Studied by 18 peopleWhat is electric current?
The flow of electrical charge.
State the equation linking charge, current and time. Give the units for the quantities involved.
Q = I t Charge (Coulombs), Current (Amperes), Time (Seconds)
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 Difference (V)
Resistance (R)
What equation should be used to calculate potential difference if current and resistance are known? State the units for all 3 quantities.
V = I R Potential Difference (V), Current (A), Resistance (Ω)
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 Dependant Resistors (LDRs)
What happens to the resistance of a filament lamp as the temperature increases? Why?
● Resistance increases ● Ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow
What is different about current flow through a diode?
● The current only flows in one direction ● Resistance is very high in the other 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 a LDR as light intensity decreases.
The LDR’s resistance increases.
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 and 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.
Why is it advantageous to connect lamps in parallel?
If one lamp blows, the rest will be unaffected and can still receive current (i.e. the circuit is still complete).
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 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 voltage of the UK mains electricity supply?
● Frequency: 50 Hz ● 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 (such as a surge or the casing becoming live), 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
State two equations for the power of a circuit. Give appropriate units.
P = I V P = I² R Power (Watts), Current (Amperes) Potential Difference (Volts), Resistance (Ohms)
State an equation linking energy transferred, power and time. Give appropriate units.
E = P t Energy (Joules), Power (Watts), Time (Seconds)
State an equation linking energy transferred, charge flow and potential difference. Give appropriate units.
E= Q V Energy (Joules), Charge (Coulombs), Potential Difference (Volts)
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
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 (like houses) ● 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 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 electric fields be found?
Surrounding any charged object.
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.
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