Electricity 2 Flashcards
The home, national grid, energy transfer, electrical fields, electricity.
What are the stages of the national grid? (steps 1 -3)
- Electric power is generated at a power station.
- Step up transformers (between power stations and transmission lines) increase voltage and decrease current.
- The electrical power is transmitted via the national grid at low current which minimises power and energy loss (efficient).
What are the stages of the national grid? (steps 4 and 5)
- Step down transformers (between transmission lines and domestic buildings) decrease voltage and increase current near our homes.
- Electric power is wired into our homes at a voltage that is relatively safe to use (230V).
Why is a high current bad when transmitting electricity?
A high current generates a lot of wasted heat because the wire heats up, and energy is lost as thermal energy to the surroundings.
What are the three wires in UK plugs?
Live wire
Neutral wire
Earth wire
What colour is the live wire?
Brown
What colour is the neutral wire?
Blue
What colour is the earth wire?
Green and yellow stripes.
How does the earth wire work to prevent an appliance becoming live?
-The earth wire is connected to the casing of the appliance.
-If the live wire touches the appliance casing, the earth wire provides an alternative pathway for electricity to flow.
-This way, we don’t get an electric shock from touching the live appliance casing.
What are the risks of surges in current?
Electric shocks, fires, damage to appliance.
Safety features of UK plugs.
-Fuse
-Circuit breakers
-Double insulation
-Earth wire
What does a fuse do?
Melts if the current through it is too high, which breaks the circuit.
What is a fuse’s rating?
The current at which a fuse breaks a circuit.
What are circuit breakers?
Fuses that can be reset and reused.
What is double insulation in terms of UK plugs?
Plastic casing, but no earth wire, so that electricity is not conducted to things the wire touches.
Where are electric fields found?
Around all charged objects.
Electrical field lines run…
…from positive to negative.
The closer to a charged object…
…the stronger the electric field (and the closer the field lines).
Ionisation (physics)
The physical process of atoms loosing or gaining electrons.
A strongly charged object can ionise the air, which means the air looses some of its electrons and conducts electricity.
What is an electrical field?
A region in which a charged object will experience a non-contact electrical force.
What is static electricity?
The build up of electrical charge on insulating materials.
Explain how rubbing an acetate rod with a cloth causes the rod and cloth to become charged (static electricity).
- Friction between the cloth and rod causes electrons to move to the cloth.
-The cloth is now negatively charged and the rod is positively charged.
Describe what happens when two electrically charged object are brought close to each other.
The charged objects exert a force on one another.
If two objects have the same type of electrical charge, they repel each other.
If they have different types of electrical charge, they attract one another.
How can we prevent static charge?
Earth two rubbing materials so that charge flows off of them to the ground.
What is an electrical conductor?
A material capable of transferring electrical energy (carrying charge) because its electrons are delocalised and can flow
A device has higher power if..?
It transfers more energy in a shorter time.
Explain why electrical power is transmitted at high voltage and low current on the National Grid.
Because the amount of electrical energy wasted as thermal energy is proportional to the square of the current, from the equation
P = I²R.
For example, if the current got 10x bigger, then 100x more energy would be lost as heat each second.
What does the live wire do?
The live wire carries the alternating potential difference from the supply.
What does the neutral wire do?
The neutral wire completes the circuit.
What does the earth wire do?
The earth wire is a safety wire to stop the appliance becoming live.
What is the pd of the earth and neutral wires?
0V
When does the earth wire carry a current?
Only if there’s a fault.
Why can a live wire be dangerous - even when a switch in the mains circuit is open?
If a person touches a metal device that is live, current would flow through them to earth.
This could be fatal.
What is the power transfer in any circuit device related to?
-The potential difference across it
-The current through it
-The energy changes over time
What are the electricity power equations?
P=VI
P=I²R
(P= E/t)
Define Power (W)
The rate of energy transfer.
The amount of energy transferred by electrical work can be calculated using what equations?
E=Pt
E=QV
(E=VIt)
What are everyday electrical appliances designed to do?
Bring about energy transfers.
What does the amount of energy an appliance transfers depend on?
-How long the appliance is switched on for
-The power of the appliance
An appliance with a higher power rating…
..transfers more energy to other energy stores each second than an appliance with a lower power rating.
Students should be able to describe how different domestic appliances transfer energy from batteries or ac mains to the kinetic
energy of electric motors or the energy of heating devices.
What is the kettle example for this?
A kettle has a power rating of 2kW and so will transfer much more energy to the thermal store of the surroundings than a lightbulb with a power rating of 10W, in the same time.
Is work is done when charge flows in a circuit?
Yes. Work is done when charge flows in a circuit.
What is the power of a circuit device related to?
- the potential difference across it
-the current through it
- the energy transferred over a given time.
A device that transfers more energy in a shorter time has a…
…higher power.
What is the national grid?
A system of cables and transformers linking
power stations to consumers.
How is electrical power transferred from power stations to consumers?
Using the national grid.
Briefly describe what happens in the national grid.
Step-up transformers are used to increase the potential difference from the power station to the transmission cables.
Then, step-down transformers are used to decrease, to a much lower value, the
potential difference for domestic use.
What is attraction and repulsion between two charged objects examples of?
Non-contact force.
Describe the process of sparking by rubbing surfaces. (steps 1 and 2)
-Electrons are transferred onto your clothes by friction.
-They will stay on your clothes because they are insulators and so the electrons cannot flow off.
Describe the process of sparking by rubbing surfaces. (steps 3 and 4)
-However, if you then touch a e.g. metal door handle, the electrons can conduct to earth as a spark.
-The electrons move off your clothes because there is a potential difference between your negatively charged clothes and the earth at 0V.
Where does the electric field from a positive point charge act?
Radially outwards
Where does the electric field from a negative point charge act?
Radially inwards.
Is electric field strength a scalar or vector quantity?
Vector, because it has both size and direction.
The arrows show the direction that an opposite charge would moved if placed in the field. (make sure your field line touch the particle!!!)
Why can materials gain electrons when they are rubbed?
-An electric field is created in the region around a charged object
-The charged object will experience a non-contact force
-A negatively charged electron placed in the electric field around a positively charged nucleus will experience a non-contact force that attracts it towards the positive charge.
-The force gets stronger as the distance between the objects decreases.
Explain how electric fields provide an explanation of the non-contact force between objects.
-An electric field is created in the region around a charged object
-The charged object will experience a non-contact force
-A negatively charged electron placed in the electric field around a positively charged nucleus will experience a non-contact force that attracts it towards the positive charge.
-The force gets stronger as the distance between the objects decreases.
When drawing field lines, the more lines…
…the stronger the electric field.
Explain what will happen to a proton that is placed in the electric field of a positive point charge.
A proton is a positively charged particle, so it will be repelled by the positive charge and move outwards.
Since the proton will experience a resultant force, it will accelerate.
explain how the motion of a charged particle changes in an electrical field when:
a) its mass changes
b) it is given the opposite charge
a) greater mass means less acceleration (from a = Flm
b) It would move in the opposite direction with the same initial acceleration.
How is a spark produced?
When there is a difference between the positive and negative charge of two objects, there is a potential difference between the two objects.
This potential difference causes a rapid transfer of charge.
Energy is stored in an electrostatic store and a spark is produced.
Where does a static electric force occur?
Between a positively and negatively charged object.
Which material becomes positively charged in the static electricity experiment?
The conductor (e.g. the rod).
Why is there a force of attraction between a rod and a cloth?
Unlike charges attract.
The amount of charge on a metal dome is increased. This causes what?
The potential difference between the metal dome and the earthed sphere to increase.
Like charges …
repel
What happens to a comb to make it attract hairs?
It becomes electrically charged.
If a comb (or another electrically charged object) is held above some small pieces of dry tissue paper, what is likely to happen?
Comb attracts paper.
If you rub your hands all over a charged object it will no longer attract your hair/another object. Explain why [2]
Charge/ electricity has gone to your body.
Explain why the student’s hair stands on end.
Each hair is negatively charged/gains electrons
Similar charges repel
What is an electric field?
A region where charges experience a force.
