P4-electricity Flashcards
What are conductors of electricity
Conductors of electricity are materials electrons flow easily through
E.g metals
What are insulators of electricity
Insulators of electricity are materials that electrons cant flow through
E.g cloth
What happens when two insulators are rubbed together
- When two insulators are rubbed together electrons can pass from one to the other
- The insulator that gains electrons becomes negativley charged, The insulator that loses electrons has an positive charge equal in size to the negative charge
What happens when a cloth is rubbed together with a plastic rod
- The cloth is covered with electrons , the electrons cannot move through the cloth as it is an insulator
- The plastic rod is also covered with electrons , the electrons cannot move through the rod as it is an insulator
- When the plastic rod is rubbed together with the cloth electrons move from the rod to the cloth.The cloth has gained electrons so has a negative charge. The plastic rod has lost electrons so has a positive charge (which is equal in size to the negative charge on the cloth).
What happens when oppositley charged objects are placed near each other
When oppositley charged objects are placed near each other they are attracted to oacb other
The force between them is a force of attraction
What type of force is a force of attraction
A force of attraction is a non contact force
When happens when like charged objects are placed near each other
When like-charged objects are placed near each other they repel each other
-There is a force of repulsion between the two objects carrying the like charges
What will happen if a charged object is placed in the electric field of another charged object
If a charge object is placed in the electric field of another charged object, it will experience a force. This will be a force of replusion if the two objects are like charged and a force of attraction if the two objects are oppositley charged
What is an electric field
An electric field is a field of charge created around any charged object(see cgp if confused)
How do scientists show electric fields
Scientists show electric fields using field lines. Feild lines are perpendicular to the surface of the object
What do the arrows on the field line show
The arrows on the field lines show the direction of the force the other object feels if it is nearby (arrows facing inwards for force of attraction and outwards for force of repulsion )
What is the relationship between the distance of the field lines and the strength of the electric field
The closer the field lines are to each other the stronger the electric field is
What is the relationship between the distance between the TWO CHARGED OBJECTS and the force between them
If you decrease the distance between the two charged objects the force between them gets stronger (force of repulsion if two like charged objects or force of attraction if two oppositley charged objects)
Draw the circuit symbol for a cell
See CGP for answer
Draw the circuit symbol for a bulb
See CGP for answer
Draw the circuit symbol for an open switch and a closed switch
See CGP for answer
What is an electric current
An electric current is the flow of electrons round a circuit
What is electric current measured in
Electric current is measured in amperes (A) which can be shortened to amps
What is a series circuit
A series circuit is a circuit with no branches. In a series circuit the current can only flow in one direction
How do you measure the current in a circuit
You measure the current in a circuit using an ammeter
What is the current like all the way around a series circuit
The current in a series circuit is the same all the way around
E.g if you measure with an ammeter at any point it will read the same amount of amps
What direction does the current flow in a series circuit
The current flows from the negative end of the cell to the positive end (however scientists draw it in the opposite direction , this is called conventional current)
Draw the symbol for an ammeter
See CGP for answer
What happens in a closed circuit with a component
In a closed circuit with a component ,electrons first leave the cell . They carry energy from the cell to the components in the circuit and pass the energy on. They then return to the cell with less energy than they started with
What is a parralel circuit
A parralel circuit is a circuit that contains multiple branches-The current can flow through all of the branches
How is the current split between the different branches
The current in the branches adds up to the total current leaving the cell ( e.g if the total current was 0.7 amps and the current on one branch was 0.4 amps the current on the other branch would be 0.3 amps )
-A1+A2=Atotal
Why is a spark caused sometimes between oppositley charged objects
- Normally, if two objects are oppositley charged, electrons in the air molecules between the two electric fields experience a force towards the positive object
- If the field is too strong, sparking happens , because some of the electrons are pulled out of air molecules by the force of the field. This causes the air molecules to ionise which makes the air conductive which means a current can flow through the air molecules
What is potential difference
- Potential difference is the driving force that pushes current round the circuit
- Potential diffenerence between two points in an electrical circuit is the energy trabsferred per coulomb of charge that passes between the points
- A potential difference of 1 volt tells us that one joule of energy is transferred for each coulomb of charge moving through the circuit
Draw the symbol for a voltmeter
See CGP for answer
-NOTE: a voltmeter needs to be connected parralel to the circuit you are measuring the potential difference of
What is the rule for potential difference in a series circuit
For a series circuit the total potential difference of the supply is shared between the components in the series( e.g if the potential difference of the supply was 9 volts and there were two components the potenetial difference of each of the components would be 4.5 volts )(if these two were lamps both the lamps would be dimmer as there is only half the volts)
What is the rule for potential difference in a parrallel circuit
- For compenents in parralel, the voltage remains the same for each component
- The voltage is the same for all the banches
- NOTE if two components are on the same branch in a parralel circuit the voltage still splits between them and it is the same total voltage as voltage of the supply( see photo off free science lessons)
What is a battery
A battery is two or more cells connected together
What is one key rule about batteries involving direction
The cells in a battery must be connected in the same direction (e.g positive ends must be on the same side )
How does the potential difference of the cells in a battery link to the total potential difference of the battery*
The potential difference of individual cells in a battery adds up to give the total potential difference of the battery
E.g if there were 3 cells in a battery and each of them were 9 volts the total potential difference of the battery 27 volts (3x9)
What happens one cell in the battery is placed the wrong direction *
If one cell in the battery is placed in the wrong direction the voltage of that cell is cancelled out from the total voltage if the battery
E.g if one cell in a three cell battery (each cell has 9 volts)is placed in the wrong direction the total voltage would be 18 (27-9)
What unit is electrical charge measured in
Electrical charge is measured in coulombs
What is the equation involving charge flow
Charge flow (Coulombs)(C)=Current(A)xTime(seconds)
Q=IxT
A current of 1 ampere = one couloumb of charge flowing per second
What happens whenever charge flows in a circuit
Whenever charge flows in a circuit work is done/energy is transferred
What is the equation that links energy transferred to electricty
Energy transferred(J) = charge flow (Coulombs)x potential difference (Volts)
What is resistance
Resistance tells us the amount of potential difference required to drive a current through a component
What is resistance measured in
Resistance is measured in ohms (♎️)
What is the equation for resistance
Resistance (ohms )=potential difference(volts)/current (Amps)
Or
Potential difference (V) = current x resistance
If a component has a lower resistance less potential difference will be required to push the current through it
What happens when electrons flow through a conductor
As electrons move through a conductor they collide with ions in the metal conductor and the electrical energy carried by them is transferred to other energy stores e.g thermal energy . This is what resistance is caused by
Draw the symbol for a resistor
See CGP for answer
What do resistors do in a circuit
Resistors add extra resistance to a circuit
Why might you use a resistor
You might use a resistor in a series circuit to make a component such as a lamp dimmer. To do this you would add a resistor to the circuit. This would cause the potential difference in the circuit to be split between the resistor and the lamp. This means less electrical energy is transferred to the lamp so it gets dimmer
What happens in an ohmic conductor
An ohmic conductor is a type of resistor where the current moving through the resistor is directly proportional to the potential difference across the resistor
What type of resistance does this type of resistor have
This resistor has a constant resistance. The resistance does not change if we increase the current .
However the resistance in an ohmic conductor will only stay constant if the temperature remains constant
How does a filament lamp work
A filament is a very tightly coiled wire . In a filament lamp the wire gets extremely hot when a current runs through it this causes it to glow and give out light
What is the relationship between current and potential difference in a filament lamp
The current through a filament lamp is not directly proportional to the potential difference
- This is because the restistance of the filament bulb increases asbthe temperature of the filament increases. As temperature increases the ions in the filament vibrate more which means collide more with the electrons in the electric current. This means less current can flow per unit of pd and there is more resistance to the current
- SEE GRAPH IN PHOTOS AND MEMORISE
- A filament lamp is not an ohmic conductor as as potential difference increases current no longer increases because resistance is increasing
What is the symbol for a diode
See CGP for answer
Which direction does a current flow through a diode
- A current flows through a diode in one direction only
- This is because a diode has a very high resistance in the reverse direction
- Diodes are very useful for controlling the flow of currents in circuits
Draw a diode current potential difference graph
See CGP for answer
Draw the symbol for an LED
See CGP for answer
What does an LED do ? Why is it useful ?
An LED emmits light when a current flows through it
LEDs are extremely energy efficient sources of light
What is the rule about resistors in a series circuit
Resistors in series add together to give the total resistance
(E.g if there were two resistors of 5 ohms the total resistance would be 10 ohms)
-This is because the current has to pass through each individual resistor in turn , it cannot bypass any resistors
What is the rule for resistors in a parralel circuit
- The total resistance of two resistors in parralel is less than the resistance of the smallest individual resistor
- This is because with a parralel circuit there is more than one route for the current to take so more total current will flow therefore reducing the reistance (R=V/I)
Draw the symbol for a light dependent resistor
See CGP for answer
In what conditions is the resistance of the LDR high ? In what conditions is the resistance of the LDR low
- In dark conditions the resistance of the LDR is very high
- In light conditions the resistance of the LDR is very low
LDR’s can be used for night lights
Draw a light intensity -Resistance graph for a LDR
See Book for answer
In what conditions will the resistance of the LDR be high
In bright light, the resistance of the LDR falls
In darkness, the resistance of the LDR is highest
Draw the symbol for a thermistor
See CGP for answer
How does the resistance of a thermistor change with temperature
- The resistance of a thermistor decreases if the temperature increases
- The resistance of the thermistor increases if the temperature drops
(They work in a similar way to LDRs with the potential difference being shared across them and a component)
Draw a resistance temperature graph for a thermistor
See CGP for answer
What can you use a thermistor for
You can use a thermistor for a car engine temperature sensor or an electronic thermostat
Why does a spark form when two charged objects are placed near each other
When one object is charged, there is a potential differnce between the object and the other object. When the potential difference is high enough the electrons will jump across the gap and a spark will form
Draw the symbol for a variable resistor
See CGP
Describe a practical to measure I-V characteristsics
- Set up the curcuit ( batteries connected by wires to component, component in series with ammeter and variable resistor and in parralel to voltmeter)
2) First use the voltmeter to measure the potential difference across the component(could be resistor, filament lamp or diode
3) then use the ammeter to measure the current through the component
Record these values in a table
4) Now adjust the variable resistor and record the readings on the ammeter and voltmeter. Do this several times to get a range of results
5) switch the direction of the battery. This means the direction of the potential difference has reversed both the voltmeter and ammeter should give negative values
6) continue taking several readings of p.d and current
7) plot an IV graph
Notes with resistor :Do not leave the circuit on for too long or the temperature of the resistor might increase ans the p.d will no longer be directly proportional to the current
Notes with diodes:with the diode in the circuit you need an extra resistor as the diodie will be easily damaged by a high current . The extra resistor will keep current low and prevent damage to a resistor. Because the current will be low a more sensitive ammeter is needed this is called a milliammeter
What factors affect the resistance of a wire
Length of wire-if the wire is longer there are going to be more ions for the electrons to flow past so a greater resistance
Larger cross section-If the cross section of the wire is bigger there will also be more ions for the electrons to flow past so more resistance
Heat-If the ions have more energy they will vibrate more a create more resistance
-conductor-if the metal is a better conductor the ions will vibrate more a create more resitance
