Electricity Flashcards
What is resistance measured in?
Ohms (Ω)
What is charge measured in?
Coulombs
What is current measured in?
Amps
What is ohm’s law?
potential difference (V) = current (I) x resistance (R)
What is the formula for charge?
Charge (Q) = current (I) x time (t)
What is the formula for efficiency?
Efficiency (%) = Useful energy (or power) output / Total energy (or power) input
What is power measured in?
Watts
What is mains electricity? (3 marks)
Mains electricity is the electricity generated by power stations and transported around the country through the National Grid.
Mains electricity is an alternating current supply.
What are hertz used to measure?
Frequency
What does one hertz equal to?
One cycle per second
Why does alternating current have no polarity (positive and negative sides)? (3 marks)
Polarity results from the fact that an electrical circuit has a negative and a positive pole. Direct current (DC) flows in one direction, resulting in a constant polarity. However, alternating current (AC) regularly changes direction and therefore has no polarity.
How are electrical appliances connected to the mains? (1 mark)
They are connected using a three-core cable
Why are wires covered in plastic? (2 marks)
Plastic is an insulator and does not conduct electricity
What is direct current? (3 mark)
A current that is steady, constantly flowing in the same direction in a circuit, from positive to negative. It has a fixed positive and negative terminal.
What is alternating current? (2 marks)
A current that continuously changes its direction, going back and forth around a circuit. It has two identical terminals.
What colour is the insulation covering the live wire?
Brown
What colour is the insulation covering the neutral wire?
Blue
What colour is the insulation covering the earth wire?
Yellow and green stripes
What are the two types of errors?
Systematic error (e.g. zero error) Random error (e.g reading off)
What is the purpose of the live wire? (1 mark)
Carries the alternating potential difference from the mains electricity supply to a circuit
What unit is potential difference measured in?
Volts
Why is voltage called potential difference? (3 marks)
Electrical energy is transferred to the electrons each time they pass through the battery. Some of the energy of the electrons is transferred to the components (e.g. a lamp or a resistor). This causes a difference in energy across the component, which is known as an electrical potential difference
What is the purpose of the neutral wire? (2 marks)
Forms the opposite end of the circuit to the live wire to complete the circuit, it has no voltage.
What is the purpose of the earth wire?
Acts as a safety wire to stop the appliance from becoming live
Why is the live wire dangerous? (1 mark)
Its potential repeatedly changes from positive to negative and back every cycle
What is the frequency and the average potential difference in mains electricity?
230V (+-325)
50Hz
What produces direct current?
Electric cells or batteries
What happens in a series circuit? (2 marks)
The current is the same at all points.
The potential difference of the power supply is shared between the individual components
What are two disadvantages of series circuits? (2 marks)
If one of the components breaks, all of the others will stop working
The components cannot be controlled (switched on and off) separately
What produces alternating current?
Electrical generators
What measures current?
Oscilloscope
What voltage does the live wire carry?
230V
What voltage does the earth wire carry?
0V, only carries a current to the ground if there is a fault in the appliance
Why is the earth wire needed? (2 marks)
Many electrical appliances have metal cases, which are conductors of electricity. The earth wire provides a low resistance path to the Earth. This prevents the appliance from becoming live and causing an electric shock.
How does the earth wire reduce the risk of electrocution? (4 marks)
The earth wire provides a low resistance path to the Earth.
This causes a surge of current in the earth wire and hence also in the live wire.
The high current through the fuse causes it to melt and break.
This cuts off the supply of electricity to the appliance, making it safe to touch.
What is the national grid? (2 marks)
It consists of a system of cables and transformers linking power stations to consumers (houses, factories and buildings).
How does electrical power reach consumers?
Electrical power is transferred from power stations to consumers via the national grid.
What is the purpose of a step-up transformer? (2 marks)
Step-up transformers increase the voltage (and reduce the current) through the wires, which reduces the energy lost by heating the wires
What is the purpose of step-down transformers? (2 marks)
Step-down transformers decrease the voltage (and increase the current) through the wires
What are the benefits of the national grid? (4 marks)
It is an efficient way to transfer energy due to the use of step-up and step-down transformers. When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss. By increasing the potential difference at which the electricity is transmitted, the same amount of power can be transmitted using a much smaller current (P = IV). This results in less heat being produced in the wire and hence less energy loss.
What does high potential difference mean for the power? (2 marks)
High potential difference means low current (less energy loss) for the same power
What does low potential difference mean for the power? (2 marks)
Low potential difference means high current (more energy loss) for the same power
Explain how the step-up transformer increases the efficiency of the National Grid (6 marks)
The useful energy is the electrical energy, the wasted energy is the thermal energy. The lower the current, the cooler the wire will be. The step-up transformer increases the voltage and decreases the current in the power cables. By decreasing the current of the transmitted electricity, the thermal energy lost through the power cables is reduced. Therefore, there is more electrical energy to be supplied to the homes which increases the efficiency of the National Grid.
What are the disadvantages of the national grid? (2 marks)
Power is wasted heating the power lines
Pylons are ugly on the landscape
What are the advantages of the national grid? (3 marks)
Power stations can be built where the fuel reserves are located
Pollution can be kept away from cities.
Power can be diverted to where it is needed
What are the advantages of overhead cables? (2 marks)
cheaper
easier to access for maintenance and repair
What are the disadvantages of overhead cables/pylons? (3 marks)
Look ugly
affected by weather
Require a lot of maintenance
What are the advantages of underground cables? (4 marks)
Less maintenance
More reliable
Less affected by weather
No visual impact
What are the disadvantages of underground cables? (2 marks)
More expensive
Harder to access for maintenance and repair
What is the formula for energy transferred using power and time?
Energy transferred (E) = power (P) x time (t)
What is the formula for energy transferred using charge and potential difference?
Energy transferred (E) = charge (Q) x potential difference (V)
What is the formula for power using potential difference and current?
power (P) = potential difference (V) x current (I)
What is the formula for power using current and resistance?
power (P) = current^2 (I^2) x resistance (R)
What is resolution of a device?
The resolution of a device is the smallest change that the device can detect (in the variable being measured)
What is the formula for total resistance?
total resistance = resistance^1 + resistance^2
What is the formula for power using time taken and energy transferred?
power (P) = energy transferred (E) / time taken (t)
What is the formula for efficiency?
efficiency = total (energy or power) output / total (energy or power) input
What is time measured in?
seconds (s)
Why do electrical appliances waste energy? (3 marks)
Electrical appliances waste energy because the resistance of the wires and the components causes the current in both the wires and the components of the appliance to generate a heating effect. Energy is transferred to heat the surroundings (energy is dissipated)
Why do electric motors waste energy due to friction? (2 marks)
Friction between the moving parts heat them up, so they transfer energy by heating the surroundings (energy is dissipated)
What happens when charge flows through a resistor? (3 marks)
Energy is transferred to the resistor, so the resistor becomes hotter. Therefore, it heats the surroundings through the transfer of energy.
What happens when a resistor is connected to a battery? (6 marks)
Work is done by the battery to make electrons pass through the resistor. Each electron repeatedly collides with the vibrating metal ions of the resistor, transferring energy to the ions. So the ions of the resistor gain kinetic energy and vibrate more. The resistor gets hotter.
What direction does current flow in a battery?
From positive (long line) to negtative (short line)
How can the fuse be determined?
You can calculate using the power rating of the appliance and its potential difference
What is a short circuit? (1 mark)
A short circuit is a low resistance path for the current to follow
What is an example of a short circuit?
If the live wire inside the appliance touches the neutral wire, a very big current passes between the two points of contact.
What is the potential difference of the human body?
0 Volts
Why might electrical appliances still be dangerous even if the power is off?
The live wire may still hold some potential difference
What is the purpose of the fuse? (3 marks)
Fuses contain a thin wire that melts if the current flowing through an appliance is too high. This breaks the circuit and stops the flow of electric current.
When are two-core cables used for?
Appliances that have plastic cases
What are three-core cables used for?
Appliances that have metal casing
Why do cables have different thickness?
Cables have different thickness for different purposes.
Why are the wires in some cables much thicker? (2 marks)
So they have less resistance. This stops the heating effect of the current from making the wires too hot.
What is the purpose of a circuit breaker? (2 marks)
Circuit breakers are special devices that switch off the flow of current if the current is higher than a certain value. However, unlike fuses they are reusable.
When does the earth wire carry a current?
Only when there is a fault in the appliance
What material are the pins in a plug made out off? (3 marks)
Pins are made of brass, which is a good conductor of electricity and doesn’t rust or oxidise.
What material are the pins in a plug made out off? (3 marks)
Pins are made of brass, which is a good conductor of electricity and doesn’t rust or oxidise.
What material is the wire in a plug made out off?
Copper, its a good electrical conductor and bends easily (malleable)
How does an appliance become live?
If the live wire touches the case of the appliance
Why should you not touch a live wire?
You would make a complete circuit from your mains supply to the ground, which would lead to a shock
