cool conditions, the resistance goes up
What can thermistors be used for?
Thermostats - the resistance of the thermistor links to the temperature of the thermistor and its surroundings
How are components connected in a series circuit?
In a line, end to end, between the positive and negative ends of the power supply
How are voltmeters connected to a circuit?
Always across a component
What happens if you disconnect a component in a series circuit?
The circuit is broken and all components stop
When is there a bigger p.d. in a series circuit?
When more cells are connected
How do you work out the total power source p.d. of a series circuit?
By adding up all the individual cell p.d.(s)
What happens to the p.d. between more than one component in a series circle?
It is shared between them (but it is not always equally shared)
What happens to the current between all the components of a series circuit?
It is equal in all of them
What is the total resistance of a series circuit?
The sum of all the resistances of the components
What does the resistance of a component determine in a series circuit?
The bigger the resistance of a component, the bigger its share of total p.d.
What are the four rules of a series circuit?
The p.d. of cells adds up to the source p.d., the source p.d. is split across the components, the current is the same in all components, and the total resistance of the circuit is the sum off all the resistances of the separate components
What do components connected in parallel have?
Their own branch in a circuit connected to the positive and negative of the supply
What will happen if you remove a component which is connected in parallel to a circuit? Why?
It will hardly affect the other components - as current still flows in a complete loop from one end of the power supply to the other through the branches that are still connected
What happens to the p.d. of components in a parallel circuit?
The potential difference is the same across all components
What is the total current flowing around a parallel circuit?
The total of all the current through the separate brunches - the total current splits into each brunch
What are the two main rules of parallel circuits?
The p.d. across each branch is the same as the source p.d., and the current is split across the branches, and the total current is the sum of the current of each branch
What happens to a circuit if it contains components connected in series and in parallel?
The different rules apply for each different bit
