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61 Cards in this Set
- Front
- Back
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Static electricity |
Static electricity is caused when charges aren't free to move, and so build up in one place. When they finally do move, it leads to sparks or shocks. |
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Build up of static |
When two insulating materials are rubbed together, electrons are scraped off one and transferred to the other. Electrons are negatively charged. This leads to positive static on the material with a lack of electrons, and negative static on the material where the electrons have been gained. |
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Insulating materials causing static |
Which way the electrons are transferred depends on the two materials involved. The classic examples are polythene and acetate rods being rubbed with a cloth duster. With polythene, electrons move from the duster to the rod. With acetate, the electrons go from the rod to the duster. |
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Moving charges |
It's only ever the negative electrons that move from material to material. When electrons are removed from particles the particles are left positively charged ions. Both positive and negative electrostatic charges are only ever produced by the movement of electrons- the negatively charged particles. The positive charges never move. |
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Charges and attraction |
Like two things with opposite charges are attracted to eachother, two things with the same electric charge will repel eachother. When the electrons are transferred between insulating materials, the one that becomes negatively charged tries to repel eachother, but can't move apart because of their fixed positions. The patch of charge that results is called static electricity. |
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Effects of static electricity |
Static electricity is responsible for some annoyances, such as: attracting dust particles which are attracted easily to anything that is charged, clinging clothes made of synthetic materials, and bad hair days when your strands of hair becomes charged and try to repel eachother so stand on edge |
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Electric current |
Electric current is a flow of charge around a circuit. Moving charge is called a current. In an electrical circuit the metal conductors are full of charges (electrons) that are free to move. Electric charge is therefore able to flow in metal conductors. Current can't flow in an insulator like plastic because there are few charges free to move.. |
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Electric circuits |
The circuit must be complete in order for current to flow- the loop between one side of battery and the other is continuous. In a complete circuit, the battery pushes the free charges through the wires. The charge flows all the way round the circuit and back to the battery |
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Current, voltage and resistance |
Current will only flow through a component if there is voltage across it. The unit for current is amperes (amps, A). Voltage (V) is the driving force that pushes the current round. It's units are volts. Resistance (R) is caused by things in the circuit, such as components, that resist the flow of charge (slows the charge down) it's units are ohms. |
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Relationship between current, voltage and resistance |
Voltage is trying to push the current round the circuit, and the resistance is opposing it. The relative sizes of voltage and resistance decide how big the current will be. The bigger the voltage, the more current will flow. The bigger the resistance, the less current will flow. |
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Power |
Power, measured in Watts or kilowatts is the rate of energy transfer. Anything supplying electricity is also supplying energy. Power supplies, such as batteries, transfer energy to the charge, which then transfers it to the components and sometimes their surroundings. Work is also done because energy is transferred. Power is the rate at which an electrical power supply transfers energy to an appliance. |
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Power ratings |
An appliance with a high power rating transfers a lot of energy in a short amount of time. This energy comes from the current flowing through it. An appliance with a high power rating will use a large current. The formula for electrical power is: power (W) = voltage (V) x current (A). Most electrical goods show their power rating. To work out the current they will draw, rearrange the equation: A = W÷V |
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Electrical circuit symbols |
You need to know the symbols for: cell, battery, power supply, switch open, switch closed, filament lamp, fixed resistor, variable resistor, ammeter, voltmeter, thermistor and light dependant resistors (LDRs) |
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Test circuits |
This is the very basic circuit used for testing components. The component, the ammeter and the variable resistor are all in series, meaning they they are put in any order in the main circuit. The ammeter needs to be placed in series with the component to measure the flow of current through it. The voltmeter can only be placed in parallel around the component under test. Varying the variable resistor alters the current through the circuit. |
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Voltage |
A voltmeter measures potential difference between 2 points. Potential difference (voltage) tells us how much energy is tranferred to or from each unit of charge as it moves between 2 points. The battery transfers energy to the charge as it passes- the "push" that moves the charge around the circuit. The voltage of a battery shows how much work the battery will do to charge that passes through it- How big of a push it gives it. |
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Voltmeters |
Components tranferred energy away from the charge as it passes, eg converts it into light and heat. When energy is transferred, work is done. So potential difference is also a measure of the work done on or by a charge as it passes between 2 points. A voltmeter is used to measure the potential difference between 2 points. It MUST BE placed in parallel with a component so that it can compare the energy of the charge before and after passing through the component. |
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Resistance in a voltage current graph |
Remember: resistance resists the flow of current. Voltage current graphs show how the current in a circuit varies as you change the voltage. The current through a component is proportional to the voltage across it when the resistance stays constant. Different resistors have different resistances- the steeper the slope the LOWER the resistance. The wires in an electric circuit have such a small resistance thay it's usually ignored. |
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Calculating resistance |
At a constant temperature the resistance of a component is steady and is equal to the inverse of the gradient of the line of a voltage current graph. Resistance= 1/gradient. The steeper the graph therefore, the lower the resistance. Alternatively, resistance can be calculated by the formula: resistance (ohms) = voltage (V) / current (A) |
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Resistors and heat |
Resistors get hot when current passes through them. When electrons move through a resistor, they collide with positive ions in the resistor. These collisions make the ions vibrate more, which causes an increase in temperature. A filament lamp contains a piece of wire with a really high resistance, so when a current passes through it, the temperature increases so much that it glows- causing the lamp to light up. |
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Resistance increases and decreases |
Increased resistance in a component is caused by either a really thin wire, or a long wire. Resistance increases in proportion to the thickness or length of a wire. This is because the thinner the wire, the more positive ions the electrons have to move through, whereas if it's thicker it doesn't have to pass through as many. The longer the wire, again the more positive ions the electrons have to collide with. |
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Light dependant resistors |
An LDR is a special type of resistor that changes it's resistance depending on how much light falls on it. In bright light, the resistance FALLS. In darkness, the resistance is HIGHEST, so the component will get hotter, and therefore will give out more light. |
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Thermistors |
Thermistors are temperature dependant resistors. It's resistance depends on temperature. In HOT conditions, the resistance DROPS. In cool conditions, the resistances INCREASES, meaning that the component will get hotter. Thermistors make useful thermostats. |
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Semiconductor devices |
Thermistors and LDRs are examples of semi conductors. Semiconductors are insulators at room temperatures but can conduct an electric current at high temperatures. Electrons absorb the extra energy and become free to move, so can then conduct an electric current. |
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Series circuits |
In a series circuit, different components are connected in a line, end to end, between the positive and negative end of the power supply (except for voltmeters, which are always connected in parallel) |
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Voltage in a series circuit |
In a series circuit, the total potential difference of the supply is shared between the various components. So the potential differences around the series circuit will always equal the potential difference of the battery. This is because the total work done on the charge by the battery must equal the total work done by the charge on the components. Total V= V1+V2+V3. |
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Current in a series circuit |
Current in a series circuit is the same everywhere. The same current flows through all parts of the circuit. A1=A2=A3. The size of the current is determined by the total PD of the cells and the total resistance of the circuit. A= V/R. This means that all the components get the same current. |
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Resistance in a series circuit |
Resistance in a series circuit adds up. The total resistance is the sum of the individual resistances. The resistance of two or more resistors in a series circuit is bigger than the resistance of just one of the resistors on its own, because the battery has to push charge through all of them. |
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Resistance and voltage in a series circuit |
The bigger the total resistance of a component, the bigger it's share of the total potential difference because most work is done by the charge when moving through a large resistor than a small one. If the resistance of one component changes, for example it's a variable resistor, then the potential difference across all the components will change too. |
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Cell voltages in a series circuit |
If you connect several cells in series all the same way (from positive to negative end) you get a bigger total voltage. This is because each charge in the circuit passes through all the cells and gets a push from each cell in return. So two 1.5V cells in series would supply 3 Van in total. Cell voltages don't add up like this for cells connected in parallel, as each charge only goes through one cell. |
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Cell current in a series circuit |
Addinf cells in a series circuit doesn't increase the current. The maximum current will be the same as if you just had one cell in the circuit. Cells connected in parallel however increase the total current of the circuit. Because the current through each cell is less than in the rest of the circuit, they join together to make the total current. |
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Parallel circuits |
Parallel circuits are much more commonly used. In parallel circuits, each component is separately connected to the positive and negative end of the supply. If you remove or disconnect one of them, it will often hardly effect the others at all. This is how most things in real life are connected, for example in cars and in household electrics. You have to be able to switch everything on and off separately. |
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Voltage in a parallel circuit |
Voltage in a parallel circuit is the same across all components. The potential difference in each component is equal to that of the battery, so the PD is the same for each component. V1=V2=V3. This means that all bulbs connected in parallel will be of the same brightness. |
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Current in a parallel circuit |
Total current in a parallel circuit is shared between branches. Current flowing from the battery is the same as the current flowing back to it- there's nowhere else for the charges to go. In parallel circuits, the current flowing from the battery is shared between the branches, so the total current leaving the battery is equal to the total current in the seperate branches. In a parallel circuit, there are junctions where the current splits or rejoins. The total current going into each junction is equal to the current leaving. A=A1+A2+A3. |
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Resistance in a parallel circuit |
The total resistance in a parallel circuit is difficult to work out, but it is always less than thay of the branch with the smallest resistance. The resistance is lower because the charge has more than one branch to take, only some of the charge will flow along each branch. A circuit with two resistors in parallel will have a lower resistance than a circuit with either of the resistors by themselves- which means that the parallel circuit will have a higher current. Total R < R1 and total R < R2. |
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Current through a comonent in parallel |
Current through a component depends on its resistance. Each component in a parallel circuit is separately connected to the battery. This means that the current through each component is the same as if that component is the only one in the circuit. The resistance of a component controls how much current the voltage is able to push through it. The component with the LEAST resistance has the LARGEST current. This is because in a parallel circuit all the components have the same PD across them- the same PD causes a larger current to flow through a smaller resistance than through a bigger one. |
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Mains electricity |
AC supply is alternating current- meaning that the current is constantly changing direction. Direct current (DC) means that the current always flows in the same direction. The UK Mains domestic electricity supply is 230 V, and is an AC supply. An ACH supply is used because it is easier to generate than DC and is easer and simpler to supply over large distances. It's produced during by generators using a process called electromagnetic induction. |
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Inducing a voltage |
You can create a voltage, and maybe a current, in a conductor by moving a magnetic in or near a coil of wire. This is called electromagnetic induction. |
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Electromagnetic induction |
As you move the magnet near a coil of wire, the magnetic field through the coil changes- this change in the magnetic field induces a voltage across the ends of the coil. If the ends of the wire are connected to make a closed circuit, then a current will flow in the wire. The direction of the voltage depends on which way you move the magnet. |
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Voltage directions |
If you move the magnet into the coil then the voltage is induced in the opposite direction from when you move it out of the coil. If you reverse the magnet's north-south polarity, so thay the opposite pole points into the coil, the voltage induced will also be in the opposite direction. |
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AC generators |
In a generator in a power station, a magnet (or an electromagnet) rotates in a coil of wire. As the magnet turns, the magnetic field through the coil changes- this change in magnetic field induces a voltage which makes a current flow in the coil. When the magnet is turned through a half turn, the direction of the magnetic field through the coil reverses. When this happens, the voltage reverses so the current flows in the opposite direction around the coil of wire. If the wire keeps turning in the same direction, the voltage reverses every half turn, making an AC current. |
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AC generators diagram |
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Factors affecting the size of induced voltage |
Four factors increase the size of the induced voltage: Adding an iron core inside the coil, increasing the strength of the magnet, increasing the speed of rotation, increasing the number of turns on the coil. To reduce the size of the voltage induced, you do the opposite. |
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Transformers |
Transformers change the AC voltage ONLY. Transformers are used to change the size of the voltage, using electromagnetic induction to step up or step down the voltage. They have 2 coils of wire, the primary and the secondary coils, wrapped around an iron core. |
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How transformers work |
The alternating current in the primary coil causes changes in the iron cores magnetic field, which induces a changing voltage in the secondary coil. Step UP transformers increase the voltage. They have more turns on the secondary coil than the primary coil. Step DOWN transformers decrease the voltage, and have more turns on the primary coil than the secondary. |
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Transformers and electromagnetic induction |
The primary coil produces a magnetic field which stays within the iron core. Because there's an AC in the primary coil, the magnetic field in the iron core constantly changes direction (100 times a second at 50Hz). The magnetic field is changing. This changing magnetic field induces an alternating voltage in the secondary coil, with the same frequency as the AC in the primary coil. This is electromagnetic induction. |
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Turns on a coil in a transformer |
The relative number of turns on the 2 coils determines whether the voltage induced in the secondary coil is greater or less than the voltage in the primary coil. If you supplied direct current to the primary coil, you'd get nothing out of the secondary coil because the magnetic field wouldn't be constantly changing so there'd be no induction. Transformers only work with AC, you need a changing field to induce a voltage |
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Transformer equation |
You can calculate the output voltage from a transformer if you know the input voltage and the number of turns on each coil. Voltage across primary coil÷voltage across secondary coil= number of turns in primary coil÷ number of turns in secondary coil. |
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Transformer equation worked example |
You can use the formula either way up. You stick in the numbers you have and work out the one that is left. Eg: a transformer with 40 turns on primary coil and 800 on secondary coil has an input voltage of 1000V. Find output voltage. Voltage in secondary÷ 1000V= 800÷40. Voltage in secondary= 1000 x (800÷40). Voltage in secondary= 20 000V. |
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Magnetic fields |
A magnetic field is a region where magnetic materials, such as iron and steel, and wire carrying currents experience a force acting on them. Magnetic fields can be shown on field diagrams, the arrows on the field lines point from the north pole magnet to the south Pole. |
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Currents carrying a wire |
A straight, current carrying wire creates a magnetic field. The field is made up of concentric circles with the wire in the centre. |
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Rectangular coils |
A rectangular coil reinforces the magnetic field. If you bend the current carrying wire round into a coil, the magnetic field looks like the picture shown. The circular magnetic fields around the sides of the loop reinforce eachother at the centre. If the coil has lots of turns, the magnetic fields from all the individual loops reinforce eachother even more. |
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Currents in a magnetic field |
Because of its magnetic field, a current carrying wire or coil can exert a force on another current carrying wire or coil, or on a permanent magnet. When a current carrying wire is put in a different magnetic field, the two magnetic fields affect one another. This results in a force on the wire. |
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Currents in a magnetic field diagram |
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Current carrying wires in a magnetic field |
To feel the full force, the wire has to be at right angles (90°) to the lines of force of the magnetic field it's placed in. If the wire runs parallel to the lines of force on a magnetic field, it won't experience any force at all. At angles between 0 and 90°, it'll feel some force. When the wire is at right angles to the magnetic field, the force always acts as right angles to both the lines of force of the magnetic field and the direction of current. |
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Fleming's left hand rule |
Using your left hand, point your First finger in the direction of the Field and your seCond finger in the direction of the Current. Your thuMb will then point in the direction of the force, and the resulting Motion. |
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The motor effect |
Magnetic fields make current carrying COILS turn. If a rectangular coil of wire carrying a current is placed in a uniform magnetic field (a magnetic field with the same strength everywhere in the field), the force will cause it to turn. This is called the motor effect. You can use Fleming's left hand rule to work out which way the coil will turn. |
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Motor effect diagram |
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The simple electric motor diagram |
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The simple electric motor explanation |
The diagram shows the forces acting on the two side arms of the coil. These forces are just the usual forces which act on any current carrying wire in a magnetic field. Because the coil is on a spindle and the forces act one up and one down, it rotates. The split ring commutator allows the contacts to swap every half turn. This reverses the direction of current to keep the coil rotating continuously in the same direction. Otherwise, the direction of force would reverse every half turn and the coil would change direction instead of fully rotating. |
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Uses of an electric motor |
Lots of devices use rotation. They all work by using an electric motor in a similar way. You link the coil to an axle, and the axle spins round with the coil. The item you want to rotate (eg a fan) is attached to the axle and spins round. For example, you attach the wheels of a car to an axle which causes them to spin round. |
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Direction of travel in a circuit |
REMEMBER- a current flows from the NEGATIVE end of the terminal of the power source to the POSITIVE END. This is because a current is a flow of electrons, there are more electrons at the negative end, which is why it is negatively charged, which move round the circuit into the positive end where there are fewer electrons so is positively charged. |
