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P2 Flashcards

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1
Q

What is needed for electrical charge to flow through a closed circuit?

A

For electrical charge to flow through a closed circuit the circuit must include a source of potential difference.

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2
Q

Define electrical current

A

Electrical current is a flow of electrical charge.

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3
Q

How does electrical current transfer energy?

A

Electric current flows from the negative end of the cell to the positive end.
Electrons are carrying energy from the cell and pass this energy to the components, where it is transferred from electrical energy to other forms of energy (e.g. thermal energy).
When the electrons return to the positive end of the cell, they are carrying less energy than when they left the negative end.

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4
Q

How do you measure current?

A

Ammeter to measure the current.

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5
Q

How to calculate charge flow?

A

Charge flow = current x time
Q (C) = I (A) t (s)

A current of 1 ampere = 1 coulomb of charge flowing per second.
The size of electric current is the rate of flow of electrical charge.

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6
Q

What does the current through a component depend on?

A

The current (l) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component.

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7
Q

What does the size of the current depend on?

A

The size of the current depends on the total potential difference of the cells and the total resistance of the circuit.

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8
Q

Define resistance

A

The resistance tells us the potential difference required to drive a current through a component.

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9
Q

What does a greater resistance mean?

A

The greater the resistance of the component, the smaller the current for a given potential difference across the component.

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10
Q

Define potential difference

A

Potential difference is the driving force that pushes the charge around. Tells us how energy is transferred in an electrical circuit.

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11
Q

What does a potential difference of 1 volt tell us?

A

A potential difference of 1 volt tells us that 1 joule of energy is transferred for each coulomb of charge that is moving through the circuit.

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12
Q

How to measure potential difference?

A

Voltmeter to measure potential difference.

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13
Q

How to calculate potential difference?

A

Potential difference = current x resistance

V (V) = I (A) R (Ω)

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14
Q

How to calculate resistance?

A

Resistance = potential difference / current

R (Ω) = V (V) / I (A)

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15
Q

8 steps to investigate how the length of the wire at a constant temperature affects the resistance

A
  1. Attach a crocodile clip to the wire level with 0cm on the ruler.
  2. Attach the second crocodile clip at the preferred length of the wire, e.g. 10cm. Only the wire that is between the 2 crocodile clips is connected to the circuit.
  3. Measure the current across the circuit and the potential difference across the wire.
  4. Move the crocodile clip along the wire, e.g. another 10cm.
  5. Record the new length, current and potential difference.
  6. Repeat this for a number of difference lengths of the test wire.
  7. Use measurements to calculate the resistance for each length of wire.
  8. Plot a graph of resistance against wire length.
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16
Q

How does the length of the wire affect the resistance of an electric circuit?

A

The resistance of the wire is directly proportional to the length. The longer the wire, the greater the resistance.

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17
Q

What 3 errors could we get when investigating how the length of the wire affects the resistance?

A

Systematic error (zero error) - the first clip is not attached at exactly 0cm so all the length readings are a bit off.

Resistance caused by the contact between the crocodile clip and the wire.

Temperature of the wire increasing the increase the resistance - to prevent this only turn on the current when taking a reading.

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18
Q

Function of a variable resistor

A

The variable resistor changes the length of the wire that the current runs through so allows us to increase the resistance and change the potential difference across the component.

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19
Q

6 steps to investigate combinations of resistors in series

A
  1. Build a circuit of a battery connected to a resistor is series with an ammeter.
  2. Make a note of the potential difference of the battery and measure the current. Use this to calculate the resistance.
  3. Add another resistor in series with the first.
  4. Measure the current and calculate the overall resistance of the circuit.
  5. Repeat steps 3 and 4 until all the resistors have been added.
  6. Plot a graph of the number of resistors against the total resistance of the circuit.
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20
Q

What happens as you add more resistors in a series circuit?

A

The resistance of the circuit is directly proportional to the number of identical resistors.
Adding resistors in series increases the total resistance of the circuit and decreases the total current through the circuit.

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21
Q

6 steps to investigate combinations of resistors in parallel

A
  1. Build the same initial circuit as the last experiment so it is a fair test (a battery connected to a resistor in series with an ammeter).
  2. Make a note of the potential difference of the battery and measure the current. Use this to calculate the resistance.
  3. Add another resistor in parallel to the first.
  4. Measure the total current through the circuit and calculate the overall resistance of the circuit.
  5. Repeat steps 3 and 4 until all the resistors have been added.
  6. Plot a graph of the number of resistors against the total resistance of the circuit.
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22
Q

What happens as you add more resistors in a parallel circuit?

A

When you add resistors, the total current through the circuit increases so the total resistance of the circuit decreases. The more resistors you add, the smaller the overall resistance becomes.

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23
Q

Function of a resistor

A

Adds resistance into a circuit and controls the potential difference across other components.

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24
Q

I-V characteristic of an ohmic conductor

A

The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor.
This means that the resistance remains constant as the current changes. Constant resistance.

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25
Define filament
Filament is a very fine, tightly-coiled wire.
26
Why is the current through a filament lamp not directly proportional to the potential difference?
The filament gets hot, which causes the resistance to increase. At high temperatures, the atoms in the filament vibrate more. The electrons in the current now collide more with the atoms. More energy is needed to push the current through the filament. Therefore, as the potential difference increases, the current no longer increases as much.
27
Define diode
A diode is a component which only allows current to flow in one direction only. This is because it has a really high resistance in the reverse direction. Arrow of the diode shows the direction that the conventional current must flow in.
28
I-V characteristic of a diode
The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction. There is only a current when the potential difference is around 0.6-0.7 volts.
29
What are diodes useful for?
Diodes are extremely useful for controlling the flow of current in circuits.
30
Define LED
Light-emitting diode which gives off light when a current flows through.
31
Define thermistor
Thermistors are used as temperature sensors.
32
What happens to the resistance of a thermistor as the temperature increases?
The resistance of the thermistor is low. It takes very little energy for the current to pass through the thermistor. This makes the potential difference across the thermistor low. This makes the other components in the series have a high potential difference.
33
What happens to the resistance of a thermistor as the temperature decreases?
The resistance of the thermistor is high. It takes a lot of energy for the current to pass through the thermistor. This makes the potential difference across the thermistor high. This makes the other components in the series have a low potential difference.
34
3 examples of uses of thermistors
Cooling fan in computers, incubator for premature babies, thermostats
35
Define LDR
Light-depedent resistor - used to detect light levels.
36
What happens to the resistance of an LDR as the light intensity increases?
The resistance of the LDR is very low. It takes very little energy for the current to pass through the LDR. This makes the potential difference across the LDR low. This makes the other components in the series have a high potential difference.
37
What happens to the resistance of an LDR as the light intensity decreases?
The resistance of the LDR is very high. It takes a lot of energy for the current to pass through the LDR. This makes the potential difference across the LDR high. This makes the other components in the series have a low potential difference.
38
2 examples of uses of LDRs
Automatic night lights (garden lamps), phone screen turning off when held by ear on call.
39
7 steps to investigate the current/voltage characteristics of a resistor or filament lamp
1. Build a circuit of a battery connected by wires to a resistor. The resistor is in series with an ammeter and a variable resistor. Voltmeter in parallel across the resistor. 2. Use the voltmeter to read the potential difference across the resistor. 3. Use the ammeter to read the current through the resistor. Record these values in a table. 4. Adjust the variable resistor and record the new readings on the voltmeter and ammeter and record the new readings on the voltmeter and ammeter. 5. Repeat several times to get a range of readings. 6. Switch the direction of the battery so that the current flows in the opposite direction. The voltmeter and the ammeter should now have negative values. 7. Plot a graph of the current against the potential difference.
40
How to investigate the current/voltage characteristics of a diode
Repeat the experiment with a diode instead. Add an extra resistor because diodes are easily damaged by a high current. A sensitive (a milliammeter) is needed as the current will be low.
41
Define series circuit
A series circuit has no branches and the current can only flow in one path and is the same all the way around.
42
Define parallel circuit
A parallel circuit has branches and the current in the branches adds up to the total current leaving the cell.
43
Describe current in a series circuit
There is the same current through each component. | I1 = I2
44
Describe current in a parallel circuit
The total current through the whole circuit is the sum of the currents through the separate components. I total = I1 + I2
45
Describe potential difference in a series circuit
The total potential difference of the power supply is shared between the components. V total = V1 + V2
46
Describe potential difference in a parallel circuit
The potential difference across each component is the same. | V1 = V2
47
Describe resistance in a series circuit
The total resistance of 2 components is the sum of the resistance of each component. R total = R1 + R2
48
Describe resistance in a parallel circuit
The total resistance of 2 resistors is less than the resistance of the smallest individual resistor.
49
Why is the total resistance of 2 components the sum of their resistance in series circuits?
By adding a resistor in series, the 2 resistors have to share the total potential difference. The same voltage is being used to push current through a harder current, therefore less current travels and the resistance increases.
50
Why is the total of 2 resistors in parallel less than the resistance of the smallest individual resistor?
There are more pathways for the current to take - more total current will flow through the circuit. An increase in current means a decrease in resistance.
51
Define direct current (DC)
Direct current (DC) is the current from a cell. It is created from a direct voltage and is always flowing in the same direction.
52
Define alternating current (AC)
``` Alternating current (AC) is a current that is constanting changing direction. The flow of electrons moves in the forward and then reserve directions. This is produced by alternating voltages in which the positive and negative ends keep alternating. ```
53
What current is mains electricity in the UK and why is it used?
Alternating current, it is easy to use a transformer to increase or decrease the potential difference.
54
What is the frequency and potential difference of AC in the UK?
50 hertz (Hz) - current switches direction 50 times a second, 230 volts
55
Function of an oscilloscope
An oscilloscope is used to see the pattern of an electrical current.
56
How to calculate the frequency of an alternating current?
Divide 1 by the difference in time between 2 peaks on an oscilloscope graph.
57
How are electrical appliances connected to the main supply?
Electrical appliances are usually connected to the main supply using three-core cable.
58
What are the three-core cables made from?
The wires are made of copper which is a good conductor of electricity. Has a plastic coating which does not conduct electricity. The insulating covering each wire is colour coded for easy identification.
59
Function of the live wire
Brown - carries the alternating potential difference from the supply (230V), connected to a fuse in the plug.
60
Function of the neutral wire
Blue - completes the wire, around 0V.
61
Function of the earth wire
Green/yellow striped - safety wire to stop the appliances from becoming live, 0V.
62
Potential difference of the Earth (ground)?
0V
63
What would happen if someone touched the live wire?
If someone touched the live wire, a current would flow through the person into the Earth and they would be electrocuted.
64
How does the earth wire prevent anyone from getting an electric shock from touching the case of an appliance?
The metal case is attached to the Earth wire which is connected to the ground with a metal rod. If the case becomes live, a huge current flows to the Earth. The fuse melts and shuts off the current.
65
How to calculate power using the potential difference?
Power = potential difference x current | P (W) = V (V) I (A)
66
How to calculate power using the resistance?
Power = (current)^2 x resistance | P (W) = I^2 (A) R (Ω)
67
How does the potential difference and current affect power?
The greater the potential difference and current, the greater the power.
68
How is work done in a current?
Work is done when charge flows in a circuit.
69
What does the amount of energy an appliance transfers depend on?
The amount of energy an appliance transfers depends on how long the appliance is switched on for and the power of the appliance. The power is the energy that is transferred per second.
70
How to calculate energy transferred using power and time?
Energy transferred = power x time | E (J) = P (W) t (s)
71
How to calculate energy transferred using charge flow and potential difference?
Energy transferred = charge flow x potential difference | E (J) = Q (C) V (V)
72
Examples of domestic appliances which transfer electrical energy to kinetic energy of the electric motors and their main purpose
Blender and fan, purpose is movement
73
Examples of domestic appliances which transfer electrical energy to thermal energy and their main purpose
Iron and kettle, purpose is heat
74
Examples of domestic appliances which transfer electrical energy to kinetic energy of the electric motors and thermal energy and their main purpose
Hair dryer and washing machine, kinetic energy for the motor and thermal energy to heat the air or water.
75
If a fan has a power rating of 20 watts, how much energy is transferred?
If a fan has a power rating of 20 watts, it transfers 20 joules of energy per second.
76
Which appliances have a high power rating?
Appliances which are designed to generate thermal energy usually have a much higher power rating than appliances designed to generate kinetic energy.
77
Define the National Grid
The National Grid is a system of cables and transformers linking power stations to consumers.
78
How do you transmit huge amounts of power?
To transmit huge amounts of power, there should be either a high potential difference or a high current (power = potential difference x current).
79
Why is it not effecient to use a high current?
When a high current is used, energy is always lost in the power cables due to the resistance of the wires. The wires heat up and energy is transferred to the thermal energy store of the surroundings. The greater the distance between the power station and the homes, the greater the energy loss.
80
How is energy loss reduced by using transformers?
The electricity passes through step-up transformers which increases the potential difference to 400,000V - less energy is lost in the transmission cables when the potential difference is very high. The electricity then passes through step-down transformers which reduces the potential difference to around 230V before the electricity passes to homes.
81
Why is the National Grid system an efficient way of transferring energy?
The National Grid distributes electricity at a high potential difference and a low current. A high potential difference means that it can distribute lots of power per second - power = potential difference x current. Using a low current reduces energy losses as the wires are not heat up as much and less energy is transferred to the thermal energy store of the surroundings.
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