| What is an electric current? | The electric current is the electrons that flow out of the cell and around the circuit once the switch is closed, it flows from the negative end of the cell to the positive end. |
| What energy change/decrease takes place in the electric current of electrons? | When the electrons return to the positive end of the cell they are carrying less energy than when they left the negative end. |
| What is the potential difference? | The potential difference is the amount of energy transferred in an electrical circuit, and the potential difference Is measured with a voltmeter.
A potential difference of 1 volt = 1 joule of energy is transferred every coulomb of charge that is moving through the circuit |
| What are the potential difference measurements in a series circuit? | When two lamps are in series, both lamps will be dimmer than the single lamp in the parallel circuit as they share the total energy carried by the current, with values that add up to the potential difference across the cell; e.g. potential difference of cell: 8 and potential difference of lamps: 4 and 4, or potential difference of lamps may be: 3 and 5 |
| What are the potential difference measurements in a parallel circuit? | In a parallel circuit, the poetical difference across each component is equal (in which the voltmeter is clipped around the components when measuring potential difference, rather than being directly connected like in a series circuit)
However, a parallel circuit may have branches with two lamps in series, in which the first lamp would be equal to the cell’s/overall potential difference, while the second and third in series lamps would equal the overall potential difference when added together, just like in a series circuit |
| What is a battery? | A battery is two or more cells connected together, and the positive ends of the cells in a battery should be connected in the same direction (both positive ends facing left or right) |
| How does the potential difference change if the placement of the cells in a battery is wrong? | Cells facing the same direction will have a direct amount of potential difference from the cell, shared between them. But when cells in a battery is facd the wrong direction, the potential difference is cancelled out for the cell beside it |
| How is electrical charge measured? | Electrical charge is measured in coulombs, and its rate of flow is the same as the size of electric current/ 1 current of ampere = 1 coulomb of charge flowing per second |
| What is the equation for charge flow? | Q = I x t
Charge flow (c) = current (A) x time (s) |
| What is the equation for resistance? | R = V/I
Resistance (Ohms) = potential difference (V) / current (A) |
| What is the equation for current? | Current = potential difference / resistance |
| What happens to the current flowing through the resistor if the potential difference is increased? | While a potential difference of 0 volts gives current of 0, a potential difference of 10 volts gives a current of 0.1 amps, and 20 volts gives 0.2 amps and so on, therefore current through a resistor is directly proportional to the potential difference and the resistance is constant: it doesn’t change if we increase the current, this kind of resistor is called an ohmic conductor. |
| When will the resistance only stay constant? | The resistance will only stay constant if the temperature is constant |
| Describe why the resistance/current of a filament lamp isn’t directly proportional to the potential difference | The current through a filament lamp isn’t directly proportional to the potential difference because the filament gets hot causing the resistance to increase. At high temperatures the atoms in the filament vibrate more causing the electrons and the current to collide more with the atoms, so more energy is needed to push the current through the filament. |
| Why is a filament lamp not a ohmic conductor? | A filament lamp is not an ohmic conductor because it’s resistance is not constant as when the potential difference increases the current also increases |
| Give the symbol for a diode and LED | The circuit symbol of a diode is a circle with an arrow and a line on the arrows edge, while LEDs have the same symbol with two arrows emitting from it |
| What are diodes useful for? | Didoes are useful for controlling the flow of current in circuits because they only allow current to flow in one direction; no current can flow in the reverse direction but in the forward direction the current increases as the potential difference increases. |
| What are LEDs useful for? | LEDs give off light when current flows through them, they are useful because they are an energy efficient source of light |
| What does the term equivalent resistance describe? | The term equivalent resistance describes when two 50 ohm resistors could be replaced by a 100 ohm resistor because resistors in series add together as it can’t avoid any component |
| Describe the total resistance produced by two resistors in parallel | The total resistance of two resistors in parallel is less than the resistance of the series individual resistor, because with two resistors in parallel, there are two pathways for the current so more total current will flow through the circuit |
| What is the circuit symbol for a light-dependant resistor (LDR)? | The circuit symbol for a light-dependant resistor is a circle with a rectangle inside |
| How can an LDR be used to detect light? | An LDR detects light, such as in phones when the screen turns off to save battery during conversations when your phone in on your cheek, and it adjust the brightness if you enter a darker or brighter environment |
| Describe how does an LDR detects light? | In the light, the resistance of the LDR is very low so it takes very little energy for the current to pass through the LDR, therefore the potential difference across the LDR is very low.
Due to the potential difference being shared between components in series the potential difference across the lamp is large so the lamp lights up the screen.
When the phone is held to someone ear the LDR is in darkness so it’s resistance rises sharply and it takes a great deal of current to pass through the LDR so the potential difference across the LDR is very high.
While less energy is available for the lamp so its potential difference is very low making it very dim, turning the phone screen off. |
| What other devices can LDR’s be found in? | LDRs can be found in garden lamps which have a circuit designed to allow the light to turn on in the darkens and switch off in the light |
| What is the circuit symbol for a thermistor? | The circuit symbol for a thermistor is a rectangle with a hockey-stick shape going through |
| How does temperature affect thermistors? | The resistance of a thermistor decreases if the temperature increases which makes them a great thermostat as they turn appliances on or off in accord with their temperatures |
| How can is overheating of computers solved, describe the process thermistors through thermostats? | Computers can often overheat, to reduce this, computer usually have a fan; when the temperature of the computer rises the fan switches on and cool it back down, (in this series the thermistor is connected in series with a cooling fan)
Under cool conditions the resistance of the thermistors is high so it takes a lot of energy for the current to pass through it, therefore the potential difference across the thermistor is high and as the potential difference is shared between components in series, the fan has a small potential difference, which means the fan operates at a very low speed
While if the computer gets hot the resistance of the thermistor falls and less energy is need for the current to pass through the thermistor so the potential difference across the thermistor is very low so more electrical energy is available for the fan; making the potential difference across the fan very high so the fan powers up to high speed; cooling the computer back down. |
| What other devices use thermistors as thermostats? | Premature babies need to be kept warm so they are placed in incubators that have systems to detect faults, if the temperature falls its sensed by a thermistor, which causes a lamp to turn on and an alarm to ring/sound |
| How does the resistance base on the length of wire? | First attach a wire to a meter ruler using tape and connect the wire to the rest of the circuit using crocodile clips so only the part of the ire between the clips is connected to the circuit, and the clips can be moved further apart to increase the length of the wire in the circuit, now the length of the wire can be changed and its resistance in the circuit can be measured. |
| After drawing up a graph what should the results of the resistance practical look like? | After drawing up a graph the results of the resistance practical should look like a straight line passing through zero, which explains that the resistance of the wire is directly proportional to the length |
| Describe how to investigate the factor of zero error that affects the resistance of electrical circuits | The resistance practical has a small resistance when the length of the wire is zero, this called zero error which is a reading on the measuring instrument when the value should be zero; it’s a systematic error that can’t be reduced by carrying out repeats. In the case resistance, the zero error needs to be subtracted from all the readings.
This systematic error is caused by the equipment because it’s impossible to get the crocodile clip on the zero position on the ruler and there is also some resistance caused by the contact between the crocodile clip and the wire. |
| Describe how to investigate the factor of heating that affects the resistance of electrical circuits | A second problem with the resistance practical is heating effects: if the temperature of the wire increases the resistance will also increase which will affect results. Heating effects can be reduced by using a low potential difference which will keep the current low, reducing any heating in the wire. Secondly the current should only be turned on when taking a readying, it should be turned off between readings which will also reduce heating effects |
| What is the circuit symbol of a variable resistor? | The circuit symbol of a variable resistor is a rectangle with an arrow going diagonally through it, pointing north. |
| What does a variable resistor do? | A variable resistor increases the length of the wire in the circuit to increase the resistance via an adjustable wipe that slides on the coil of wire.
A variable resistor can be used to control the potential difference across a lamp. By increasing the resistance that would make the lamp dimmer. |
| Describe the current/PD characteristic of a resistor practical | In the PD characteristic of a resistor practical there is a battery connected to a resistor in series with an ammeter and a variable resistor, and a voltmeter is in parallel across the resistor.
Firstly, use the voltmeter to read the potential difference across the resistor then use the ammeter to read the current through the resistor and record these values in a table.
Then adjust the variable resistor and record the new readings on the voltmeter and ammeter, do this several times to get a range of readings.
Now switch the director of the battery so the direction of the potential difference is reversed and both the voltmeter and ammeter have negative values.
Continue to take several readings of the potential difference and current. |
| Describe what data should be drawn up at the end of the current/PD characteristic of a resistor practical | After the current/PD characteristic of a resistor practical plot a graph of the current against the potential difference. The results should be a straight line passing through zero meaning the current us directly proportional to the potential difference.
And if the directions of the current or potential difference are switched their results will still pass through zero.
However, if the temperature of the resistor is not kept constant this experiment will no longer be directly proportional, so it is important to switch the circuit off between readings. |
| Describe current/PD characteristics of a filament lamp practical? | Just like in the resistor practical, adjust the variable resistor and read the potential difference and current, then do this for a range of values with the battery in the forward and reverse direction.
Plot a graph of the current against the potential difference, the results should be a straight line passing through zero with a curve at the beginning and the end of the line, this is because when the current increases the temperature of the filament lamp increases. An increased temperature that causes the resistance to increase.
And the same shaped graph will occurs when the direction of the potential difference is changed. |
| Describe the current/PD characteristics of a diode practical | The circuit for the current/PD characteristics of a diode has the same components as the circuit for the resistor, however there is an extra resistor because diodes are very easily damaged by high current so the extra resistor keeps the current low to protect the diode.
As the current will be low, an extra sensitive ammeter is needed: a milliammeter. Now adjust the variable resistor a number of times while recording the potential difference and the current, with the battery in the forward and reverse direction.
Now plot a graph of the current against the potential difference, in the case of a diode, there is only a current when the potential difference is around 06-0.7 volts and as the potential difference increases beyond this point the current rises sharply. Moreover, with a diode there is no current when the potential difference is reversed because in the reverse direction a diode has a high resistance |
| What is the power of components equation? | P = V x I
Power (W) = potential difference (V) x current (A) |
| How can power be calculated when given then current through a component and resistance across it? | When given the current through a component and resistance across it, power can be calculated using the equation:
Power (W) = current squared (A) x resistance (ohms) |
| Define what DC and AC are | The current from a cell is a direct current (DC), the electrons only ever travel in one direction.
Mains electricity in the UK is an alternating current (AC), in AC the current is constantly changing direction |
| What is the benefit of AC? | The main benefit of an alternating current is that it’s very easy to use a transformer to increase or decrease the potential difference compared of the alternating current, then it is to increase or decrease the potential difference of a direct current |
| What is the frequency and potential difference of mains electricity? | In the UK AC has a frequency of 50 hertz, it has a potential difference of around 230 volts |
| What do scientists use to see the pattern of an electrical current? | Scientists can use an oscilloscope which shows an alternating current with a potential difference that rises and falls showing the current is going backwards and forwards. The height of the peak is the maximum potential difference. Oscilloscopes can also be used to display direct currents, which do not have a changing poetical difference |
| How can an oscilloscope be used to calculate the frequency of an alternating current? | In an AC from one peak to the next peak, the current has changed direction and change again so the time from one peak to the next peak is one complete cycle, the time measured between the two peaks is the time it takes for the current to change direction and change back.
As the frequency is the number of cycles in one second, it can be calculated by dividing one by the time intervals of the first and second peak. |
| How are electrical appliances connected to the mains supply in the UK? | In the UK electrical appliances are connected to the mains supply using a three-core cable, which consists of three wires that are all made of copper which is a good conductor for electricity and the coatings are made of plastic which isn’t’ an electrical conductor and therefore makes it safe for use |
| What is the function of the earth wire? | The earth wire is green and yellow and it’s a safety wire that stops the appliance from becoming live |
| What is the function of a live wire? | The live wire is brown, it causes the alternating potential difference from the supply (usually around 230 Volts) and it’s connected to a fuse in the plug |
| What is the function of the neutral wire? | The neutral wire is blue, it completes the circuit with the live wire and it has a potential difference of 0 Volts |
| What’s the circuit symbol for a fuse? | The circuit symbol for a fuse is a rectangle with a horizontal line going through it |
| What makes electrical appliances dangerous? | The live wire is a plug can be fatal if touched; because the live wire is connected to the earth and the poetical difference of the earth is 0 V, so if someone were to touch the live wire a current would flow through them into the earth causing electrocution. |
| Give an example of an electrical appliance which can cause electrocution | Appliances with a metal case can be dangerous; if the live wire becomes lose and touches the metal case, the case can become live/it will contract a poetical difference of 230 volts, which could give someone an electric shock if the touch the live case.
The metal case is attached to the live wire and the live wire is connected into the ground with a metal rod so if the case becomes live a huge current flows the earth, the fuse metals and shuts down the current preventing someone from getting an electrical shock from touching the case |
| How is electricity distributed by the National Gird? | The nation grid consists of a system of transformers and high-voltage cables; it distributes electricity as it connects the power station to homes (and only the transformers in the cables are called the National Gird) |
| What is the main problem with sending electricity to homes through the National Gird? | The main problem with sending electricity to homes is that energy is always lost in the power cables due to the resistance of the wires and the bigger the distance between the power stations and the homes; the greater the energy loss |
| How can the problem of energy loss of the National Gird be prevented? | To reduce the energy loss of the National Gird power stations can be build nearer to homes or through the problem can be solved through the use of transformers. |
| State how transformers can be used to reduce energy loss and in doing so state the uses of step-up and step-down transformers | Transformers can be used to reduce energy loss through the following process; first electricity passes through step-up transformers which increase the potential difference to several hundred thousand volts because less energy is lost in the power cables when the potential difference is very high.
Now the electricity passes through high-voltage cables which cannot yet be passed to homes as the potential difference is too high, so finally the electrical passes through step-down transformers which reduce the poetical difference to around 230 Volts, then the electricity is passed to homes |
| What data should be drawn up after the current/PD characteristic of a resistor practical? | After the current/PD characteristic of a resistor practical plot a graph of the current against the potential difference. The results should be a straight line passing through zero meaning the current us directly proportional to the potential difference.
Although if the directions of the current or potential difference are switched their results will still pass through zero.
Hworver, if the temperature of the resistor is not kept constant this experiment will no longer be directly proportional, so it is important to switch the circuit off between readings. |
