Electricity Flashcards
What is electric current?
The flow of electrical charge
What can be said about the value of current at any point in a single closed loop?
Current is the same at all points in a closed loop
What two factors does the current in a circuit depend on?
Potential differene (V)
Resistance (R)
What is an Ohmic conductor? State the condition required
A conductor for which current and potential difference are directly proportional
Resistance remains constant as current changes
Temperature must be constant
List four components for which resistance is not constant as current changes
Lamps
Diodes
Thermistors
Light dependent resistors
What is different about current flow through a diode?
The current only flows in one direction
Resistance is very high in the opposite direction, preventing current flow
State what happens to the resistance of a thermistor as temperature increases
The thermistor’s 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
State what happens to the resistance of an LDR as light intensity decreases
The LDR’s intensity increases
Give an application for an LDR?
Street lights often use LDR’s
When light levels become too low, the light gains sufficient current to turn on
What are the two ways that a component can be connected in a circuit?
Series (same loop)
Parallel (adjacent loop)
How does the potential difference across two components vary when connected in series or parallel?
Series: Total P.D is shared between each component
Parallel: P.D across each component is the same
If two resistors are connected in parallel, what can be said about their combined total resistance?
Their total resistance is less than the smallest of the two individual resistances
If two resistors are connected in series, what can be said about their total resistance?
Their total combined resistance is equal to the sum of the two individual resistances
Describe the current in a series circuit
In a series circuit, the current is the same at all positions since the charge only has one path to flow through
Describe the current in a parallel circuit
In a parallel circuit, the current is shared between the different branches. When the charge reaches a junction it splits
How should you connect an ammeter in a circuit to measure current?
Ammeters should be connected in series with the component that they are measuring current through
How should you connect a voltmeter in a circuit to measure potential difference?
Voltmeters should be connected in parallel to the component that they are measuring the potential difference of
Is mains electricity an a.c supply or a d.c supply? What do each of these stand for?
Mains electricity is an a.c supply
a.c : alternating current
d.c : direct current
Define alternating current
Current that continuously changes direction at a specific frequency
Define direct current
One directional current flow
What is the frequency and voltage of the UK mains electricity supply?
Frequency : 50Hz
Voltage : 230V
How many wires are usually in the cables connecting electrical appliances to the mains? Name these wires
Live wire
Neutral wire
Earth wire
State the insulation colour used on the Earth wire
Green and yellow stripes
State the insulation colour used on the live wire
Brown
State the insulation colour used on the neutral wire
Blue
Explain when the Earth wire does and doesn’t carry a current
Under normal circumstances, no current flows through the Earth wire
If a fault occurs in the appliance, current will flow to the ground
What potential is the neutral wire at?
0 volts
State the potential difference between the live and earth wires
230 volts
What is the purpose of the neutral wire?
To complete the circuit by connecting the appliance back to the mains supply
For metal appliances, where is the Earth wire connected to? Why?
Earth wire is connected to the metal casing of the appliance
If live wire becomes loose and touches the casing, the current will flow through the Earth wire, preventing electrocution
What two main factors does the amount of energy transferred by an appliance depend 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 as well as waste energy in the form of heating
Describe the energy transfers in a battery powered motor
Battery converts chemical energy into electrical energy
Motor converts electrical energy into kinetic energy as well as waste energy in the form of heating due to friction
What three things determine the power of a circuit device?
The potential difference across the circuit
The current through the circuit
The amount of energy transferred in a given time
What is the purpose of the National Grid?
To link power stations to consumers so that they have access to a source of electricity
What are the two types of transformers used in the National grid?
Step-up transformers
Step-down transformers
Where are step-up transformers found in the National grid? What do they do?
Step-up transformers are used when connecting power stations to transmission cables
They increase the potential difference
Where are step-down transformers found in the National grid? What do they do?
Step-down transformers are used in connecting transmission cables to domestic buildings
They decrease the potential difference
Why do transmission lines transfer electricity at high potentials?
A high potential results in a low current
The lower the current, the less energy that is wasted as heat
Therefore it is more efficient
Why does the potential need to be decreased between transmission lines and houses?
Lower potentials are safer for domestic use and reduces the likelihood of severe electrocution
Appliances are designed for 230V
What can happen when insulating materials are rubbed together?
They can become (statically) electrically charged
Why can insulators become electrically charged when rubbed together?
Electrons are rubbed from one material onto the other
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 onto 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 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 electric fields be found?
Surrounding any charged objects
Describe the electric field around a charged particle
Strongest closest to the object
Decreases in strength as you move away from the object
What happens to the force between two charged objects when they are moved closer together?
The force between them becomes stronger as the separation reduces
In situations where sparks are unwanted, what precaution must be taken to prevent the build up of static charge?
Any surfaces that are rubbing against each other should be earthed to allow the charge to flow off the materials
