Electricity -Topic 2 Flashcards
what is an electric current
the flow of electric charge
state the equation linking charge current and time
Q = It —- charge = current x time
what two factors does the current in the circuit depend on?
- potential difference
- resistance
Equation for potential difference using current and resistance
V= IR —- P.D = current x resistance
what is an ohmic conductor?
- a conductor for which current and potential difference is directly proportional.
- resistance remains constant as the current changes.
4 components where resistance is not constant as current changes
- lamps
- diodes
- thermistors
- light dependant resistors
what happens to the resistance of a filament lamp as the temperature increases
- resistance increases
- ions in the metal have more energy, so vibrate more, causing more collisions with electrons as the flow through the metal creating greater resistance to current flow.
How does current flow through a diode?
- the current flows in only one direction
- resistance is very high in the other direction preventing current flow.
what happens to the resistance of a themistor as temp increases
the thermistors resistance decreases
give two examples of when a thermistor may be used
- In a thermostat to turn a heater on below a certain temperature.
- In a freezer to turn on a cooler when the temperature becomes too high.
What happens to the resistance of an LDR when the light intensity decreases?
The LDRs resistance increases.
Examples of when LDRs are used?
- Street lights sometimes use them
- When light levels become too low the light gains sufficient current to turn on.
what are the two ways that components can be connected in a circuit?
- series (same loop)
- parallel (adjacent loop)
how does the potential difference two components vary when connected in series and parallel?
- in series the total P.D is shard between each component.
- in parallel P.D across each component is the same.
If two resistors are connected in parallel what is their total combined resistance?
The total resistance is less than the smallest of the two individual resistors.
If two resistors are connected in series what is their total resistance?
- Their total combined resistance is equal to the sum of the two individual resistances.
Describe the current in a series circuit
the current is the same in all positions as the current only has one path to flow through
describe the current in a parallel circuit
the current is shared between the different branches and then is combined again before going back to its power supply
how should you connect the ammeter in a circuit to measure the current
in series with the component they are measuring the current through.
how should voltmeters be connected in a circuit to measure the potential difference?
connected in parallel to the component they are measuring the potential difference of.
what is an advantage of connecting a lamp in parallel
if the lamp blow the rest will be unaffected and still receive current. (if the circuit is still complete)
what type of current is the mains electricity?
the mains is an a.c supply (alternating current)
define alternating and direct current
- alternating current: current that continuously changes direction at a specific frequency.
- direct current: one directional current flow
what is the frequency and potential difference of the UK mains supply?
- frequency:50 Hz
- P.D: 230V
what wires are in the cables connecting electrical appliances to the mains?
- live wire
- earth wire
- neutral wire
colours used for earth wire
green and yellow stripes
colour of live wire
brown
colour of neutral wire
blue
explain why the earth wire doesn’t carry a current
- normally no current flows through the earth wire.
- is a fault occurs in the appliance current will flow to the ground.
- As the earth wire has virtually no resistance a large current flows, this causes the fuse to blow preventing further current.
P.D at neutral wire
0V
P.D between live and earth wires
230 V
what is the purpose of the neutral wire
to complete the circuit by connecting the appliance back to the mains supply.
In metal appliances where is the earth wire connected to? Why?
- the earth wire is connected to the metal casing of the appliance.
- If the live wire becomes loose and touches the metal casing the current will flow through the earth wire preventing electrocution.
two equations for the power of a circuit
- P = IxV
- P = (I)2 x R
equation linking energy transfer power and time
E = P x t
equation linking energy charge and P.D
E = Q x V
two main factors that energy transfer in an appliance depends on
- how long the appliance is being used for
- the power of the appliance
describe the energy transfers in a battery powered torch
- battery converts chemical energy into electrical energy.
- bulb converts electrical energy into light energy and waste energy in the form of heat.
energy transfers in battery powered motor
- battery converts chemical energy in to electrical energy.
- motor converts electrical energy into kinetic energy and waste energy in the form of heat due to friction.
three things that determine the power of a circuit device.
- potential difference across the circuit
- current through the circuit
- The amount of energy transferred in a given time
what is the explain why the earth wire doesn’t carry a current purpose of the national grid
to link power stations to consumer so they have access to a source of electricity.
what are the two types of transformers used in the national grid
- step - up transformer
- step - down transformer
where are the step up transformers found on the national grid? What do they do?
- used when connecting power station to transmission cables
- increase the P.D and decrease the current
where are step down transformers found in the national gird? what do they do?
- used when connecting transmission cables to domestic buildings (e.g houses)
- decrease P.D increase current
why do transmission lines transfer electricity at high P.D
- High P.D results in a low current
- The lower the current the less energy wasted as heat
- Therefore it is more efficient
why does P.D need to be decreased between transmission lines and houses?
- lower P.D is safer for domestic use and reduces the likelihood of serious electrocution.
- Appliances are designed for 230 V
what can happen when insulating materials are rubbed together?
they become electrically charged
why can insulators become electrically charged when rubbed together?
- electrons are rubbed form one material onto another.
- the material gaining electrons becomes negatively charged.
- the material losing electrons becomes equally positively charged.
what happens when two electrically charged objects are brought close together?
they exert a force on each other
what happens when two identically charged objects are brought close together
They exert a repulsive force on each other and repel
what happens when two oppositely charged objects are brought close together
they exert an attractive force on each other and attract
give an example of a non contact force
the repulsive or attractive force acting between two electrically charged objects
what is an electric field
a region in which a charged object will experience a non contact electrical force
where can electrical fields be found
surrounding any charged object
