10 Electricity Flashcards
What is an electrical current?
A flow of charge.
What is the symbol for charge?
Q
What is the symbol for current?
I
What does the Q for charge stand for?
Quantity
What does the I for current stand for?
Intensity
What are electrons?
Fundamental particles with an approximate diameter of 10-¹⁸m.
What are electrons and ions called?
Electron carriers
What is charge measured in?
Coulombs (C).
What charge are electrons?
Negative
What is the definition of current?
A current of 1 ampere (1A) means that 1 coulomb (1C) of electrons flows past any point in the circuit every circuit.
What is 1A equal to?
1 C s‐¹
What is electrical current measured in?
Ampere
In which direction do electrons flow in a circuit?
From positive to negative
In a cell, which side is negative?
The short side
In a cell, which side is positive?
The long side
What are circuit rules for current?
- The current leaving the power supply is equal to the current returning to the power supply.
- The current entering a component is equal to the current leaving a component.
- Components in series have the same current flowing through them.
- The total current entering a junction is equal to the total current leaving a junction.
What happens to current as it goes through a junction?
It splits.
What is charge always?
Conserved
Explain how charge is conserved?
The number of coulombs of electrons leaving the power supply every second is equal to the number returning every second.
Where does chemical action inside a cell move electrons?
From one terminal to the other.
What is potential difference?
If the potential difference between the two points in the circuit is 1 volt then 1 joule of potential energy is lost per coulomb of electrons as they pass between those points.
What is 1V equal to?
1 J C‐¹
What is the definition of potential difference?
Energy change per unit charge.
What is used to measure potential difference?
A voltmeter
What is used to measure current?
Ammeter
What are the circuit voltage rules?
- The pd’s across components in parallel are the same
- The pd’s across individual components in series add up to equal the pd across all of them.
What are resistors used for?
To control the size of an electrical current.
What is the size of a resistor called?
Resistance
What is resistance measured in?
Ohms(Ω)
If a resistor is connected to a power supply, what is the potential difference across to resistor divided by in order to find its resistance?
The current flowing through the resistor.
What is the definition of resistance?
The potential difference per unit current.
What is the possible percentage variation in the resistors value called?
The tolerance.
What tolerance do most resistors have?
±5%
What are some random errors in circuits?
-Resistance values quoted on resistors may be different from their actual Resistance.
-the meters used to measure current and potential difference are only as accurate as the precision on their scales.
-electrical circuits have many points of electrical connection. A bad electrical connection has significant contact Resistance.
What is the Resistance at a point of connection called?
Contact resistance.
What does contact resistance do?
Adds to the circuits resistance.
Why can contact resistance cause a random error?
Each time you replace a component in your circuit the contact resistance may change.
Compare the total resistance of two parallel 10Ω resistors with a single 10Ω resistor?
Should have halved.
Compare the total resistance of two series 10Ω resistors with a single 10Ω resistor?
It is double.
What is the current leaving a component equal to?
The current entering a component.
What is Vt equal to in a series circuit?
V1+V2+V3
IR1+IR2+IR3
What is Rt equal to in a series circuit?
R=R1+R2+R3
What is the total current entering a junction equal to?
The total current leaving a junction.
What are the potential differences across components in a parallel?
The same.
What is Rt equal to in a parallel circuit?
1/R = 1/R1 + 1/R2 + 1/R3
What are the 2 different types of resistors?
-wire-wound resistors
-carbon resistor
How are wire-wound resistors made?
A precise length of wire is wound onto a ceramic rod to create the resistance value required.
How is a carbon resistor made?
There is carbon inside a ceramic rod.
What is the resistance of a wire directly proportional to?
The length of the wire.
What can the proportion sign be replaced by? (Inverse proportion)
= constant / value.
What is resistance inversely proportional to?
The cross sectional area.
What quantity does multiplying the resistance by the cross sectional area and then dividing by the length give?
Resistivity (ρ)
What is the definition of resistivity (ρ)?
The resistivity of a material is its resistance multiplied by its cross-sectional area divided by its length.
What is the symbol for resistivity?
ρ
What are the units for resistivity?
Ω m
What quality of metals have higher conductors?
Small resistivity values.
What does cooling metal with ice reduce?
The vibrations of the metal lattice ions.
Why is it easier for electrons to flow past lattice ions after cooling?
The rate of electrons colliding is less.
What happens to the resistance of metal when cooled?
It decreases.
Why is it harder for electrons to flow past lattice ions after heating?
The rate of electrons colliding is more.
What happens to the resistance of metal when heated?
It increases.
What is the critical temperature? (Super conductor)
The temperature at which a superconductor’s resistance becomes zero.
Once flowing, what will the electrical current in a superconductor circuit flow without ?
A power supply
How much resistance do super conductors have?
0Ω
When are super conductors used?
When scientists need to create the very strong magnetic fields required in particle accelerators such as the Large Hadron Collider.
What is a superconductor?
A material that has zero resistance when it is cooled to a temperature equal to its critical temperature.
What does NTC stand for (semiconductor)?
Negative temperature coefficient
What is this?
A thermistor
As the temperature of a semiconductor, what happens to its resistance?
It increase
What group of the periodic table are semiconductors in?
Group 4
What is the temperature of a semiconductor inversely proportional to?
The resistance.
What is likely to happen to the resistance of an NTC semiconductor over a long period of time if the temperature of the room is constant?
As the current goes round the the battery will heat up the circuit and the resistance will decrease.
What 2 effects does heating a semiconductor have?
•many of the orbital electrons trapped within the atoms become free to move around.
•the lattice ions vibrate more.
Why does a semiconductor become a better conductor when heated?
It’s resistance decreases.
What does an NTC thermistor graph look like?
What is the definition of power?
The energy transfer (or work done) per unit time.
Rate of dissipating energy.
What is power measured in?
Watts (W).
What is 1W equal to?
1W = 1 J s‐¹
What is the symbol for energy change the same as?
Work done.
What is the equation for energy transfer (electricity)?
E= VIt
What is this?
A potentiometer.
What piece of equipment can be used as a variable resistor and a potentiometer?
A rheostat
What is a graph of current vs pd describes as?
The component’s currents/pd characteristics.
What is ohms law?
Current is directly proportional to p.d., provided that there is no change in external conditions i.e. temperature.
What are components which obey ohms law called?
Ohmic conductors.
What type of current vs pd graphs do homicide conductors produce?
A straight line through the origin providing it has a constant resistance.
What are components not obeying ohms law called?
Non-ohmic conductors
What is the resistance of a non-ohmic conductor like?
Constant.
What is this?
A diode
In what direction does the current flowing through a diode?
The direction of the arrow.
Is a diode an ohmic conductor? Why?
No. The current is not directly proportional to the pd.
What happens to the diodes resistance as the pd across it increases?
The resistance decreases.
When the diode is conducting a significant current, what is the pd across it?
About 0.6V.
What is this?
A bulb
What happens to a bulbs resistance as the pd across it increases?
It increases.
What graph is this?
Is it an ohmic conductor?
Resistor - ohmic conductor
What graph is this?
Is it an ohmic conductor?
Diode - non-ohmic conductor.
What is this?
Is it an ohmic conductor?
Filament bulb - non-ohnic conductor.
Is a filament light bulb and ohmic conductor?
No.
How can a rheostat be used as a potentiometer?
Use all 3 sockets.
What is this?
A variable resistor.
How do you use a rheostat as a variable resistor?
Use the top socket and only one of the bottom sockets.
What is this?
A light dependant resistor (LDR).
When does the resistance of an LDR decrease?
When more light is incident upon it.
Is an LDR an ohmic conductor?
Yes.
Describe a problem with using a variable resistor in a circuit to control the speed of a motor.
You can’t get the current to zero.
What happens to the reading on an ammeter when the light incident on an LDR is increased?
It increases
What happens to the reading on a voltmeter when the light incident on an LDR is increased?
It decreases
Is an LDR a semi-conductor?
Yes.
Why does the resistance of an LDR increase when more light incident is upon it?
When light shines on it releases electrons which increases the number of electrons to carry the current. Hence, as the light increases the current increases resulting in a reduction in resistance.
Why does the resistance of an LDR increase when less light incident is upon it?
In the dark, no extra electrons are available so the current experiences a greater resistance.
What is this?
A light emitting diode (LED).
When do LED emit light?
When conducting.
Are LED’s efficient?
Yes. Very. As they waste little energy as thermal energy.
Are filament bulbs efficent?
No. They waste a lot of energy through thermal energy.
What is this? What does it do?
A resistor.
Controls electrical current.
What is this? What does it do?
An LDR.
When light intensity increases, the resistance decreases.
What is this? What does it do?
A thermistor.
The more heat the less resistance.
What is this? What does it do?
A potentiometer.
Varies the potential difference.
What is this and what does it do?
A variable resistor.
Adjusts the resistance by moving the threshold.
What is this? What does it do?
Diode.
Only allows the current to flow when forward bias.
What is this? What does it do?
As light increases, current increases so the resistance decreases (when forward bias).
What is this? What does it do?
Lamp / filament bulb.
Electric fire allows light to be emitted.
What is the definition of current?
The amount of charge flowing through a cross sectional area of wire per unit time.
What is a potential divider circuit made up of?
At least 2 series resistors connected to a fixed voltage. The voltage is then divided between the resistors.
What is this? What does it use?
A fixed potential divider.
Uses resistors of a fixed value.
What is this? What does it use?
A variable potential divider.
Uses resistors in series but with one or more of the resistors can change in values.
What is the definition of an electromotive force(ϵ)?
The electromotive force of a cell is the potential energy supplied per unit charge.
What is the symbol of electromotive force?
ϵ
What is E.M.F short for?
Electromotive force (ϵ)
What is emf(ϵ) measured in?
Volts.
How is the emf of a cell (ϵ) found?
By connecting a high resistance voltmeter across the terminals of the cell when it is not connected to anything else i.e open circuit.
What is internal resistance?
Loss of potential difference per unit current inside the cell.
How will you know if it is an internal resistance question?
No negligible internal resistance.
What is the symbol for internal resistance?
r
Why is the symbol for internal resistance a lower case ‘r’?
Because it is small.
When a cell supplies a current to a circuit why does the cell heat up?
Because the electrical current flows inside the cell as well as around the circuit.
For electrical potential energy to be changed to heat inside the cell what must the materials inside the cell have? What is it called?
some resistance.
The cells internal resistance (r).
Explain the conservation of energy in terms of internal resistance (r)?
Energy supplied per coulomb by the cell(ϵ) = energy changed per coulomb following in the external circuit (V) + energy changed per coulomb following through the cells internal resistance (v).
So: ϵ = V + v
What is the v in ϵ = V + v?
Voltage for the resistance in battery. Lost volts.
Give an equation for ϵ (VIR).
(ϵ) = V + Ir
What are the ends of the batteries called?
Terminals of batteries.
What is the terminal pd?
Potential difference taken from the battery terminals.
What is the equation for internal resistance?
(e.m.f - terminal pd) / current
((e.m.f - V) / I)
What is the symbol for terminal pd?
V
What is the gradient equal to in a terminal pd vs current graph?
-r (modulus of cells internal resistance)
What is the y intercept of a terminal pd vs current graph equal to?
ϵ
What is the potential divider formula?
V out = V in x (R/Rt)
What are the rules for combining cells in series?
E.m.f between X and Y = ϵa + ϵb + ϵc
Internal resistance between X and Y = ra +rb +rc
What are the rules for combining cells in series but the wrong way round?
E.m.f between X and Y = ϵa - ϵb.
Internal resistance between X and Y = ra + rb.
What are the rules for combining identical cells in parallel?
E.m.f between X and Y = ϵ
Internal resistance between X and Y = (1/r + 1/r + 1/r)‐¹
Describe some advantages of combining cells in parallel?
Reduces internal resistance while keeping e.m.f the same.
Higher max current. Lasts longer. Terminal pd is closer to e.m.f.
What is the equation for the maximum current?
ϵ/r
How is a cell short circuited?
By connecting a lead from one terminal to the other.
If a cell is short circuited, what will the current be?
The maximum current
What is the definition of the power supplied by a cell?
The total electrical potential energy converted to other types of energy in the circuit and inside the cell energy second.
What is the equation for power supplied to a cell?
P = ϵ × I
P = I²R + I²r
What is the equation for energy in a circuit?
E = VIT
What is the rate of dissipation of energy?
Power
