1
Q
What are ions?
A
When electrons are removed from particles the particles are left positively charged - these charged particles are called ions.
2
Q
Can electrons and positive charges move?
A
Positive and Negative charges are produced by the movement of electrons.
The positive charges do not move.
A positive static charge is always caused by electrons moving away elsewhere.
3
Q
Repel and attract?
A
- Two things with opposite electric charges are attracted to each other.
- Two things with the same electric charge will repel each other.
4
Q
How is static electricity caused?
A
When you rub two insulating materials together a load of electrons get dumped together on one of the insulators. Which become negatively charged.
They try to repel each other but can’t move apart due to their fixed position.
The patch of charge that results is called static electricity because it can’t move.
5
Q
What is electric current?
A
- Electric current is a flow of charge.
- In an electrical circuit the metal conductors are full of charges (electrons) that are free to move.
- Electric charge flows in metal conductors because the electrons are free to move around.
- Current can’t flow in an insulator (like plastic) because there are few charges free to move.
- In a complete circuit the battery pushes the free charges through the wires.
- The charge flows all the way around the circuit and back to the battery. It is not used up.
6
Q
Current?
A
Current will only flow through a component if there’s a voltage across that component.
It’s units are amperes (amps) A.
7
Q
Voltage?
A
Voltage is the driving force that pushes the current round.
It’s units are volts V.
8
Q
Resistance?
A
Resistance is caused by things in the circuit that resist the flow of charge.
It’s units are ohms O.
9
Q
Link between current and voltage?
A
If you increase the voltage- then more current will flow.
If you increase the resistance- then less current will flow.
10
Q
Power?
A
- Anything that supplies electricity is also supplying energy.
- Power supplies all transfer energy to the charge which then transfers it to the components.
- Work is also done because energy is transferred.
- Power is the rate at which an electrical power supply transfers energy to an appliance.
11
Q
What is power measured in??
A
Power is measured in watts (W) Or kilowatts (kW)
12
Q
An appliance with high power?
A
An appliance with a high power transfers a lot of energy in a short time.
This energy comes from the current flowing through it. This means that an appliance with high power will use a large current.
13
Q
Electrical power equation?
A
Power (W) = Voltage (V) * Current (A)
14
Q
Circuit symbols??
A
(Look it up)
15
Q
The standard circuit?
A
- Very basic circuit for testing components.
- The component, the ammeter and the variable resistor are all in series which means they can be 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 flowing through the circuit.
16
Q
Potential difference?
A
- Potential difference (voltage) tells us how much energy is transferred to or from each unit of charge as it moves between two points.
- The battery transfers energy to the charge as it passes.
- Components transfer energy away from the charge as it passes.
- 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 two points.
- The voltage of a battery shows how much work the battery will do to charge that passes through it.
- A voltmeter is used to measure the potential difference between two points.
- A voltmeter must be placed in parallel with a component so it can compare the energy the charge has before and after passing through the component.
17
Q
Voltage current graphs?
A
- 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 small resistance that it’s usually ignored.
18
Q
Resistance equation?
A
Resistance = voltage (V) / current (A)
19
Q
What happens when a resistor gets hot?
A
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.
20
Q
LDR??
A
A light dependant resistor or LDR is a type of resistor that changes its resistance depending on how much light there is:
- In bright light, the resistance falls.
- In darkness, the resistance is highest.
This makes it useful for various electronic circuits.
21
Q
Thermistor??
A
A thermistor is like an LDR but it’s resistance depends on temperature.
- In hot conditions, the resistance drops.
- In cool conditions, the resistance goes up.
Thermistors make useful temperature detectors.
22
Q
Series circuits??
A
In series circuits the different components are connected in a line, end to end between the positive and negative of the power supply.
23
Q
Potential difference is shared in series circuits?
A
- In series circuits the total potential difference of the supply is shared between the various components.
- The potential difference round a circuit always add up to equal the potential difference across 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.
24
Q
Current is the same??
A
- In series circuits the same current flows through all parts of the circuits.
- The size of the current is determined by the total potential difference of the cells and the total resistance of the circuit.
- This means all the components get the same current.
25
Resistance adds up??
* In series circuits, the total resistance is just the sum of the individual resistances.
* The resistance of two resistors in series 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.
* The bigger the resistance of a component the bigger its share of the total potential difference because more work is done by the charge when moving through a large resistance than through a small one.
* If the resistor of one component changes then the potential difference across all the components will change too.
26
Cell voltages add up?
* If you connect several cells in series all the same way. (Positive to negative) . You get a bigger total voltage.
* Because each charge in the circuit passes through all the cells and gets a "push" from each cell in turn.
* Cell voltages don't add up like that for cells connected in parallel. Each charge only goes through one cell.
27
Cell current is the same in each cell?
* Adding cells in series increases the voltage without increasing the resistance- so the total current in the circuit increases.
* The current through each cell is the same as the total current in the circuit.
* Cells connected in parallel also increase the total current in the circuit.
* However, the current through each cell is less than in the rest of the circuit because they join together to make the total current.
28
Parallel circuits?
* In parallel circuits each component is separately connected to the positive and negative of the supply.
* If you remove or disconnect one of them it will often hardly affect the others at all.
29
Potential differences in all components?
* In parallel circuits the P.D across each component is equal to the P.D of the battery so the P.D is the same across all components.
* This means that identical bulbs connected in parallel will all be at the same brightness.
30
Total current is shared between branches?
* In all circuits the current flowing from the battery is the same as the current flowing back to it- there is no where else for the charge 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 of the currents in the separate branches.
* In parallel circuits there are junctions where the current splits or rejoins.
* The total current going into a junction equals the total current leaving.
31
Resistance in a circuit??
* The total resistance of a parallel circuit is always less than that 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 the parallel circuit will have a higher current.
32
The current through a component depends on its resistance??
* Each component in a parallel circuit is separately connected to the battery.
* This means the current through each component is the same as if that component was 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 P.D across them- the same P.D causes a larger current to flow through a smaller resistance than through a bigger one.
33
Mains supply is AC, battery supply is DC??
* The UK mains domestic electricity supply is 230 volts.
* Its produced by generators using a process called electromagnetic induction.
* Mains electricity is an AC supply (alternating current) the current is constantly changing direction.
* Batteries supply direct current (DC). This means the current always flows in the same direction.
* AC is used for mains electricity because it's easier to generate than DC and is easier and simpler to distribute over long distances.
34
Moving a magnet into a coil to induce voltage??
* You can create a voltage and maybe a current in a conductor by moving a magnet in or near a coil of wire. This is called electromagnetic induction.
* As you move the magnet, 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.
35
AC generators??
* In a generator, a magnet rotates in a coil of wire. As the magnet turns, the magnetic field through the coil changes- this change in the magnetic field induces a voltage, which makes a current flow in the coil.
* When the magnet is turned through half a 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 magnet keeps turning in the same direction then the voltage keeps on reversing every half turn and you get an AC current.
36
4 factors that affect the size of the induced voltage??
* Add an iron core inside the coil.
* Increase the strength of the magnetic field.
* Increase the speed of rotation.
* Increase the number of turns on the coil.
37
Transformers??
* Transformers are used to change the size of the voltage- they use electromagnetic induction to "step up" or "step down" the voltage.
* They have two coils of wire the primary and the secondary coils, wound around an iron core.
* The alternating current in the primary coil causes changes in the iron cores magnetic field which induces a changing voltage in the secondary coil.
38
Step up transformers?
They step the voltage up (increase it). They have more turns on the secondary coil than the primary coil.
39
Step down transformers?
They step the voltage down (decrease it). They have more turns on the primary coil than the secondary.
40
Transformers work by electromagnetic induction?
* The primary coil produces a magnetic field which stays within the iron core.
* As there's an alternating current (AC) in the primary coil, the magnetic field in the iron core constantly changes direction.
* This changing magnetic field induces an alternating voltage in the secondary coil with the same frequency as the alternating current in the primary. Electromagnetic induction of voltage.
* The relative number of turns on the two could 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 (DC) to the primary coil. There will be no induction in the secondary coil. Because you need a changing field to induce a voltage.
* Transformers only work with AC. They don't work with DC at all.
41
Transformer equation?
Voltage across primary coil/ voltage across secondary coil=
Number of turns in primary coil/ number of turns in secondary coil
42
What is a magnetic field?
A magnetic field is a region where magnetic materials (like iron and steel) and also wires carrying currents experience a force acting on them.
43
Current carrying wire??
There is a magnetic field around a straight, current carrying wire.
The field is made up of concentric circles with the wire in the centre.
44
Rectangular coil??
If you bend the current carrying wire round into a coil the magnetic field looks like this.
The circular magnetic fields around the sides of the loop reinforce each other at the centre.
If the coil has lots of turns the magnetic fields from all the individual loops reinforce each other even more.
45
The current in a magnetic field experienced a force?
* Because of its magnetic a current carrying wire or coil can exert a force on another current carrying wire or coil or on a per memenet magnet.
* When a current carrying wire is put in a different magnetic field the two magnetic fields affect one another. The result is a force on the wire.
* To feel the full force the wire has to be at right angles (90) to the lines of the force of the magnetic field its placed in.
* If the wire runs parallel to the lines of force of the magnetic field it won't experience any force at all. And at angles in between 0 and 90 degrees it will feel some force.
* When the wire is at right angles to the magnetic field the force always acts at right angles to both the lines of force of the magnetic field and the direction of the current.
46
Flemings 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 (motion)
47
What is the motor effect?
If a rectangular coil of wire carrying a current is placed in a uniform magnetic field the force will cause it to turn. This is called the motor effect.
48
The simple electric motor?
* The forces are acting on the two arms of a coil.
* These forces are 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 swaps the contacts very half turn.
* This reverses the direction of the current every half turn to keep the coil rotating continuously in the same direction.
* Otherwise, the direction of the force would reverse every half turn and the coil would change direction every half turn instead of fully rotating.
49
Anything that rotates can be powered by electric motors?
* Lots of devices use rotation. They all work by using an electric motor in a similar way.
* Link the coil to an axle and the axle spins round.
For example,
In a DVD player the axles attached to the bit where the DVD sits on to make it spin.
And electric cars and trains have their wheels attached to axles.
50
The build up of static?
* When two insulating materials are rubbed together, electrons are scrapped off one and dumped on the other.
* Electrons are negatively charged.
* So this leaves a positive static charge one and a negative static charge on the other.
* Which way the electrons are transferred depends on the materials involved.
