2. Electricity Flashcards
What direction is ‘electron flow’ in?
Opposite direction of the conventional current
What is charge?
The electrons that are always in a wire - whether connected to a cell or not
What is current?
The flow of electrons in a conductor - when it’s connected to a cell in a complete circuit
What is current measured in?
Amps (A)
What is voltage?
The push and pull of electrons in a wire - makes electrons flow
What is voltage measured in?
Volts
What is electrical energy?
Energy carried by current and converted to heat and light energy
Is the current the same all the way round in series?
Yes
Why do electrical charges move through metals?
They have some electrons that are free to move from their atoms
What is an electric current?
A movement of charge
What is electrolysis?
When current flows in a liquid, carried by positive and negative ions
What happens when electrical current flows through a resistor?
Electrical energy is transformed into heat energy
What does the energy transferred depend on?
Amount of charge carried by electrons and p.d. Pushing the charge around
What charge do electrons have?
Negative
What is the unit for power?
Watt (W)
What happens to power as current increases?
Increases
What happens to a lamp when there is lower resistance?
the lamp is brighter
What do electrons have to do when passing through a wire?
Push their way through vibrating electrons
What does the resistance in a circuit do?
Limit the current
Ohms law?
The current through a resistor at a constant temperature is directly proportional to the potential difference across the resistor
What is the p.d. of cells?
The sum of the p.d. of each cell
In a series circuit…
- the same current passes through the components
- the p.d. of the voltage supply is shared between the components
- the total resistance of components is equal to the sum of the resistance of each component
In a parallel circuit…
- the total current through the whole circuit is the sum of the currents through the separate components
- the p.d. across each branch is the same
Is the voltage across each branch in parallel the same?
Yes
What happens to current as resistance of a filament lamp increases?
Increases
What does a current - p.d. graph with a straight line show?
The resistance is constant, obeying Ohm’s law.
An example of an ohmic conductor?
A wire
What does a current - p.d. graph with a curve show?
The resistance isn’t constant, it increases as the temperature increases.
What does a current - p.d. graph with a forward/reverse bias show?
The resistance is not constant, it rapidly decreased and the current increases
What component will make a forward bias?
Diode
What component will make a curve?
Filament lamp
What component will make a straight line?
A wire
What happens to resistance as light intensity increases?
It increases
What is static electricity?
When charge builds up on an object and doesn’t move e.g. rubbing a rod
What will happen when two different rods are held close to each other?
They will attract if they have different charge
What happens to materials that are oppositely charged?
They attract
What will happen when two rods of the same material are held close to each other?
They will repel as they have the same charge
What happens to materials that have the same charge?
They will repel each other
Are electrostatic forces contact or non-contact?
Non-contact forces that can act at a distance
What is an electric field?
The region where a charged object would experience a force
How is an electric field represented?
Using field lines
What does the direction of a field line show?
The direction of the force that would act on a small positive charge at that point
What does the spacing of field lines show?
The strength of the force.
The closer together the lines, the stronger the force
What is a uniform field?
A field for a force that has a constant size
What direction do field lines point when an object is positively charged?
Field lines point outwards because like charges repel
What direction do field lines point when an object is negatively charged?
Field lines point inwards because unlike charges attract
What happens to field lines as you move away from the charged object?
Field lines spread out as you move away, so the force becomes weaker
How does a ‘Van de Graff’ generator work?
A charge builds up on the dome due to electrons being rubbed off by the belt. If charge is big enough then the voltage comes high enough to ionise the air molecules so electrons jump down to earth = electric current
Dangers with static electricity when refuelling?
Fuel gains electrons from pipe so pipe is positive and fuel negative. Resulting voltage may cause a spark.
What is direct current?
Current travels in one direction
An example of direct current?
Simple circuit
What is an alternating current?
Current repeatedly reversed its direction
What measures the waves of an alternating current?
Oscilloscope
What current do cells and batteries alway supply?
Direct current which always passes in the same direction
What type of supply is mains electricity?
a.c. supply
What is the voltage of the live wire in an alternating current?
Alternates between +325V and -325V = 230V d.c.
What is the frequency of the mains a.c. in the U.K.?
50Hz
What is a metal wire made up of?
A lattice of positive ions, surrounded by delocalised electrons
What happens to electrons when a wire is attached to the battery?
The free electrons are repelled by the negative terminal and attracted to the positive one
What direction is electron flow?
Opposite to the direction of the conventional current
How often do electrons change direction in an a.c. circuit?
Every 0.01 seconds
How does a mains-powered appliance gets its energy if the electrons do not travel all the way from the power source?
The power sources causes all electrons in a circuit loop to vibrate at once; energy is transferred from electrons to appliance
Why is the casing of a plug made from plastic?
Because it doesn’t conduct, it’s a good insulator
What is the casing of a plug made from?
Plastic
What do cables contain?
Copper wires
Why do cables contain copper wires?
Because copper is a good conductor
How is copper arranged in a wire?
The copper is made into lots of thin strands twisted together rather than one thick wire as this makes the cables more flexible
What are the pins in a plug made from?
Brass
Why are the pins in a cable made from brass?
It is a good conductor, it is stiff and does not corrode unlike copper
Where is the fuse in a plug?
Between the live pin and the live wire
What does the fuse do?
If current gets too high the fuse melts and breaks the circuit, cutting off the live wire
Which pin is the longest in a plug?
The earth pin
Why is the earth pin the longest in a plug?
To make contact with the earth wire of the socket, before contact with the live wire
What colour is the live wire?
Brown
What does the live wire do?
Alternates between +325V and -325V which is equivalent to a 230V d.c.
What colour is the neutral wire?
Blue
What does a neutral wire do?
Completes the circuit
What colour is the earth wire?
Yellow/green
What does the earth wire do?
Prevents the appliance becoming ‘live’
What type of cables are less likely to overheat?
Thick cables
What will thin wires be used for?
Lighting
What will thick wires be used for?
Electric cookers
What thickness of wire is used when?
Thin wires will be used for lower currents whereas thicker wires will be used for higher currents
How is the amount of power for an object altered when connected to the mains electricity?
Current will be altered
Describe a circuit when a wire has short-circuited one out of three bulbs
- the resistance of the wire is almost zero
- current flows through wire instead of lamp 1
- current passes through two bulbs
- loop is smaller and current is higher
- bulbs are brighter
Describe a circuit when a wire has short-circuited three bulbs
- all 3 bulbs switch off
- wire gets very hot
- resistance of loops is zero
- current is a lot higher
- wire overheats
What is a short circuit?
An electrical circuit that allows current to travel along unintended paths with no or very low resistance
Resistance low = current very high
Describe the resistance and the current in a short circuit
No or very low resistance, very high current
Why do lamps blow when they have just been turned on?
The lamp is cold so low resistance, high current flows - sometimes enough to melt the wire in the bulb
What happens to resistance as you add components in a series circuit?
more components = more resistance
What happens to resistance as you add components in a parallel circuit?
more components = less resistance
What unit do electrical companies bill customers in?
Kilowatt hours (kWh)
What is 1 kWh?
The energy delivered when a 1kW device is used for 1 hour
What is 1 kWh equal to in J?
3.6 million J
What does the cost of using a particular device depend on?
How fast it uses electrical energy (the power of the device) and how long it’s being used for
What does a fuse contain?
A thin wire that heats up and melts if too much current passes through it
What happens when a fuse ‘blows’?
The thin wire inside the fuse heats up and melts when too much current passes through it
What happens if the current rating is too large?
The fuse will not blow when it should, and the heating effect could cause a fire in the appliance
What size fuse are domestic appliances often fitted with?
3, 5 or 13A
How do you work out which fuse to use for a particular appliance?
Need to know the power rating and working p.d. of the device so that you can calculate its working current
When are earth wires used?
If the appliance has a metal case
What happens when the live wire touches the case, in terms of the earth wire?
The large current surges down the earth wire and the fuse blows
What happens if a fuse doesn’t blow because of incorrect rating?
The current will flow down the earth wire and the user won’t be electrocuted by touching the case
What is the National Grid?
A network of transporters and power lines which ensure electricity is carried to where it is needed
What happens to heat loss as current increases?
It increases
What happens to voltage at the start of the national grid in power stations?
The voltage is ‘stepped-up’ (increased) using a transformer
Why is voltage increased to transfer energy across the national grid?
Increasing the voltage reduces the current in the power lines which reduces energy in the cables, and heat energy produced
What happens to voltage at the consumer end of the national grid?
Transformers ‘step-down’ voltage to safer levels
Advantages of overhead cables?
Cheap to install and repair
High, out of reach
Advantages of underground cables?
Out of sight
Safe from weather conditions
Disadvantages of overhead cables?
Unattractive
More likely to get damaged in storms
Disadvantages of underground cables?
Not as far from people e.g. digging roads
Expensive - digging tunnels
Difficult to know where they are when repairing
What is the voltage from a power station before it is stepped up?
20 kV
What is the voltage after it’s been stepped up?
400 kV
What is the voltage after it’s been stepped down?
230 V
What is a thermistor affected by?
A change in temperature
What is an LDR affected by?
A change in light intensity
Practically, how could you change the resistance of a thermistor?
Use different temperatures of water
Practically, how could you change the resistance of a LDR?
Alter the distance between LDR and light source
In a thermistor, what is the link between resistance and temperature?
Resistance increases as temperature decreases
In a LDR, what is the link between resistance and light intensity?
Resistance increases as light intensity decreases
What is the graph of a thermistor at a constant temperature? What does this mean for the resistance?
A straight line so its resistance is constant
What is the graph of a LDR at a constant LI? What does this mean for the resistance?
A straight line so its resistance is constant
What happens when temperature is increased in a circuit with a thermistor?
Resistance decreases. Current increased so a component may be turned on
What happens when LI is increased in a circuit with a LDR?
Resistance decreases. Current increased so a component may be turn on
What are thermistors and LDRs examples of?
Ohmic conductors
In a potential divider, what happens to voltage when two resistors are equal?
They share applied voltage equally
In a potential divider, a current passes through a resistor then through a thermistor. How is output voltage increased?
The thermistor cools. The resistance of the thermistor rises, taking a larger share of input voltage - so output voltage rises
In a potential divider, a current passes through a thermistor then through a resistor. How is output voltage increased?
Temperature of the thermistor is increased and resistance decreases. The resistor takes a larger share of the input voltage - increasing the output
In a potential divider, a current passes through a resistor then through a LDR. How is output voltage increased?
LI decreases, resistance increases, taking a larger share of the input voltage. So output voltage rises.
What is power?
The rate at which energy is transformed
Another phrase for energy transferred?
Work done
