topic 2 - electricity Flashcards
What must be included for electrical charge to flow through a closed circuit?
a source of potential difference must be included
What is electric current?
the rate of flow of electrical charge
State the equation linking charge, current and time
Q = IT
charge = current x time
What is 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
- resistance
what occurs to the current when the resistance increases?
- the greater the resistance the smaller the current for a given potential difference across the component
What equation should be used to calculate the potential difference if current and resistance are known?
potential difference = current x resistance
V = IR
What is a zero error?
- reading on a measuring instrument when the value should be zero
- this is a systematic error. We cannot reduce it by carrying repeats
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
How does the resistance change with the current?
- As current increases, electrons have more energy
- when electrons flow through a resistor, they collide with the atoms in the resistor
- this transfers energy to the atoms, causing them to vibrate more
- this makes it more difficult for electrons to flow through the resistor
-> so as the current increases, the resistance decreases
How does the resistance change with temperature
- in hotter temperatures, the thermistors resistance decreases
How does the resistance change with length?
- the greater the length, the greater the resistance and the lower the current
- electrons have to make their way through more resistor atoms, so it is harder than using a shorter wire
How does the resistance change with light?
- the greater the intensity of light, the lower the LDR’s resistance
- so the resistance is greatest when it is dark
How does the resistance of a diode change with voltage?
- diode allows current to flow freely in one direction
- in the opposite direction, it has a very high resistance so no current can flow
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 is an application for an LDR?
- street lights often use LDRs
- 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)
state the rules for current, p.d., and resistance in a series circuit
- the current is the same through each component
- the total potential difference of the power supply is shared between the components
- the total resistance of two components is the sum of the resistance of each component
When two resistors are connected in series, how can u calculate the total resistance?
Rtotal = R1 +R2
Explain why in a series circuit, if one bulb goes out all of them will get out.
- An electron will pass through every component on its way around the circuit.
- If one of the bulbs is broken then current will not be able to flow round the circuit
- If one bulb goes out, they all go out
What happens to the resistance of a parallel circuit when another resistor is added in parallel?
- the resistance decreases
State the rules of current, potential difference and resistance in parallel circuits
- the p.d across each component is the same
- the total current through the whole circuit is the sum of the currents through the separate components
- the total resistance of two resistors is less than the resistance of the smallest individual resistor
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 a voltmeter in a circuit to measure p.d?
Voltmeters should be connected in parallel to the component that they are measuring the p.d of
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 the current through
Describe how to perform an experiment to compare the total resistance in series and parallel arrangements
- set up the circuit, turn the power supply on and close the switch
- Record the voltmeter and ammeter readings and calculate the resistance of the resistor using R = V/I, where R is resistance, v is the potential difference and I is current
- change the resistor and repeat step two to find the resistance of a second resistor
- arrange the two resistors in series and close the switch
- record the voltmeter and ammeter readings once again and determine the total resistance of both resistors in series using R = V/I
- arrange the two resistors in parallel and close the switch
- Record the voltmeter and ammeter readings once again and calculate the total resistance of both resistors in parallel
What is the difference between the total resistance when resistors are added in series and parallel circuits?
- In series, the resistance of the network is equal to the sum of the two individual resistances
- In parallel, the resistance of the network is less than either of the two individual resistances
What is the mains electricity?
- Mains electricity refers to the power that is brought into people’s homes
Is mains electricity an a.c supply or a d.c supply? What does each of these stand for?
Mains electricity is an a.c supply
a.c - alternating current
d.c - direct current
What is alternating current?
a current that continuously changes direction at a specific frequency
What direct current?
One directional current flow
What is the frequency and voltage of the UK mains electricity supply?
- frequency: 50 Hz
- Voltage: 230 V
What are the wires that are usually in the cables connecting electrical appliances to the mains?
- live wire
- neutral wire
- earth wire
what are the two colours of the earth wire
green and yellow stripes
what is the colour used on the live wire?
brown
what is the colour used on the neutral wire?
blue
what is the purpose of the live wire?
- the live wire carries the alternating potential difference from the supply
what is the purpose of the neutral wire?
the neutral wire completes the circuit
what is the purpose of the earth wire?
it is a safety wire to stop the appliance becoming live
What is the potential difference of the earth and neutral wire
0V, it only carries a current if there is a fault
Explain why a live wire may be dangerous even when a switch in the mains circuit is open
The first part of the live wire would still contain a voltage/p.d
- if someone touched the live wire, a current would flow through the person, into the earth and they’d be electrocuted
Why are appliances with a metal case dangerous?
- If the live wire comes loose and touches the metal case, the case can become live
- this could give someone a fatal electric shock if they touch the live case
For appliances with a metal case, where is the Earth wire connected? Why?
- the earth wire is connected to the metal casing of the appliance
- if the live wire becomes loose and touches the casing, the current will flow through the earth wire, preventing electrocution
What are the two equations for the power of a circuit?
power = current x potential difference
power = (current)^2 x resistance
State the equation linking energy transferred, charge flow and potential difference
energy transferred = charge flow x potential difference
E = Q X V
What two main factors do the amount of energy transferred by an appliance depend on?
- how long the appliance is switched on for
- the power of the appliance
Describe the energy transfers in a battery-powered torch
- Battery converts chemical energy into electrical energy
- Bulb convert electrical energy into light energy as well as waste energy in the form of heat
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 energy is transferred in a kettle?
electrical energy is transferred to thermal energy
When is work done?
When charge flows in a circuit
what three things determine the power of a circuit device?
- the p.d across the circuit
- the current through the circuit
- the amount of energy transferred in a given time
What is the National Grid?
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 the two types of transformers used in the national grid?
- step-up trnasformers
- step-down transformers
What is the purpose of step-up transformers?
- step-up transformers are used to increase the potential difference from the power station to the transmission cables
what is the purpose of step-down transformers?
- step-down transformers are used to decrease, to a much lower value, the pd for domestic use
Why is the National Grid system an efficient way to transfer energy?
- when electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss
- by increasing the potential difference at which the electricity is transmitted, the same amount of power can be transmitted using a much smaller current
- this results in less heat being produced in the wire and hence less energy loss
what is an electrical insulator?
an insulator does not allow electrical charges to flow through a material
what is a static charge?
an electrical charge that accumulates on an insulated object
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 an attract
What is an electric field?
A region in which a charged object will experience a non-contact electrical force
What do all electrically charged objects have around them?
An electric field
Where is the electrical field stronger?
the closer to the charged object
How do the electrical field lines show how strong the field is?
the closer together the field lines, the stronger the field is
How can a build-up of static charge lead to a spark
- an object with a large static charge has a large potential difference between it and the earth
- a large potential difference means a strong electric field. this ionises the air and makes it conductive
- this allows charge to flow in the form of a spark
What precautions 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
