P2 - Electricity Flashcards
What is needed for electrical charge to flow through a closed circuit?
The circuit must include a source of potential difference
What is electric current?
The flow of electrical charge
What are ammeters placed in and what do they measure?
Ammeters are placed in series and measure current
What are voltmeters placed in and what do they measure?
Voltmeters are placed in parallel and measure voltage/p.d.
What is potential difference/voltage?
Difference in electrical potential from one point in a circuit to another
What is the equation that links charge flow, current and time?
Q = It, where
Q is charge flow (C), I is current (A) and t is time (s)
What is electricity?
The flow of electrons between atoms
In a single closed loop, what value does the current have?
A current has the same value at any point in a single closed loop.
What is resistance?
The measure of how much a component in a circuit resists charge
What is the effect of greater resistance?
The smaller the current becomes for a given potential difference across the component
What is the effect of lower resistance?
Rate of flow of charge increases so current increases. Less power is converted to heat and lost to surroundings
Explain what causes resistance (3)
As electric current flows and electrons move through a conductor, the moving electrons can collide with other metal ions in the metal, making it difficult for current to flow
Describe how work is done when a charge flows through a circuit
Whenever a charge flows, it has to overcome the resistance of the circuit, thus requiring energy
What is the equation that links current, potential difference and resistance?
V = IR , where
V = potential difference (V)
I = current (A)
R = resistance (Ω)
Why does increasing the potential difference in a circuit also increase the current?
Current and potential difference are directly proportional
Why are materials with a low resistance chosen for power cables?
Less power is converted to heat and lost to the surrounding environment
So more of the supplied power is used efficiently.
RP3: Describe a method to investigate how the length of wire affects resistance
Attach an ammeter, a voltmeter, a ruler, a cell, crocodile clips and wires to form a circuit
Set the power supply to 6 V
Connect one crocodile clip to the zero end of the meter ruler connected to the wire
Clip the second crocodile clip to the 10 cm mark
Record the ammeter and voltmeter readings
Switch the power supply off as soon as the readings have been taken
Calculate the resistance for each measurement using R=V/I
Repeat the investigation three more times each time increasing the length of wire and repeating steps 3 to 4
Determine a mean resistance for each length and plot a graph of resistance against the length of wire, Including a line of best fit
What factors affect the resistance of a given length of wire?
The type of MATERIAL
LENGTH as longer wires have greater resistance
THICKNESS as smaller diameter wires have greater resistance
TEMPERATURE as heating a wire increases resistance
Suggest why it is important that ammeters have very low resistances
Suggest why voltmeters have high resistances
So that the ammeter affects the current as little as possible
Little current flows through the voltmeter. Most current flows through the component
In required practical 3, explain why the student should open the switch after each reading
The wire would get hot if the current was left flowing, so the resistance would increase
In required practical 3, suggest a reason for an anomalous result
Measurement of voltage was too small.
measurement of current was too small.
incorrect calculation of resistance.
Thermometer misread
In required practical 3, explain why there is a small amount of resistance when the length of the wire is 0cm. What type of error is this and how do you fix it?
There is resistance between the crocodile clip and wire
This is a zero error, which is a form of systematic error that cannot be fixed by repeats. You must subtract the zero error from all readings
Explain why there is a zero error in the RP3 experiment
It is difficult to attach the clip precisely to the zero end of the wire
So there will be some contact resistance
Why are measurements not taken below 20 cm in RP3?
The resistance is very low so the wire would heat up, which is a safety issue and will change the resistance of the wire
Explain why temperature must be kept constant throughout the RP3 investigation. Describe how to reduce the effect of temperature on this investigation
An increase in temperature causes an increase in resistance
This can be reduced by using a low potential difference, which keeps current low and reduces heating in the wire
You can also only turn on current when reading is taking place
What is a resistor?
A resistor is a component that restricts the flow of electrical current
What is an ohmic conductor?
An ohmic conductor is a conductor in which voltage is directly proportional to the current flowing through it at a constant temperature
Explain how temperature affects resistance
As electrons move through a metal some collide with other metal ions
These collisions cause resistance and generate heat
Heating the metal causes atoms to vibrate more
Which makes it more difficult for the electrons to flow, increasing resistance and decreasing current
What are the I-V characteristics of an ohmic conductor?
The current at a constant temperature is directly proportional to the potential difference across the resistor. This means that the resistance remains constant as the current changes, so the graph will have a linear relationship.
What is the difference between the resistance of components such as lamps, diodes, thermistors, LDRs and ohmic conductors?
In an ohmic conductor, resistance remains constant as current changes, whereas in the other components, the resistance is not constant as it changes with the current through the component
Describe the IV characteristics of a filament lamp
As the current increases, the temperature of the lamp increases, so the resistance increases
This means less current can flow per unit potential difference so the graph gets shallower resulting in a curve
Describe the IV characteristics of a diode
The current through acdiode flows in One Direction only. The diode has a very high resistance in the reverse direction
How does the resistance of a thermistor change?
In hot conditions, the resistance decreases
In cool conditions, the resistance increases
Give two uses of thermistors
Used for thermostats
Used for car engine temperature sensors
How does resistance change in a light dependent resistor (LDR)?
In bright light, the resistance decreases
In darkness, the resistance is high
Give two uses of light dependent resistors
Switching the lights on when it gets dark (automatic night lights)
Burglar detectors
What is a series circuit and what are its properties?
A series circuit is a circuit with a single loop
For components connected in series:
there is the SAME CURRENT through each component.
The TOTAL P.D of the power supply is SHARED between the components
the TOTAL RESISTANCE of two components is this SUM of the resistance of each component
What is a parallel circuit and what are its properties?
A parallel circuit is a circuit with more than one loop
For components connected in parallel, the potential difference 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
Explain why adding resistors in series increases the total resistance whilst Adding resistors in parallel decreases the total resistance
Adding resistance in parallel forms another circuit loop so decreases the total resistance and increases the current flowing
Adding resistors in series increases resistance as it means that the current has to pass through another resistor, decreasing the rate of flow of charge
Explain the design and use of DC series circuits for measurement and testing purposes
Compare series and parallel circuits
What are the two types of electricity supplies?
DC, direct current
AC, Alternating current.
What type of electricity supply is Mains electricity?
AC
What is direct current DC?
Current that always flows in the same direction used to supply cells and batteries
What is alternating-current AC?
Current that constantly changes direction
How are alternating currents produced?
Alternating currents are produced by alternating voltage in which the positive and negative ends keep alternating
In the United Kingdom, the domestic electricity supply has a frequency and voltage of what?
The domestic electricity supply has a frequency of 50 Hz and is 230 V
What is the difference between direct current and alternating current?
Most electrical appliances are connected to the mains using three-core cable. What are they?
Live wire, which is brown
Neutral wire, which is blue
Earth wire, which has green and yellow stripes
What is the function of live wire, neutral wire and Earth wire?
The livewire carries the alternating potential difference from the supply
The neutral wire completes the circuit
The Earth wire is a safety wire to stop the appliance becoming live
What is the potential difference of the Earth wire and neutral wire?
The neutral wire is at, or close to, earth potential, which is 0 V
The Earth wire is at 0 V. It only carries a current if there is a fault
Explain why a live wire is dangerous even when a switch in the mains circuit is open
Explain the dangers of providing any connection between the LiveWire and the earth
What is power?
Power is the rate at which energy is transferred or work is done
What is the equation that links power , potential difference and current?
P=IV , where
P = power (W)
V = p.d. (V)
I = current (A)
What is the equation that links power, resistance and current?
P = I^2R, where
P = Power (W)
I = Current (A)
R = resistance (Ω)
What does the amount of energy an appliance transfers depend on?
How long the appliance is switched on for and the power of the appliance
Students should be able to describe how different domestic appliances transfer energy from batteries or ac mains to the kinetic energy of electric motors or the energy of heating devices.
What is the equation that links energy transferred, power and time?
E=Pt
E = energy transferred (J)
P = Power (W)
T = time (S)
What is the equation that links energy transferred, charge flow and potential difference?
E=Qv
E = energy transferred
Q = charge flow
V = voltage
Students should be able to explain how the power of a circuit device is related to:
• the potential difference across it and the current through it
• the energy transferred over a given time.
Students should be able to describe, with examples, the relationship between the power ratings for domestic electrical appliances and the changes in stored energy when they are in use.
What is the National Grid?
The National Grid is a system of cables and transformers linking power stations to consumers
Explain why the National Grid is efficient
The National Grid uses a high potential difference and a low current
The low current wastes less energy as less energy is wasted to heating
It is cheaper to increase the potential difference and keep the current as low as possible
For a given power, a high potential difference is needed for a low current
What is a transformer?
A transformer is a device that can change the voltage of an alternating current
What are step up transformers used for?
Used to increase the potential difference from the power station to the transmission cables
What a step down transformers used for
Used to decrease, to a much lower value, the potential difference for domestic use
Describe and explain how rubbing materials against each other can get them to become charged (4)
When two insulating materials rub together they become electrostatically charged
Friction causes the electrons to gain energy to leave the atom
Electrons now move to the other insulating material
Material that gains electrons becomes negatively charged and conversely, the material that loses electrons becomes positively charged
Describe why a charged rod can attract a piece of paper that is electrically neutral
When you bring the rod close to the paper, the changes within it are not free to move about as a current
So the negative charges repel the rod
How does static cause a spark?
As electric charge builds up on an object, the potential difference between the object and the Earth increases
If the potential difference gets large enough, electrons can jump across the gap between the charged object and the Earth, causing a spark
What is an isolated object
An object with no conducting path
What does a charged object create?
An electric field around itself
Where is the electric field strongest?
The electric field is the strongest close to the charged object. the further away from the charged object, the weaker the field
A second charged object placed in an electric field experience what?
A second charged object place in electrical field experience a force. the force gets stronger as the distance between the objects decreases
