1
Q
What is its electric characteristic like of a filament lamp and why is this?
A
- The electrical characteristic is a curve that starts steep but gets shallower as the voltage rises
- The resistance of the metal increases as temperature increases
1. A standard tungsten filament lamp transfers electrical energy into light and heat as the current flows through it
2. As the current increases, so does the frequency of the electron collision with the positive ion cores of the tungsten lattice, transferring more kinetic energy
3. The positive ion cores vibrate with a greater amplitude and so the resistance increases
4. A higher current leads to a higher temperatures, which in turn leads to a high resistance - The electrical characteristic shows the ration V/I (x axis is V) increasing and therefore the resistance increasing
2
Q
What is an equation to find out current?
A
- Change in Current / Change in time
- The area under a current-time graph is the charge transferred
3
Q
How do ammeters affect the circuit?
A
- They affect the size of the current as any device put into the circuit in series will have a resistance
- The extra resistance of the ammeter will therefore reduce the current in the circuit and this effect cannot be overcome
4
Q
How are semi-conductors used in sensors? What are some examples?
A
- Semiconductors are nowhere near as good at conducting electricity as metals
- This is because they have far fewer charge carrier available
- However, if energy is supplied to the semiconductor, more charge carriers can be released
- This means that they make excellent sensors for detecting changes in the environment
Examples: thermistors and diodes
5
Q
What is Kirchhoff’s first circuit law?
A
- At a circuit junction, the sum of the currents flowing into the junction equals the sum of the current flowing out of the junction
- At a circuit junction, the sum of the charge flowing into the junction equals the sum of the charge flowing out of the junction (per second)
6
Q
What is a non-ohmic component?
A
- Is a component that does not obey Ohm’s law; i.e. current is not directly proportional to the potential difference applied across it
- E.g. filament lamp, their electrical characteristics are non-linear
7
Q
How do you calculate resistance in parallel?
A
1/RT = 1/R1 + 1/R2 + 1/R3…+1/Rn
8
Q
How does an old analogue ammeter work?
A
-Passing the current through a coil of wire which generates a magnetic field that interacts with a permanent magnetic field, causing the coil to turn; the current is then measure by a point on an analogue scale
9
Q
How do changes in voltage and current impact resistance?
A
- Using R=V/I:
1. For a fixed pd (V), a small measured current (I) implies that the component has a large resistance (R) and vice versa
2. For a fixed current (I), we need a large pd (V), to drive the current through a large resistance, (R)
10
Q
How does pd and current split up in a parallel circuit?
A
- The pd across each component is the same
2. The total current through the whole circuit is the sum of the currents through the separate components
11
Q
What is an electrical characteristic?
A
- Is a graph (usually I-V) that illustrates the electrical behaviour of the component
- They show how the potential difference and current vary when the component is connected in both forward and reverse bias (when the current passes one way and then in the other direction)
- ALWAYS CHECK AXIS!
- Look at notes for diagrams!! and check specification!
12
Q
Which way does conventional current flow?
A
From positive to negative
13
Q
What is an electrolyte?
A
- A conducting solution, usually containing positive and negative salt ions dissolved in water
- Not all electric currents involve the flow of electrons, charged ions in a solution (an electrolyte) can also flow and create a current
14
Q
What is electrical power?
A
- Power is defined as the rate of transfer of energy
- Measured in watts (w), where 1 watt is equivalent to 1 joule per second
- P = E/t
- The electrical power transferred by a circuit is the sum of all the power transferred by the individual electrical components in the circuit
15
Q
What is going on in resistance?
A
- When current flows through the material of a circuit, such as the metal of the connecting wires, the material of the circuit gets in the way of the flow of charge
- As the electrons flow though the metal they COLIDE with the vibrating positive ion cores of the metal structure
- The collisions between the electrons and the positive ion cores transfer electrical energy from the electrons to the structure of the metal
- This causes the metal ion cores to vibrate more, thus heating up the wire
16
Q
What are some useful conversions of m?
A
- 1m = 1000 mm
- 1m2 = 1000000mm2
17
Q
Where are thermistors used?
A
- Thermistors are widely used to sense temperature (temperature sensors) changes and then to control devices. The change in the resistance of the thermistor affects a current, which can be used to switch devices on or off
- They can be found in thermal cut-out circuits to prevent devices from overheating (such as a hairdryer), central heating circuits, digital thermometers and engine-management circuits
- Lower resistance more current can flow through
18
Q
What is the internal resistance?
A
- Resistance comes from electrons colliding with atoms and losing energy to other forms
- Battery, chemical energy used to make electrons moves and as they do they collide with atoms inside the battery, so batteries have an internal resistance (this makes arteries and cells warm up when they are used)
- Amount of electrical energy battery produces per colour of charge is emf
- The pd across the load resistance is energy transferred when one coloumb of change flows through the load resistance
- If no internal resistance, the terminal pd would be the same as the emf, however in real power supplies always some energy lost overcoming internal resistance
2. As the current flows through the power supply, the internal resistance creates a potential difference (as V=IR) that leads to electrical energy being transferred to thermal energy inside the power supply
3. The internal resistance of most batteries and power supplies is very low (IGNORE internal resistance unless question explicitly states it)
19
Q
Which way do diodes conduct?
A
- Diodes only conduct in forward bias (in the direction of the arrow on the symbol)
- Most diodes require a threshold voltage of about 0.6V in the forward direction before they will conduct - Diodes do not conduct in reverse bias; this means that the resistance if the diode in reverse bias is infinite and diodes have very low resistance in forward bias, (in reverse bias, the resistance of the diode is very high and the current that flows is very tiny)
20
Q
How do you increase the current in a wire?
A
- Use a wire with more free electrons to increase n
- Increase the voltage so more of a ‘push’ to increase V
- Use a thicker wire to increase A
21
Q
How does pd and current split up in a series circuit?
A
- The potential difference provided by cells connected in series is the sum of the pd of each cell (depending on the direction in which they are connected)
- The total resistance is the sum of the resistance of each component
- There is the same current through each component
- The total pd of the supply is shared between the components
22
Q
Why are superconductors important? What could they be used for?
A
- Using superconducting wires you could make
1. Power cables that transmit electricity without any loss of power
2. Really strong electromagnets that don’t need a constant power source (for use in medical applications and Maglev trains)
3. Electronic circuits that work really fast, because there is no resistance to slow them down - If there are zero-resiatnce superconductors electronic devices and computer units would not generate heat and so don’t need cooling fans or batteries that last for an extremely long time on one charge, portable MRI scanners, cheap magnetic levitation
- Super-strong electromagnets and electrical power transmission lines that don’t waste energy
- Applications of superconductors to include the production of strong magnetic fields and the reduction of energy loss in transmission of electric power
23
Q
How do voltmeters work?
A
- Voltmeters are always connected in parallel with other components and both analogue and digital voltmeters work in very similar ways to ammeters
- However a small current is drawn from the circuit that passes through a set, known, very high resistance resistor so that the current is proportional to the pd. High quality voltmeters therefore have very high resistance
24
Q
What happens when you replace R2 with a ntc thermistor?
A
- The effect of the pd across the fixed resistor R1:
1. As the temperature increases, the resistance of the thermistor, R2 decreases and so V1 rises
2. thus increasing temperature produces increasing pd
3. If the voltmeter is connected across the thermistor, the opposite will happen: increasing the temperature will produce a decrease in pd - In most cases, applications require the pd to increase with temperature, so the voltmeter is usually connected across the fixed resistor
25
How is resistivity dependent on temperature?
1. The resistivity of metals increases with increasing temperature
2. The resistivity of many semiconductors, such as silicon and germanium, decreases with increasing temperature
3. The resistivity of a superconducting material decreases with decreasing temperature above its critical temperature (like a metal), but its resist drops to zero below the critical temperature
26
What do potential dividers consist of? How are they connected?
1. A power supply (such as a cell)
2. A fixed resistor
3. A resistive component, whose resistance can be fixed (fixed resistor) or variable (variable resistors, thermistors, LDRs etc.)
- All of these components are connected in series and the emf of the power supply is shared across the two resistive components
27
What is the equation for resistivity?
rho=RA/L
28
What does the resistivity of a material depend on?
- Some of the intrinsic properties of the material:
1. It relates directly to the number of free, conducting electrons that can flow through the structure
2. The arrangement of the atoms in the conductor and any distribution of impurities affects this mobility as does the temperature of the material
29
How would you find the resistivity of a wire?
- You need to find its resistance
1. Before you start, you need to know the cross sectional area of the test wire. Assume that the wire is cylindrical, and so the cross-section is circular and then find cross sectional area using pir^2
2. Use a micrometer to measure the diameter of the test in at least three different point along the wire. Take an average value as the diameter and divide by 2 to get the radius (make sure this is in m). Plug it into the equation for cross-sectional area
3. The test wire should be clamped to a ruler with the circuit attached to the wire where the ruler reads zero
4. Attach the flying lead to the its wire 0 the lead is just a wire with a crocodile clip at the end to allow connection to any point along the test wire
5. Record the length of the test wire connected in the circuit, the voltmeter reading and the ammeter reading
6. Use your readings to calculate the resistance of the length of the wire using R=V/I
7. Repeat this measurement and calculate an average resistance for the length
8. Repeat for several different lengths, for empale between 0.1 and 1.00m
9. Plot your results on a graph of resistance against length, and draw a line of best fit
- The gradient of the line of best first is equal to R/l = rho / A, So multiply the gradient of the line of best fir by the cross sectional area of the wire to find the resistivity of the wire material
10. The resistivity of a material depends on its temperature so you can only find the resistivity of a material at a certain temperature. Current flowing in the test wore can cause its temperature to increase, which an lead to random error and invalid results. Try to keep the temperature of the test wire constant, e.g. by only having a small current flowing through the wire
30
What are some equations for power?
P=
- IV
- I2R
- V2/R
31
What are examples of resistivity?
1. At RT (20 degrees C), good insulating material e.g. ABS plastic has extremely high resistivity
2. Good metallic conductors have very low resistivity
3. Superconductors hae zero resistivity below their critical temperature
32
What is the resistivity of a substance?
- It is the intrinsic resistance of the material that the substance is made from; it is independent of the dimensions of the substance such as length and cross-sectional area
- Resistivity also varies with temperature, although the resistivity of metals changes only gradually with temperature
- Units is ohm meters (omegam)
33
What is the potential difference (pd)? What is an example?
- Is the amount of electrical work done per unit charge moved (flowing through a component)
- Is the work done BY a unit of charge in a component
- Converting electrical energy into light energy
- Symbol V, measured in volts (V)
- To measure put voltmeter in across the component in parallel with it
- V = W/Q
34
How do you measure the internal resistance and emf of a real power supply?
1. Set up a circuit with a variable resistor with a voltmeter across it, an ammeter and a battery or real power supply
2. Due to KII the emf must be the same as the sum of the pds in the circuit and these are across the the external variable resistor and the other is the pd across the internal resistor, and although this cannot be measured directly, the pd across this resistor is equal to Ir as V=IR so:
- emf= V+IR
3. Since you can measure I using an ammeter and emf and r are just constants the equation can be re-written
- V=-rI+emf
4. As this is the same form of y=mx+c where the gradient is negative
5. If an electrical characteristic is drawn using values of V and I from various values of R (the external load resistance) then the y intercept of the graph is the emf and the gradient is negative and equal to -r, the internal resistance
35
What is another equation for the current flowing through wire?
I=enAV
1. If there is a charge on the ion e.g. 2+ or 2- you must multiply the e (charge on an electron) by two. If there are both .g. SO42- and Cu 2+ ions in solution you would times by four
- I is the current, measured in amperes (A)
- e is the charge on an electron (1.6 x10-19)
- A is the cross sectional area of the wire (m2)
- V is the velocity at which the electrons move (ms-1)
- n electrons per count volume (number density of e- (n))
36
What is the electromotive force (emf)? What is an example?
- Is the amount of electrical work done per unit charge by a power supply such as a cell in an electrical circuit. The power supply transfers the other form of energy such as chemical energy into electrical energy
- Is the work done ON a unit of change passing through the seat of emf
- In a battery it is the chemical energy converted to electrical energy for each coulomb
- Symbol epsilon, measured in volts (V)
- Epsilon = E/Q
37
What is an LDR?
1. These are components that change their resistance with light intensity, which makes them incredibly useful as light sensors
2. A light dependent resistor is made of semi-conducting materials; as the light shines on the material, electrons are freed from the structure and can flow through it reducing the resistance of the LDR
3. In the dark, LDRs can have a very high resistance, of the order of mega-ohms, and yet in the light their resistance can drop as low as a few hundred ohms
38
What is an ohmic conductor? What happens when you double the pd?
- They obey Ohm's law
- The current flowing through an ohmic conductor is directly proportional to the pd applied across it
- The resistance is constant
- Doubling the pd doubles the current
- E.G fixed resistors and metal wires at a constant temperature obey Ohm's law across their current range and their electrical characteristics are linear
39
Wat happens when you replace R2 with an LDR?
1. If an LDR is connected into a potential divider in place of R2 in the potential divider circuit then, as the light intensity INCREASES, then so will the output pd V1 across the fixed resistor
40
What is a potential divider? What is it used for?
- They are simple, three component circuits designed to control the potential difference in a circuit
- They are used to supply constant or variable potential difference from a power supply
41
How do you work out the emf of multiple cells in parallel?
ETotal = E1 = E2 = E3
- Identical cells in parallel in a circuit, the total emf of the combination of cells is the same size as the emf one acc individual cells
- This is because the current will split equally between identical cells
- The charge only gains emf from the cells it travels through - so the overall emf in the circuit does not increase
42
What is Kirchhoff's second circuit law?
- In a closed circuit loop, the sum of the potential differences is equal to the sum of the electromotive forces
- Look at loops and remember that current is lazy and so takes the path of least resistance
43
What happens to resistance as temperature increases?
- The resistance of the metal when it is heated increases as the temperature increases
- The vibrating, positively charged, ion cores move around much more as the temperature increases, thus getting in the way of electron flow
- This opposes the flow of the gas of electrons moving through the structure
44
What is electric current?
The rate of flow of electric charge
45
How do you calculate resistance in series?
RT=R1+R2+R3....+Rn
46
What is the electric characteristic for a fixed resistor like?
- (For an ohmic conductor) Straight line through the origin
- When it is current on x axis and V on y axis, for components following Ohm's law then the gradient of the line is the same value as the resistance off the component
- High-resisatnce components have large, steep gradients and low resistance components have small, shallow gradients
- BUT watch out for which way axis is drawn
47
How are potential dividers used as sensors?
- Used in sensor circuits
- If the variable resistor R2 in the potential divider circuit is replaced with a component whose resistance varies with an external physical variable such as temperature or light intensity, then the pd across the fixed resistor, R1 will also vary with the external physical variable
- The voltmeter connected across R1 can then be calibrated in terms of the value of the external physical variable. The circuit can then be and e to act as an electronic thermometer or an electronic light sensor
48
What is a volt?
The potential difference across a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component
49
What is Ohm's law?
1. The current through an ohmic conductor is directly proportional to the potential difference across it, providing that physical conditions such as temperature remain constant
2. The current (I) flowing through the wire is directly proportional to the potential difference (V) across the wire, providing that the temperature (and other physical variables) remained constant
3. A special case where I alpha V under constant physical conditions
50
What is the potential divider equation?
- V1=emfR1 / (R1+R2)
- From this equation it can be seen that if epsilon and R1 are fixed then V1 only depends on R2. In fact as R2 increases, V1 decreases and vice versa
51
How do you work out the emf of multiple cells in series?
ETotal = E1 + E2 + E3...
- For cells in series in a. circuit, you can calculate the total emf of the cells by adding their individual emfs
- This makes sense if you think about it, because each change goes through each of the cells and so gains emf (electrical energy) form each one
52
Why are semi-conductor diodes different?
1. Components such as a fixed resistor or filament lamp have the same electrical characteristic independent of the direction of current flow and their V-I graphs produce the same shapes in forward and reverse bias
2. Semi-conductor diodes generally act as one-way gates, preventing current from flowing back through the circuit e.g. very useful in mains power supply where they can be used in circuits to convert ac into dc
53
How does pd and emf relate to the law of conservation of energy?
In a series circuit where the components are connected one after another in a complete loop, the total electrical energy per coulomb transferring into the circuit (the sum of the emfs in the circuit) must equal the energy per coulomb transferring into other forms of energy (the sum of the pds)
- LOOK AT THE EXAMPLE IN NOTES
1. The amount of electrical energy the battery produces for each coulomb of charge is called its emf
2. The potential difference across the load resistance (R) is the energy transferred when one coulomb of charge flows through the load resistance, This potential difference is called the terminal pd (V)
3. If there was no internal resistance, the terminal pd would be the same as the emf, however in real power supplied there is always some energy lost overcoming the internal resistance
4. The energy wasted per coloumb overcoming the internal resistance is called the lost volts
- Energy per coulomb supplied by the source = energy per colour transferred in load resistance + energy per colour wasted in internal resistance
54
What is a superconductor?
- Is a material whose resistance drops to zero below a specific temperature, called the critical temperature T little c
- Different superconducting materials have different critical temperatures (tungsten has lowest known)
55
What is resistance defined as?
1. The resistance of a conductor is the opposition of the conductor to electric current flowing through it
- Unit is the ohm (omega sign)
2. R=V/I
3. Component with very high resistances let very little current through them, and are considered to be electrical insulators
56
What is a thermistor? What is ntc?
1. Is component whose resistance varies with temperature (it is a semiconductor component)
2. Many thermistors used in electrical circuits have a negative temperature coefficient (ntc) which means that their resistance decreases with increasing temperature
3. The vast majority of ntc thermistors also have a non-linear response to changing temperature and so a graph of resistance against temperature is a curve
4. Increasing the current through the thermistor increase its temperature, the increasing gradient of this tells you that the resistance is decreasing (I on Y and V on X), the gradient of the V/I graph does the opposite
57
How does Kirchhoff relate to the law of conservation of energy?
- Kirchhoff's seconds circuit law applies to any circuit but in the case of parallel circuits, the circuit must be considered a succession of individual series cities with the same power supply
- This law is based on the conservation of energy: the energy per coulomb transferred to the charge by the battery (emf), is then transferred to other forms of energy by the charge as it flows through the circuit components
58
What happens to the emf in cells?
- When identical cells are connected in parallel to form a battery, the emf of the resultant battery is just the emf of the individual cells
- Connecting cells in parallel is generally not a good idea as they are rarely identical, and one cell will force current back into the other, causing damage to the cells
- Cells in parallel have a higher capacity for storing and transferring electrical energy than single cells, but it is better to just buy and use a bigger single cell
- Several cells joined in series or in parallel are called a battery
59
Describe superconductors
- Normally all materials have some resistivity, and so that resistance means that whenever electricity flows through them, they heat up and some of the electrical energy is wasted as thermal energy and you can lower the resistivity of many materials like metals by cooling them down
- If you cool some materials (like mercury) down to blow a 'transition temperature' their resistivity disappears entirely and they become a superconductor
- Without any resistance, none of the electrical energy is turned into heat, so none of its wasted. This means that you can start a current flowing in a circuit using a magnetic field, take a way the magenta and the current would carry on flowing forever
1. Most 'normal conductors' e.g. metals, have transition temperatures below 10 kelvin and getting things that cold is hard and expensive
2. They are trying to develop room-tmertaure superconductors and so fair a metal oxide tings to superconduct at about 140K which is much easier temperature to get down to
1. A current set up in a loop of superconducting material carries on flowing indefinitely
2. Superconductors also exclude magnetic fields inside them and this allows a strong permanent magnetic to be repelled and held above the superconductor
3. The ultimate goal is to develop superconductors with critical temperatures that are around room temperature
60
What are the equations with EMF and internal resistance?
- E is emf
- V is terminal pd
- v is lost volts
- I is current
- R is load reliance
- r is internal resistance
- E = V + v
- V = E-v
- Vm= E-Ir
- E = I(R + r)
61
How do modern digital ammeters work?
- They use an integrated circuit within the meter to measure the current, which is then displayed on a numerical display
- They are designed to have very low resistances and are calibrated to take into account the reduction of the current due to the resistance of the meter
62
What aspects affect resistance?
1. The resistance of the conductor increases with increasing length, as there are more positive ion cores in the way of the gas of electrons moving through the conductor. When l is doubled so is the R so l is proportional to R
- the longer the wire the more difficult it is to make current flow
2. The resistance of the conductor decreases with increasing cross sectional area A, as there are more conduction pathways through the conductor for the electrons to move through. If the A doubles then R halves so R is inversely proportional to the A
- the wider the wire the easier it will be for electrons to pass along it
3. Resistivity, which depends on the material. The structure may make it easy or diffract for charge to flow. In general, resistivity depends on the environmental factors as well, like temperature and light intensity
63
Why does the current through the lamp increase rapidly between 0 and 50ms before dropping to a steady value?
1. The metal in the filament lamp is initially cold, and this means that its resistance is low and a larger amount of current can flow through the filament
2. The filament heats up as more current passes through it and the resistance increases, gradually limiting the current to a maximum value
64
Write a suitable numbered method that would allow you to obtain accurate and reliable measurements of the resistance of the wire over s range of temperatures between 0 and 100 degrees
1. Set up apparatus with ammeter, cell, voltmeter across coiled metal wire which is in a water bath which has a thermometer inside it
2. Adjust temperature to 0 degrees Celsius using ice
3. Measure and record current pd temperature
4. Repeat this step
5. Repeat for 10 degrees Celsius temperature increments up to 100 degrees
6. Calculate values of R for each temperature using R=V/I
65
What is the critical temperature of an electrical conductor?
The temperature at and below which a material becomes superconducting
66
What is the experiment to determine if an electrical component is an ohmic conductor?
1. Adjust the power supply so that the voltmeter reads 1.0V
2. Measure and record pd, V and current I
3. Alter pd, V across the component in increments of 1.0V
4. Reverse bias of the component
5. Repeat steps 1-4, reaching values as negative
6. Calculate values of R=V/I, for each current, pd pair
7. If the component is 'ohmic' then the value of R will be very similar with no trend
67
Describe how the resistance of the filament lamp changes as the pd across it changes using the current voltage characteristic
-Resistance = V/I, as V increase so does I but proportionately less than the increase in V and this indicates that the rate R = V/I increases with V
68
Describe the method you could use to enable you to collect data to plot the IV curve of the semiconductor diode
1. Adjust the power supply so that the voltmeter reads 0.5V
2. Measure and record pd, V and current I
3. Alter pd, V across the component in increments of 0.5V up to 3.0V
4. Repeat 2
5. Reverse bias the component
6. Repeat 1 recoding the values as negative
69
What would happen to the voltage in a series circuit by adding a 10 ohm internal resistance battery
-Higher resistance, int he circuit will result in a smaller current and hence a lower value of V
70
What are superconductors?
- Superconductivity means a material has zero resistance
- Resistivity decreases with temperature
- Becomes superconducting when you reach the critical temperature
71
What happens to the resistance of the cable when the embedded filaments of wire are made superconducting?
The resistance decreases to zero
The copper still has deists cd
But this is in parallels with filaments which have zero resistance
And hence total resistance is zero
72
How would you determine resistivity of putty?
- Length with a ruler
- Thickness/diameter with vernier capillary
- Measure voltage
- Measure Current
- Calculate Resistance
- Use IV graph
- Use of diameter to calculate cross sectional area of precision so use vernier capillar
- Flat metal electrodes at each end to improve connection
73
How do you get an IV curve for a semiconductor?
- Connect circuit up
- Measure current and voltage
- Vary resistance
- Obtain a range of results
- Reverse connection to power supply and repeat
- Plot a graph of pd against current
74
If two currents have the same current what does that mean of their brightness?
They have the same brightness
Z A Level Physics
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