5.1 Flashcards
What is electric current? state its units
Net flow of charged particles. Amps.
What is potential difference?
The work done moving a unit charge between 2 points in a circuit
V = W / Q
What is resistance?
How difficult it is for current to flow through an appliance.
What is the metal wire made up of?
A lattice of positive ions, surrounded by ‘free electrons’
How do ions move in a metal wire?
They vibrate about fixed positions
How do electrons move in a wire?
They are free to move from one ion to another
What happens to electrons when a battery is connected to a wire?
Free electrons are repelled by the negative terminal and attracted to the positive
Draw a Ohmic conductor resistor I-V graph
Draw a semi conductor diode I-V graph
Draw a filament lamp I-V graph
Conventional current
the flow of positive charge from the positive terminal
of a cell to the negative terminal
Ohm’s law
Current is directly proportional to the p.d at a constant temperature
How can you measure the current in a circuit?
With an ammeter connected in series with the component
How can you measure the potential difference across a component?
Using a voltmeter, connected in parallel across the component being measured
Why does the current increasing on a filament lamp cause an increase in resistance?
As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increasing amplitude. This blocks the movement of electrons through the lamp as they collide with ions
The resistivity equation shows that:
- The longer the wire, the greater its resistance
- The thicker the wire, the smaller its resistance
What is Resistivity is measured in?
Ω m
Why does the flow of electric current decrease in a material when electrons collide with atoms?
Electrons collide with the vibrating atoms which impede their flow, hence the current decreases.
How does temperature affect the vibration of atoms in a material?
The higher the temperature, the faster these atoms vibrate.
How does an increase in temperature affect the resistance of a metallic conductor that obeys Ohm’s law?
An increase in temperature causes an increase in resistance
Does a thermistor follow the same resistance-temperature relationship as a metallic conductor that obeys Ohm’s law?
No, for a thermistor, an increase in temperature causes a decrease in resistance.
What is a thermistor?
non-ohmic conductor and sensory resistor where resistance varies with temperature.
What happens to the resistance of a thermistor if the temperature increases?
If the temperature increases, the resistance of the thermistor decreases (and vice versa).
What are some examples of applications where thermistors are used?
fire alarms, and digital thermometers
How does the resistance of a thermistor change as it gets hotter or cooler?
- As it gets hotter, resistance decreases,
- As it gets cooler, resistance increases.
What is a superconductor?
material with no resistance below a critical temperature.
How does a material become superconductor?
If a material is cooled below a temperature called the critical temperature, its resistivity disappears entirely
What are some applications where superconductors are useful? (2)
- production of strong magnetic fields - MRI scanners
- Power cables, which would reduce energy loss i through heating to zero during transmission
Unless stated in the question, should you assume that voltmeters to have zero resistance or infinite resistance?
Infinite resistance. Current
What is an LDR
A semiconductor that is sensitive to light. As light intensity increases, its resistance decreases.
What is the current like in a series circuit?
the current is the same for all components.
What is the current like in a parallel circuit?
the current is split across the different branches (or junctions).The total current into a junction must equal the total current out of a junction.
What is kirchof’s first law
All of the current going into a junction is equal to the current leaving that junction. Charge is conserved.
What is kirchof’s second law
for any loop of a circuit, the sum of all the p.d must equal to total emf of the circuit. Energy is conserved
What is power?
The rate of energy transfer
What is the purporse of a potential divider
- To provide variable potential difference
- To provide a constant specifc potential difference
what is emf?
electrical energy transferred by a power supply per unit charge
What happens to the potential difference across resistance R if the potential difference across the thermistor increases?
the potential difference across the resistance R must decrease, to keep the same overall total e.m.f.
what are diodes?
diodes only let current flow in one direction
What is an NTC?
A negative temperature coefficient of resistance so resistance decrease as temp increases.
Is there a p.d. when a material is a superconductor?
No - as a current flows but resistance = 0
When is a potentiometer useful?
- When you need to change the voltage continuously
What is a potentiometer?
A type of potential divider that uses a single variable resistor instead of two fixed resistors to give a desired voltage output.
what are the equations for emf not found on the data sheet?
ϵ= I (r+R) found on data sheet
ϵ= V + v
V = ϵ - Ir
ϵ= e.m.f
V= terminal pd
v= lost volts
I = current
r= internal resistance
n the experiment to find the e.m.f. and internal resistance of a cell, how should the V and I values be plotted and why?
- V = ε - Ir
Rearranges to: - V = -rI + ε
- y = mx + c
Therefore: - Plot V on the y and I on the x
- Gradient = -r
- y-intercept = ε
Explain how and why the resistance of a filament lamp changes as potential difference is increased.
- As potential difference increases, so does current.
- Increasing current means that some electrical energy is transferred into heat energy, increasing the metal’s temperature (due to electron-ion collisions).
- Positive ions in metal vibrate more -> More difficult for charge carriers to pass -> Current can’t flow as easily -> Resistance increased
Explain how and why the resistance of a filament lamp changes as current is increased.
- Increasing current increases the temperature (due to electron-ion collisions)
- Positive ions in metal vibrate more -> More difficult for charge carriers to pass -> Resistance increased.
IV graph levels off at high current for filament lamps.
Why isn’t a filament bulb an ohmic conductor (in terms of pd and temperature and resistance)?
The wire heats up with current and potential difference, causing the resistance to increase.
How can you investigate the resistance of a thermistor?
Use water bath:
Put the thermistor into a beaker and pour boiling water into it, covering the thermistor.
Measure the temperature of the water using a digital thermometer.
Record the current through the circuit with the ammeter.
The potential difference needs to be kept constant throughout the experiment.
Keep recording current and temperature for every 5 degree drop.
From the values for current and potential difference, work out resistance for each temperature.
For NTC resistors, as temperature decreases, resistance increases (current will decrease).
Describe an experiment to calculate the e.m.f. and internal resistance of a cell.
1) Connect the cell in series with an ammeter and variable resistor + connect a voltmeter across the cell
2) Vary the current using the variable resistor - start at highest resistance (open the switch and close it again to get two more sets of I and V values and find mean)
3) Record the voltage at each current.
4) Plot a graph of voltage (y) against current (x).
5) y-intercept = ε
Gradient = -r
Make sure external factors are kept the constant like temperature.
