Electricity Flashcards
What does EMF stand for?
Electromotive force
What is a ideal cell?
A source of EMF
What is a real cell?
A source of EMF with an internal resistance.
Explain why a real cell has internal resistance.
- The current produced has to flow through the cell itself.
- The chemicals in the cell have an electrical resistance.
- The resistance inside the cell forms part of the circuit.
Calculate the current that would flow from a 1.5V AA cell when short circuited and describe the heating effect. (R=0, r=1))
- 1.5A
- The cell will get warm.
Calculate the current that would flow from a 1.2V AA NiMH cell when short circuited and describe the heating effect. (R=0, r=0.1))
- 12A
- Substantial heating effect.
Calculate the current that would flow from a 12V car battery when short circuited and describe the heating effect. (R=0, r=0.01))
- 1200A
- Explosive heating effect.
Which is higher v(emf) or V (terminal pd)?
v
Define emf
The potential difference across the cell terminals when no current is drawn from the cell.
What property must a voltmeter have to measure emf?
Infinitely high resistance
What two instruments can be used to measure emf?
- Voltmeter
- Oscilloscope
What does the term rated mean?
The max p.d. that can be applied to a component without damaging it.
Give five equations that relate to emf.
- ε = V + v
- V = IR
- v = Ir
- ε = I( R+r )
- ε = V + Ir
What is a conductor?
A component with a very small resistance and allow current to pass through with ease.
What is an insulator?
A component with a high resistance making it hard for current to pass through.
What is a semiconductor?
Materials that can change resistance by electrical means or by external factors.
Define resistance
R = V/I
where: R is the components RESISTANCE measured in OHMS
V is the p.d. across the component measured in VOLTS.
I is the current through the component measured in AMPS.
What is Ohm’s Law?
For a metallic conductor, the current flowing through the conductor is directly proportional to the potential difference across the conductor provided the temperature remains constant.
What is a diode?
A component that only allows current to flow in one direction.
What is forward bias?
When a diode allows current through with little or no resistance.
What is reverse bias?
When a diode doesn’t allow current though due to almost infinite resistance.
What is an LED?
- Light emitting diode.
- Emits light in forward bias.
What does LDR stand for?
Light dependent resistor
Describe how the resistance of an LDR changes.
The resistance of the LDR falls as the light intensity increases.
Explain how an LDR works.
- Made from CdS.
- CdS forms a covalent structure with almost all available electrons in covalent bonds.
- When a photon hits, the energy is transferred to an electron giving it enough energy to break the bond.
- The electron is then free for conduction.
- More electrons means a lower resistance.
Describe how the resistance of a thermistor changes.
The resistance falls as the temperature increases. (ntc)
What does ntc stand for?
Negative temperature coefficient
Explain how an ntc thermistor works.
- Made from silicon.
- Silicon forms a giant covalent structure.
- Heat energy causes bonds to break releasing electrons.
- More electrons means a lower resistance.
Explain how the reading of the voltmeter will change as the temperature of the thermistor falls.
- As the temperature increases, the resistance of the thermistor falls.
- As the resistance of the thermistor falls the total resistance in the circuit falls.
- The current in the circuit will increase.
- The pd across the fixed resistor increase so the pd across the thermistor decreases.
What is the relationship between pd and current of an Ohmic conductor?
They are directly proportional as long as the temperature remains constant.
How is the resistance calculated from the graph of a non-Ohmic conductor?
V/I
Not the gradient
How is charge flow calculated?
ΔQ = IΔt
How are electrons arranged in an insulator?
An electron is attached to an atom and cannot move away from the atom.
How are electrons arranged in a metallic conductor?
Most electrons are attached to electrons but the ones that are not can carry charge.
How are electrons arranged in a semiconductor?
The number of charge carrying electrons increases with temperature.
Define potential difference.
Work done per unit charge
How is resistivity calculated?
ρ = RA/L
What is the unit of resistivity?
Ohm metre
What is a superconductor?
A wire or device with zero resistivity at or below a critical temperature.
What can be used to measure the variation of current with pd for a component?
- Potential divider
- Variable resistor
Give an advantage of using a potential divider.
Pd and current can be reduced to zero.
What is the current passing through a diode in reverse bias?
Extremely small
What is the current passing through a diode in forward bias?
The current increases up to about 0.6V when the increases exponentially.
How does the resistance of a metal change with temperature?
As the temperature increases, the resistance of the metal increases.
How does the resistance of a thermistor change with temperature?
A thermistor has a negative temperature coefficient.
What are the three main rules for current?
- At any junction, the total current leaving the junction is equal to the total current entering the junction.
- The current entering a component is equal to the current leaving the component.
- The current passing through two or more components in series is equal in each component.
What are the three main rules for potential difference?
- The total pd across components is series is equal to the sum of the pd of each component.
- The pd across components in parallel is equal.
- In a loop, the sum of the emfs is equal to the sum of the potential drops round the loop.
What is the total resistance of a set of resistors in series?
R1 + R2 + R3…
What is the total resistance of a set of resistors in parallel?
1/Rtotal = 1/R1 + 1/R2…
How is the rate of heat transfer calculated?
P=I^2R
How is emf calculated?
ε = E/Q
When is maximum power delivered to the load?
When the load resistance is equal to the internal resistance of the source.
How is cell current calculated?
cell emf / total circuit resistance
How is the pd across each resistor in a circuit calculated?
current x resistance of each resistor
How is the current through each resistor in parallel in a circuit calculated?
pd across the parallel combination / resistors resistance
What are the three ways a potential divider can deliver pd?
- A fixed pd between zero and the source pd.
- A variable pd.
- A pd that varies with a physical condition.
What is the ratio of the pds across each resistor equal to?
The resistance ratio of the two resistors.
Give two uses of potential dividers.
- A volume control
- Vary the brightness of a bulb.
What is a temperature sensor circuit made from?
A thermistor and variable resistor
What is a light sensor circuit made from?
An LDR and variable resistor
What is the frequency of mains electricity?
50Hz
How is time period calculated?
1/freq
What is the peak value of an alternating current?
The maximum current or pd which is the same in either direction.
What is the peak pd in a mains circuit?
325V
How does increasing the output pd affect an oscilloscope screen?
The trace gets taller.
How does increasing the frequency affect an oscilloscope screen?
The number of cycles on the screen increases.
What does I0 represent?
Peak current
What is the root mean square value of AC?
The value of direct current that would give the same heating effect as the AC in the same resistor.
How is Irms calculated?
I0/√2
How is Vrms calculated?
V0/√2
Which value is used to calculate mean power of an AC circuit?
rms
If a pd is applied across the X-plates of an oscilloscope, how will the spot deflect?
Horizontally
If a pd is applied across the Y-plates of an oscilloscope, how will the spot deflect?
Vertically
