P2 Flashcards
What must be included in a circuit for current to flow?
a source of potential difference
What is current?
the flow of electrical charge
equation linking charge flow, current & time?
charge flow(Q) = current(I) x time(t)
What is charge measured in?
coulombs
What happens when a component has a greater resistance?
It has a smaller current
equation that links potential difference, current and resistance?
potential difference(V) = current(I) x resistance(R)
What factors affect the resistance of an electrical circuit?
- length of wire( at a constant temp)
- combination of resistors in series and parallel
What does a resistor do?
oppose the flow of electric current through the circuit
What is ohms law?
the voltage across a conductor is directly proportional to the current flowing through as long as all physical conditions remain constant
Describe the current, potential difference and resistance through an ohmic conductor
- the current through an ohmic conductor (at a constant temp) is directly proportional to the potential difference across the resistor
- the resistance remains constant as the current changes
Give examples of non-ohmic conductors?
- filament lamp
- diode
What happens to the the resistance of a filament lamp as the temperature increase?
the resistance increases
What happens to the resistance of an LDR as light intensity increases?
the resistance decreases
Give a use of an LDR
light sensing circuits e.g. street lamps
Describe the current in a diode
the current flows in one direction only and the resistance is very high in the reverse direction
What happens to the resistance of a thermistor as the temperature increases?
the resistance decreases
Give a use of a thermistor
a sensor circuit of a thermostat
What is a series circuit?
A circuit that has all its components connected in one single loop
What is a parallel circuit?
A circuit where components are connected on separate branches of wire to the energy source
Describe the current, potential difference and resistance in series circuit
- the same current flows through each component (I1=I2=I3 etc…)
- the the total potential difference is shared between the components (Vtotal= V1+V2 etc…)
- the total resistance is the sum of the resistance of each component (Rtotal= R1+R2 etc…)
Describe the current, potential difference and resistance in a parallel circuit
- the total current is the sum across separate components
- the potential difference is the same across all components
- the total resistance is smaller than the resistance of the smallest resistor
What is an equivalent resistor?
In a series circuit, when two resistors are connected they can be replaced by one equivalent resistor which has a resistance equal to the sum of the ones being replaced
What will happen to a bulb that is connected in series with a thermistor, as the temperature on the thermistor changes?
- at high temperatures, the bulb grows brighter
- at high temperatures, there is a higher potential difference across the bulb
What is Mains electricity in the UK?
an alternating current supply with a frequency of 50Hz and voltage of 230V
What is the difference between direct and alternating current?
Alternating current constantly changes direction and is used in mains electricity whilst direct current always flows in the same direction and is used in solar cells & batteries
What does an oscilloscope screen show?
Voltage against time
What are different colours in the three-core cable?
Neutral wire
-blue = completes the circuit and carries current way from the appliance
- close to earth potential difference= 0v
Earth wire
- yellow/green = a safety wire to stop the appliance becoming live and only carries a current if there is a fault
- has a 0v potential
Live wire
-brown = provides the alternating potential difference from the mains electricity supply
- carries a 230v potential difference
What is a fuse?
a safety measure that involves a thin wire that melts if the current flowing through an appliance is too high.
This breaks the current and stops the flow of current
What is a circuit breaker?
A special device that switches off the flow of current if the current is higher than a certain value. They do the same job as fuses but can be used multiple times
What are the three available fuse ratings?
3A, 5A and 13A
Why do metal cases need earthing?
If there is a fault in the appliance and one of them touches the case due to metals being conductors, anyone touching the case could receive an electric shock
Why can the live wire and earth wire never be in contact?
This would remove the safety feature of the earth wire
What happens if you touch the live wire?
Your body would supply a link between the supply and the ground and cause an electric shock
What is the National Grid?
A system of cables and transformers that connects power stations to consumers
Explain the function of a step-up transformer
A step-up transformer has more turns on the secondary coil so the voltage increases
Explain the function of a step-down transformer
A step-down transformer has fewer turns on the secondary coil so the voltage decreases
How can static electricity be produced?
through friction- by rubbing two electrical insulators against one another
- one would lose electrons- become positive
- the other would gain these electrons- become negative
Can a static charge build up on a conductor?
yes- if the conductor is isolated
What is a spark?
when an electrical charge builds up on an object the potential difference between the object and earth increases, if the potential difference is large enough the electrons can jump across the gap and produces a spark
What do charged objects create around them?
Electrical fields
Describe what may effect the strength of an electric field
- the closer an object is to the charged object, the stronger the electric field
- the higher the charge of the object, the stronger the electric field
