1
Q
What is potential difference?
A
- To make a current flow in a circuit you need a potential difference (p.d.)
- Inside a cell or battery there are substances that react to seperate the changes
- This makes one side of the battery positively charged and the other side negatively charged
- The positive terminal of the cell is at a higher electrical potential than the negative terminal
- The potential difference, and the current that a cell can deliver, depends on the construction of the cell, not it’s size
- Potential difference is measured in volts (V)
- The longer line on the symbol for a cell is positive, and the shorter line is negative
2
Q
How can you set up an electric field in a wire?
A
- By applying a p.d. between the ends of a piece of wire
- The field is set up very quickly (close to the speed of light in a vacuum), so the charged electrons start to move straight away
3
Q
What can you measure potential difference with?
A
A voltmeter
4
Q
What is the link between potential difference, energy and charge? (Equation)
A
Energy transferred (J) = potential difference (V) x charge (C)
5
Q
How do you measure current in series circuits?
A
- A series circuit has only one loop, and the current is the same everywhere
- You can measure the current in a circuit with an ammeter
6
Q
How do you measure potential difference in series circuits?
A
- A voltmeter measures the potential difference
- You need to connect the voltmeter to both sides of a component
- A voltmeter measures the ‘rise in potential’ across a cell or battery
- It can also measure the ‘drop in potential’ across a component
- You end up at 0V, so the readings on the voltmeters always add up to the reading across the cell
7
Q
What happens in parallel circuits?
A
- In a parallel circuit, each loop can be worked independently
- If you measure the current at different points in a parallel circuit, the currents in the loops add up to the current near the battery
8
Q
What does the current depend on?
A
The current in a circuit depends on the potential difference of the cell or battery and the resistance (ohms) of the components
9
Q
What is the equation for potential difference, current and resistance?
A
Potential difference (V) = current (A) x resistance (ohms)
10
Q
Why does the resistance of some components change?
A
- A metal is made up of positively charges ions arranged in a regular pattern
- The ions are formed when electrons leave the outer shells of metal atoms
- These delocalised electrons are free to move through the structure of the metal
- Resistance is produced when electrons collide with the ions in the lattice
11
Q
What is variable resistor?
A
A circuit component you can use to change the amount of wire or other resisting material
12
Q
What is a characteristic graph?
A
A graph of current against potential difference
13
Q
How do you get measurements for a characteristic graph?
A
- By taking measurements of the current flowing through a component for different potential differences
- Then reverse the battery or power supply connections and repeat the measurements
14
Q
What can you do with current and potential difference measurements?
A
Calculate resistance for each pair of measurements and plot a characteristic graph
15
Q
What is the characteristic graph for a resistance wire or a resistor?
A
- The wire is a linear circuit element
- Its resistance does not change as you change the potential difference
16
Q
What can you use if you need the resistance in a circuit to be constant?
A
A resistance wire or a resistor
17
Q
What is Ohm’s Law?
A
The current is proportional to the potential difference if the temperature does not change
18
Q
What is the characteristic graph for a lamp and why?
A
- If the wire gets hot then the resistance varies and the graph is not a straight line
- For a non-linear circuit element the resistance is not constant
- The electrons in a wire collide with the ions in the wire
- The ions vibrate more as the wire gets hotter
- As that happens there are more collisions
- The wire heats up more
- The current increases as the potential difference increases, but at a slower rate
19
Q
What is a diode?
A
- A diode is a component that only allows a current to flow one way
- Some diodes emit light
- There are LEDs in most of the electronic devices that you use
- The long leg of LEDs should be connected to the positive terminal of a battery
20
Q
What does a diode’s characteristic graph look like?
A
- As you apply a potential difference in the ‘forward’ direction, very little current flows
- Then the current suddenly increases
- If you reverse the potential difference there is no current
- You can use a diode if you want the current to flow only in one direction
21
Q
What is a thermistor?
A
- At the end of many digital thermometers is a thermistor
- The resistance of a thermistor depends on temperature
- Thermistors come in different shapes and sizes, and detect different ranges of temperatures
- A thermistor is not like a piece of wire
- It is made of a semiconducting material
- Electrons in the atoms of a semiconductor do not need much energy to escape from the atom to from the current
22
Q
How does the resistance of a thermistor change with temperature?
A
- The resistance of the thermistor can be measured by measuring the current through it and the potential difference across it
- If you put a thermistor in a beaker of cold water and heat the water up, the resistance of thermistor decreases
- As you heat the thermistor many electrons gain enough energy to escape from the atoms in the semiconductor
23
Q
For a certain potential difference: (and why?)
A
- At low temperatures, the current is small, so the resistance of the thermistor is high
- At higher temperatures, the current is high, so the resistance of the thermistor is low
- If there are more electrons moving per second for a particular potential difference, the current will be bigger
- A bigger current for the same potential difference means the resistance is smaller
24
Q
What are some uses of thermistors in your house?
A
They are used to monitor the temperature in ovens and refrigerators, and help to prevent your laptop or tablet from overheating
25
What is an LDR?
- The resistance of light-dependent resistor (LDR) changes with light intensity
- It is a circuit component that is also made of a semiconducting material
- In the LDR, light causes electrons to be released into the circuit to increase the current
26
How does the resistance of an LDR change with light intensity?
- As the intensity of the light increases, more electrons are released in the semiconductor and the resistance decreases
- (The shape of the graph for an LDR is the same as the shape of the graph for a thermistor)
27
What is a use of LDRs?
Automatically control lights in a building
28
What would happen if you changed the components, or the arrangement of the components in a circuit?
The current and net resistance would change
29
What would happen if you add another identical lamp in a series circuit?
The current decreases, and the net resistance increases
30
What would happen if you add another identical lamp in a parallel circuit?
The total current increases, and the net resistance decreases
31
What do you know about a series circuit?
- The current is the same everywhere
- The p.d.s across each component add up to the p.d. across the battery
32
What do you know about a parallel circuit?
- The p.d. is the same across each loop
- The current in each loop adds up to the current near the battery
33
How can you measure temperature with a circuit?
- Thermistors can be used in sensing circuits to produce a potential difference that changes with temperature
- The output potential difference will depend on the potential difference of the battery and the magnitude of each resistor
- V(out), as measured by a voltmeter, will depend on the temperature
34
How do you make other measurements of the environment?
- Circuits measure temperature because it contains a thermistor
- If you replaced the thermistor with a different component then you could monitor a different quantity
35
What do thermistors, LDRs, variable resistors, and pressure sensors measure?
Temperature, light level, how much it turns, the force applied to it
36
What is power?
- The energy transferred per second
- Power is measured in watts (W)
- Large powers are measured in kilowatts (kW) or megawatts (MW)
- You might choose a higher-power appliance because you can use it for a shorter time to get a job done
37
What is current?
- In an electrical circuit, the current is the rate of flow of charge, it tells you the amount of charge flowing through a component per second
- Current = charge / time
38
What is potential difference?
- In an electrical circuit, the potential difference tells you the energy transferred by each charge
- Potential difference = energy transferred / charge = current x resistance
39
What is the equation for power?
Power (W) = potential difference (V) x current (A)
40
What is the equation for energy transferred?
Energy transferred (J kWh) = power (W) x time (s)
41
How is power related to current and resistance?
Power = I^2 x R = current squared x resistance
Physics
flashcards
Decks in class (16)
# Cards
-
P1.1 The particle model6
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P1.2 Changes of state17
-
P1.3 Pressure16
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P2.1 Motion21
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P2.2 Newton's Laws45
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P2.3 Forces in action30
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P2 (Extra)17
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P3.1 Static and charge14
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P3.2 Simple circuits41
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P4.1 Magnets and magnetic fields9
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P4.2 Uses of magnetism28
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P5.1 Wave behaviour14
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P5.2 The electromagnetic spectrum10
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P5.3 Wave interaction0
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P6.1 Radioactive emissions17
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P6.2 Uses and hazards3
