Electricity Flashcards
2 main types of current
Describe each
What is the frequency of electricity from the Mains supply
What voltage delivers to homes
Direct Current & Alternating
Direct in one direction
Alternating regularly changes direction between negative and positive charges
Mains. Supply = 50 hz
230 volts delivered to homes
6 components of a plug
Briefly describe each
Outer insulation -All three wires in the cable are bundled together and there is extra plastic insulation wrapped round them all for safety
Cable grip - This holds the cable tightly in place so that wires do not become loose
Live wire - Copper wire coated with brown plastic along which the current enters the device
Fuse -A glass or ceramic canister containing a thin wire that melts if the current gets too high
Neutral wire -Copper wire coated with blue plastic that also connects to the cable in the wall and completes the circuit
Earth wire - Copper wire coated in striped plastic that provides a path for current to flow from the case of the device to the ground if there is a fault
Describe Earthing
Without the earth wire, if a fault occurs and the live wire becomes loose, there is a danger that it will touch the case.
The next person who uses the appliance could get electrocuted.
The earth wire is therefore connected to the case and is attached to a metal plate or water pipe underground. As the wire is made of copper, the earth wire provides a low resistance path to the ground.
In the event of a fault, the live current passing through the case will follow this path to the ground instead of passing through a person.
Describe a Fuse
A fuse provides a built-in fail-safe to the electrical circuit for a device.
The fuse contains a thin wire that will melt if the current gets too high.
If there is a fault that causes the casing of the device to become live, a large current will flow through the low-resistance earth wire.
This high current will cause the fuse to melt.
Once the fuse has melted, the circuit is broken and no more current flows through the device. This means the case of the device is no longer live and there is no more risk of electrocution.
How do electrical appliances work
What does the amount of energy transferred depend on
What’s the equation for energy transferred by appliance and what are the units of measurement in the equation
All electrical appliances transfer energy from one store to another, for example chemical energy in the fuel in power stations. This is transferred into kinetic energy in a fan or heat energy in a cooker.
Amount of energy transferred depends on on the power (the energy transferred each second) and the amount of time the appliance is switched on for.
The energy transferred by an appliance can be calculated using the equation:
energy = power × time
E = p x t
- energy (E) is measured in joules (J)
- power (P) is measured in watts (W)
- time (t) is measured in seconds (s)
What is 1 Watt
One watt is the power when one joule of energy is transferred in one second.
How much energy is transferred by a 1,500 W hair dryer in 15 minutes?
E = p x t
1,500 x (15x 60)
E = 1,500 x (900)
E - 1,350,000 Joules or 1.35 mj
How is power calculated, using both equation
P = V x I
Power (P) is measured in watts (W)
Potential difference (V) is measured in volts (V)
current (I) is measured in amps (A)
P = e \ t
Power = energy transferred \ time
What is the power of an electric heater that draws a current of 8 amps (A) when plugged into the mains?
P = V x I
Power (P) is measured in watts (W)
potential difference (V) is measured in volts (V)
current (I) is measured in amps (A)
P = 230 x 8
P= 1,840 W
What is the power of an electric heater that draws a current of 8 amps (A) when plugged into the mains?
If the electric heater is used for 20 minutes, how much energy is transferred in that time?
P = V x I
Power (P) is measured in watts (W)
potential difference (V) is measured in volts (V)
current (I) is measured in amps (A)
P = 230 x 8
P= 1,840 W
E = p x t
E = 1,840 x (20 x 60)
E = 1,840 x (1,200)
E = 2,208,000 j or 2.21 mj
Describe Tranformers
- what do they do
-how formed
- how does National Grid use them . What does increase/decrease in voltage do .
Transformersare used to change voltages and currents in transmission lines
. A transformer is formed from two coils of wire around a magnetic core. The number of coils determines whether the transformers will step-up or step-down the voltage.
As the power transferred must stay the same:
- increasing voltage decreases current
- decreasing voltage increases current
In the National Grid, a step-up transformeris used to increase the voltage and reduce the current.
The voltage is increased from about 25,000 Volts (V) to 400,000 V causing the current to decrease. Less current means less energy is lost through heating the wire.
To keep people safe from these high voltage wires, pylons are used to support transmission lines above the ground.
Before reaching the end user, a step-down transformer reduces the voltage from the transmission voltage to the safer voltage of 230 V for home use.
Power in transformers
If the transformer is 100% efficient, the power in each coil will be the same - this means that:
potential difference across primary coil × current in primary coil = potential difference across secondary coil × current in secondary coil
Electricity is generated in a power station at 25,000 V, and to transmit the electricity across the country on the
National Grid
, transformers increase the potential difference to 400,000 V.
This means that the transformer increases the potential difference by a factor of 16. So if there are no energy losses in the system, the current would also be reduced by a factor of 16 as:
power = potential difference × current
Reducing the current by a factor of 16 means that the heating effect of the current is reduced by a factor of 256 (162), as:
power = current2 × resistance
This means that the energy is transferred more efficiently, as less energy is dissipated as heat.
Transmission lines
As an electric current flows through the thick cables held up by the pylons, they will get hotter and dissipate energy to the surroundings. The electrical power dissipated depends on current and resistance:
power = current2 × resistance
This is when:power (P) is measured in watts (W) current (I) is measured in amps (A)
resistance (R) is measured in ohms (Ω)
To ensure that the minimum amount of power is lost from the cables:
the cables are thick so that their resistance is low high voltages are used to reduce the current through the transmission lines
A low resistance and a low current mean that the transmission wires will not heat up as much. As a result, most of the power is delivered to the consumer, and not lost through the wires.
Potential difference and resistance
-what does current flowing through a component depend on
- how you measure potential difference . What is it also known as
The current through a component depends on both the resistance of the component and the potential difference
across the component. To measure the potential difference across a component, a voltmeter must be placed -in parallel
with that component in order to measure the difference in energy from one side of the component to the other. Potential difference is also known as voltage and is measured in volts (V).
