✅ 2. Electricity Flashcards
What are the 2 things needed for charge to flow through a circuit?
For charge to flow:
- The circuit must be closed
- There must be a source of potential difference (e.g. battery/cell)
What is electric current?
Electric current is the flow of electrical charge (flow of electrons?)
Define the size of an electric current?
The size of the electric current is the rate of flow of electrical charge
What is the equation linking charge flow, current and time
What can be said about the value of current at any point in a single, closed loop?
A current has the same value at any point in a single closed loop (in a series circuit)
What is potential difference?
Potential difference is a measure of the energy transferred for each coloumb or charge/electron that passes through a component
What does the potential difference of 1 volt tell us?
The potential difference of 1 volt tells us that 1 joule of energy is transferred for each coloumb of charge/electrons that moves through the circuit
What 2 factors does the size of current through a component depend on?
The current through a component depends on:
- The resistance of the component
- The potential difference across the component
(As V=IR)
The greater the resistance of a component…
The greater the resistance of a component, the smaller the current for a given potential difference across the component
R = V / I
What is the equation linking current, potential difference and resistance
How should you connect a voltmeter to a circuit?
You should connect a voltmeter to a circuit in parallel to the component you are trying to find the potential difference of
How should you connect an ammeter to a circuit?
You should connect a ammeter to a circuit in series
What does Ohm’s Law state?
The current through an ohmic conductor at a constant temperature is directly proportional to the potential difference across it
What is an ohmic conductor?
An ohmic conductor is a conductor for which the current and potential difference are directly proportional
What happens to the resistance in an ohmic conductor?
The resistance remains constant as the current changes
Give one example of an ohmic conductor?
A wire
Describe the resistance of a filament lamp?
The resistance of a filament lamp increases as the temperature of the filament increases
What is different about current through a diode?
- The current only flows through one direction
- The resistance is very high in the reverse direction
Describe the resistance of a thermistor
The resistance of a thermistor decreases as the temperature increases
Give one application of where thermistors may be used?
Thermistors may be used in thermostats
Describe the resistance of a LDR
The resistance of an LDR decreases as light intensity increases
Give one application of where a LDR may be used?
A LDR may be used when switching lights on when it gets dark
List 4 examples of non-ohmic conductors?
- Lamps
- Diodes
- Thermistors
- LDRs
Describe the current in a series circuit
In a series circuit, the same current passes through each component
(The current is the same at all points of the circuit)
Describe the potential difference in a series circuit
In a series circuit, the total potential difference of the power supply is shared between the components
What is the total resistance of a series circuit?
The total resistance of a series circuit is equal to the sum of the resistance of each component
Describe the potential difference for components connected in a parallel circuit
For components connected in a parallel circuit, the potential difference across each component is the same
Describe the current in a parallel circuit
In a parallel circuit, the total current through the whole circuit is the sum of the currents through the separate components
Describe the total resistance of two resistors in parallel
The total resistance of two resistors in parallel is less than the resistance of the smallest individual resistor
What are the differences between series and parallel circuits?
Explain the effects of adding resistors in series and parallel circuits
Adding resistors in series increases the total resistance, whilst adding resistors in parallel decreases the total resistance
Explain why adding resistors in parallel decreases the total resistance?
Adding resistors in parallel decreases the total resistance as:
- Resistance = Potential Difference / Current (R = V/I)
- Each parallel resistor creates an extra path along which the current can flow
- There is more current flowing whilst the battery potential difference is the same, so there is less resistance
What is the advantage of connecting lamps in parallel?
If one lamp blows, the rest will be unaffected and can still recieve current
Define Direct Current (DC) and Alternating Current (AC)
- Direct Current (DC) is current that flows in one direction only
- Alternating Current (AC) is current that continuously changes its direction at a specific frequency
Give one example of Direct Current (DC)
Cells and batteries
What type of current supply is the UK mains electricity?
The UK mains electricity is an alternating current supply (AC)
What is the frequency and potential difference of the UK mains electricity?
The UK mains electricity has a frequency of 50Hz and potential difference of 230V
How are most electrical appliances are connected to the mains?
Most electrical appliances are connected to the mains using three-core cable
What are the colours of the three-core cables connecting the electrical appliances to the mains?
Live wire – brown
Neutral wire – blue
Earth wire – green and yellow stripes
What is the function of the live wire?
The live wire carries the alternating potential difference from the supply
Why may live wires be dangerous?
A live wire may be dangerous, even if mains circuit is off, as current may still be flowing through it
What is the function of the neutral wire?
The neutral completes the circuit formed by the live wire
Why is the neutral wire less dangerous than the live wire?
It has a potential difference of 0V - which is a lower voltage than the live wire
What is the function of the Earth wire?
- The earth wire is a safety wire to stop the appliance becoming live
- The potential difference is 0V, only carries current if there is a fault
Explain how the Earth wire works
- The metal casing of an appliance is attached to an Earth wire
- The Earth wire is connected to the Earth
- If the metal casing becomes live, the current will flow through the Earth wire, preventing electrocution
What is the danger of providing any connection between the live wire and earth
The danger is that if someone touched the live wire, a current would flow through the person to the Earth and they’d be electrocuted
What is the potential difference between the live wire and earth (0 V)?
The potential difference between the live wire and earth (0 V) is about 230 V
What is the equation linking power, potential difference and current?
power = current x potential difference
P = I V
P in watts (W)
V in volts, (V)
I in amps (A)
What is the equation linking power, current and resistance?
power = current2 x resistance
P = I2R
P in watts (W)
R in ohms, (Ω)
I in amps (A)
What are everyday electrical appliances designed to do?
Everyday elecrical appliances are designed to transfer energy
What does the amount of energy an everyday electrical appliance transfers depends on?
The amount of energy an everyday electrical appliance transfers depends on:
- How long the appliance is switched on for
- The power of the appliance
What are the common ways energy is usually transferred in electrical appliances?
Energy is usually transferred in electrical appliances from batteries or ac mains to the kinetic energy of electric motors or the energy of heating devices
When is electrical work done?
Electrical work is doen when charge flows through a circuit
What are the 2 formulas for energy transferred?
Explain the role of the National Grid?
- The National Grid is a system of cables and transformers linking power stations to consumers
- Electrical power is transferred from power stations to consumers using the National Grid
What are step-up transformers used for?
Step-up transformers are used to increase the potential difference from the power station to the transmission cables
What are step-down transformers used for?
Step-down transformers are used to decrease the potential difference to a much lower value for domestic use
Why is the National Grid system an efficient way to transfer energy?
- The step up transformers increase the voltage / potential difference by several thousand volts
- This means that the same amount of power can be transferred using a much smaller current (as P = I V)
- The lower the current, the less energy that is wasted due to the resistance heating
What happens when certain insulating materials are rubbed against each other?
- When certain insulating materials are rubbed against each other they become electrically charged objects
- Negatively charged electrons are rubbed off one material and on to the other
- The material that gains electrons becomes negatively charged
- The material that loses electrons is left with an equal positive charge
What happens when two electrically charged objects are brought close together?
- When two electrically charged objects are brought close together they exert a force on each other
- Two objects that carry the same type of charge repel
- Two objects that carry different types of charge attract
Give an example of a non-contact force
Electrostatic forces of attraction and repulsion between two charged objects
How do sparks occur?
- Sparking occurs when enough charge builds up, and the objects are close but not touching
- The charge jumps through the air from the highly negative object to the highly positive object, to balance out the charges
How do we prevent sparks?
To prevent sparks, the objects are earthed to prevent any charges from building up and leading to spark
How do we know if objects are charged or not?
Charged objects exert forces of attraction or repulsion on one another when not in contact
What does a charged object create?
A charged object creates an electric field around itself
Describe the electric field of a charged object
- The electric field is strongest close to the charged object
- The further away from the charged object, the weaker the field
What are the electric field patterns for an isolated charged sphere?
