Electricity Flashcards
Define the volt
The voltage in volts is the energy in Joules divided by the charge in Coulombs
What is a battery?
A combination of two or more electrical cells.
Current always flows from
The positive terminal to the negative terminal - conventional
Ammeter- 0.01A
An ammeter is an instrument that measures current and is always connected in series. It has very low electrical resistance. The resolution is 0.01 A.
Resolution
It is the smallest detectable value that an instrument can measure.
If a resolution is detected by decimal points, what does this reduce?
percentage uncertainty
voLtmeter- 0.01 V
A voltmeter is always connected in parallel across a component. The voltmeter measures potential difference (voltage). It has a very high electrical resistance. The resolution is 0.01 V.
Resistance
The opposition to the flow of charge. Resistance in Ohms is the voltage in volts divided by the current in Amps
resistivity
Different materials have different resistances
What affects the resistance?
- the length of a wire
- type of material/ wire (eg: copper has a lower resistance than tungsten)
- cross-sectional area (thickness of wire), diameter
- temperature (in most cases, as the temperature increases, resistance increases)
Resistors in series
- The current is the same at every point in the circuit at the same time
- One electrical route/ pathway
- total resistance increases (R1 + R2)
- voltage (p.d). - The larger resistor has the higher voltage
- When resistance increases, the current decreases
- voltage in series are called potential dividers
Resistors in parallel
- more than one electrical pathway
- Total resistance decreases
- voltage (p.d.) remains the same across parallel resistors
- the resistors in parallel are called current dividers
Thermistor
A temperature sensor that detects changes in temperautre
Thermistor- when temperature increases…
resistance decreases, current increases
LDR- when light intensity increases….
The resistance of the LDR decreases
Determine the resistance of a resistor (6 marks)
- A battery provides a source of electrical energy/ power/ voltage supply
- An ammeter is connected in series to measure current
- open switch will conserve energy. Use an open switch when recording data because an increase in temperature will affect the resistance.Important to obtain current when needed.
- variable resistor to change the resistance
- voltmeter connected in parallel to measure the potential difference across the fixed resistor
- wire to establish good electrical contact
METHOD
1, Set the rheostat/ variable resistor at zero and close the switch.
2, Read off the ammeter and voltmeter reading.
3, Open the switch and move the slider to a suitable position away from zero, close the switch and repeat step 2.
4, Perform steps 2 and 3 for different positions on the variable resistor (rheostat). At least five readings and three repeats and then calculate the mean.
5, The dependent variable is the current and the independent variable is the voltage. The control variables are temperature and the same unknown resistor. Control variable in any investigation help to arrive at a VALID CONCLUSION. Only one independent variable can change in any investigation at any time. - to determine the resistance of the unknown component, use Ohm’s law Rx= V (average)/ I (average) (of the repeats)
- Plot the results on a graph and determine the resistance
SAFETY (risk assessment) - ensure that the cables/ wires do not overheat which could cause possible burns
IMPROVEMENT
Use an ohmmeter from a multimeter- this will prevent overheating of the wires and ensure a more constant temperature.
What is the importance of a control variable
A control variable in any investigation helps to arrive at a valid conclusion. Only one independent variable can change in any investigation at a given time.
metaL current voltage graph - fixed resistor
Linear graph.
Q: Explain how the resistance from a graph can be determined (4 marks)
1, Choose a point on the curve and construct a tangent
2, Use the gradient change in y/ change in x
3, Change in current/ change in voltage = I/R
4, R/I = rsistance
Potential difference plotted against current
As the potential difference increases, the current increases. Ohm’s law- Current is directly proportional to the potential difference at a constant temperature.
Resistor graph - p.d (y axis) and current
Resistance is constant. Resistance= potential difference / current. The steeper the graph, the lower the resistance.
What effect does an increase in temperature have on the resistance of a thermistor?
It decreases
Explain how thermistors can be used to make a fire alarm work
If there is a fire, the thermistor of the fire alarm will detect the change/ increase in temperature. This causes the resistance to drop. Current can flow through the alarm to set the alarm off.
Diode graph - current voltage
Current starts to flow when the potential difference reaches about 0.7 volts/ a small voltage is present. It only allows the current to flow in one direction.
What are diodes commonly used for?
They are commonly used for battery operated appliances. If the battery is connected the wong way round, the current starts to flow and protects the environment.
LDR- resistance (y axis, light intensity graph)
Resistance decreases as light intensity increases. This causes the current to decrease.
Describe a use for a LDR and explain how it works
It is used in digital cameras to control the total amount of light that enters the camera.
Filament lamp (current voltage graph)
A filament lamp does not follow Ohm’s law. This is because the lamp is kept on so the temperature of the lamp will increase, thus it does not follow Ohm’s law- The resistance not constant. This can be seen from the way the graph curves. As the temperature increases, the resistance increases.
Filament lamp (current voltage graph)
Resistance in the filament lamp is not constant. As the temperature of the filament increases, the resistance increases.
Filament lamp (current voltage graph)
Resistance in the filament lamp is not constant. As the temperature of the filament increases, the resistance increases.
Filament lamp (current voltage graph)
Resistance in the filament lamp is not constant. As the temperature of the filament increases, the resistance increases.
Explain why a filament lamp does not follow Ohm’s law
The further increase in voltage will not allow current to flow through because the density and temperature increases.
cell/battery
Provides the circuit with a source of potential difference. A battery is two or more electrical cells.
switch
to turn the circuit on (closed) or off (open)
Fixed resistor
A resistor that limits the flow of current. A fixed resistor has a resistance it cannot change.
Variable resistor
Can be altered to change its resistance. Often used in dimmer switches and volume controls.
Diode
Allows current to flow in one direction only. Used to converts AC to DC current .
Light-emitting diode
This is equivalent to a diode and emits light when a current passes through it.They are used for aviation lighting and displays (TVs, road signs)
What do electrical circuit diagrams require to work effectively?
- an energy source: This is a source of potential difference so a current can flow. This can be a cell, battery or power supply.
- a closed path or complete circuit: Electrons need to flow in a complete pathway for a current to flow. A circuit can be open and closed using a switch
- electrical components: These could act as sensors that respond to the environment (LDR, thermistor) , or measure a value (ammeter, voltmeter), or transfer electrical energy to other forms of energy (LED, lamp) . These must be drawn with the correct circuit symbol.
What are the differences between series and parallel circuits?
Series:
- When resistance increases, current decreases
- voltmeters in series-potential dividers
- only one electrical pathway/route
- the current is the same at every point in the circuit at the same time
- total resistance increases (RT= R1 + R2)
- voltage is shared in a series circuit
- the larger resistor has the higher voltage.
Parallel:
- more than one electrical route/ pathway
- the current splits and rejoins
- the total resistance decreases (1/R1 = 1/R1 +1/R2)
- voltage (pd) remains the same across parallel resistors
- the resistors in parallel are called current dividers
Resistance
Resistance is the opposition to the flow of charge. (slows down flow of current) Resistance in Ohms is the voltage in volts divided by current in amps.
-measured in ohms
Current
- It is the rate of flow of charge (electrons)
- It is measured in Amps (A) using an ammeter
Voltage/ potential difference
- It is the difference of electrical potential energy between two points of a circuit
- it is the pushing/ driving force for current in a circuit
(measured in Volts (v) using a voltmeter)
What are the 4 requirements needed for a circuit to work?
1, Power supply (cell/battery)
2, Components (bulb/meter)
3, Complete loop of wire (so current can flow)
4, Avoid short circuits (a path with no resistance)
Ohms’ Law
” Voltage is directly proportional to the current provided the resistance and temperature in the circuit remains constant.”
(if the voltage increases or decreases, the current would do the same thing).
What is an ohmic conductor?
It is a device which follows Ohms Law.
P=
I^2R
Direct current (DC)
This is current that only flows in one direction, this current is NOT able to travel long distances efficiently.
eg: batteries and cells
Alternating current (AC)
This is current that will constantly change direction, this current is able to transfer energy over long distances efficiently
eg: mains electricity
Series and parallel
- series: when resistance increases, current decreases
- resistors in series - potential dividers
- only one electrical pathway/route
- the current is the same at every point in the circuit at the same time
- total resistance increases (RT= R1 + R2)
- voltage is shared in a series circuit
- the larger resistor has the higher voltage
Parallel: - more than 1 electrical route/ pathway
- the current splits and rejoins
- total resistance decreases (1/RT= 1/R1 + 1/R2)
- voltage (pd) remains the same across parallel resistor (same in each branch)
- the resistors in parallel are called current dividers
- current will split and rejoin at branches
- if one component stops working, the rest will remain functioning as they are on individual pathways.
- voltmeters are conencted in parallel
Mains electricity
- This is the electricity that is carried on the national grid and is distributed acorss the nation and comes to the household.
- The potential difference = 230- 240 V
- AC is used for mains electricity- because AC can travel long distances efficiently
- Frequency (Hz)- the number of oscillations that pass through a point pr second
- Frequency of AC- 50 Hz–> 50 cycles of current per second.
Fuse
It is a safety devise inside plugs. This fuse has a low resistive wire inside of it. This wire will melt when too much current flows through. This breaks the circuit and it needs replacing.
Live wire (brown)
This is the wire that takes current from the power supply to the component
neutral wire (blue)
This is the wire which is used to complete the circuit and allows the backflow of current.
earth wire (yellow/green)
this is the safety wire which gets rid of excess charge and grounds it
Transformers
- It is a device which is used to alter the voltage and the current in a circuit.
Step- up transformer
- It is used to increase the voltage and decrease the current in the circuit
- In a step up transformer, it has more coils in the secondary coil than the primary coil.
Step- down transformer
- It is used to decrease the voltage and increase the current in the circuit
- In a step-down transformer, it has more coils in the primary coil than the secondary coil.
Static electricity
- It is the build up of charge when two insulating materials are rubbed against each other, and electrons are transferred from one material to another material
- occurs with insulating materials
- the material that gains electrons becomes negatively charged
- the material that loses electrons becomes positively charged
- static - non-flowing electricity
When rubbing a balloon to someone’s hair
As the hair and the balloon are both insulating materials when they are rubbed against each other, there will be a build- up of charge. One way this charge could be transferred is by electrons transferring from the hair to the balloon. This makes the balloon negatively charged.
- The same thing could happen but electrons could be transferred to the person’s hair instead of the balloon. This means the balloon is positively charged. The balloon will still stick to the wall as the negatively charged electrons will move in front of the positive particles and it would attract the positively charged balloon.
Earthing
- earthing is the process where the excess charge is grounded (taken down to the earth) and makes the object neutral again
- to prevent too much charge building up, we use an earth wire
- if too much charge is built up, it could result in a spark. This spark could lead to an explosion.
examples of earthing
- carpets have a low resistive wire inside it so when charge is being build up when walking on the carpet, it will automatically get earthed
- planes flying in the air experience air resistance, this causes a build-up of charge, this means when these air crafts are refuelled they initially have to be discharged using an earth wire so the charge doesn’t cause an explosion.
uses for static electricity
- photocopiers and printers
- filtering smoke in factories
- in heart defibrillators
- spray painting
A charged object creates
an electric field around itself
Field lines
- point away from positive charges
- point towards negative charges
All objects are initially
electrically neutral, meaning the negative (electrons) and positive charges are evenly distributed
Sticking a balloon to the wall
- Rubbing a balloon on a woolen jumper transfers electrons onto the balloon by friction
- The balloon is now negatively charged whilst the jumper is left positively charged
- The wall is still neutral, however, when the balloon is placed near the wall, the positive charges in the wall are brought to the surface because they are attracted to the negative charge of the balloon
- Since opposite charges attract, the balloon sticks to the wall from only the electrostatic attraction
Where are static electricity experiences most common?
- These experiences are most common in dry air conditions
- This is because it is much harder for the charges to dissipate so they build up instead, and make static electricity more noticeable
Sparking
- The build-up of electrostatic charge can be quite dangerous and can cause sparking (also known as an electric shock)
- A static electric spark occurs when two objects which are charged by friction and become oppositely charged and have a surplus of electrons so large that the electrons ‘jump’ across to an object that is neutral
- Since a current is the flow of electrons, this causes a small current to flow between the objects, called a spark
- An example of sparking is the small electric shock felt from touching a door handle, or another person, after walking on a vinyl floor or nylon carpet with rubber shoes or whilst wearing socks
Lightning
- An extreme example of sparking is lightning
- In a storm, clouds move over each other causing them to become charged when electrons are transferred between them
- Since the ground is neutral, the negative charge jumps to meet the positive charges on the ground creating a big spark
- This is what is known as lightning
- Sparks can become quite dangerous and can cause a fire by igniting flammable gases and liquids, such as petrol
What does the strength of an electric field depend on?
- distance from the object creating the field:
- The field is strongest close to the charged object – this is shown by the field lines being closer together
- The field becomes weaker further away from the charged object – this is shown by the field lines becoming further apart
Objects in an electric field will experience
an electric force
What does the size of the force depend on?
- the strength of the field at that point
The force becomes stronger as
the distance between the two charged objects decreases
the force becomes weaker as
the distance between the two charged objects increases
What is the direction of electric field lines?
From positive to negative
electric field lines for a spherical conductor
- positvely charged spherical conductor: field lines away from the centre of the conductor
- negatively charged spherical conductor: field lines towards the centre of the conductor
A uniform electric field, such as that between two parallel plates,
are straight parallel lines from positive to negative
electric field
-The electric field helps to explain the non-contact force between charged objects since the electric field cannot be seen, but can be detected by another charged object that moves within that field due to the electric force
-This is a non-contact force because the charged objects do not touch for the force to be exerted
If an electric field becomes strong enough, the charges are forced through insulators such as air, creating a spark
-This is what happens for example, when a charged person touches a conductor
-The charged particles will travel towards areas with the opposite charge
