Electricity Flashcards
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
Q
How is resistivity dependent on temperature?
A
- The resistivity of metals increases with increasing temperature
- The resistivity of many semiconductors, such as silicon and germanium, decreases with increasing temperature
- 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
Q
What do potential dividers consist of? How are they connected?
A
- A power supply (such as a cell)
- A fixed resistor
- 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
Q
What is the equation for resistivity?
A
rho=RA/L
28
Q
What does the resistivity of a material depend on?
A
- 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
Q
How would you find the resistivity of a wire?
A
- 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
