P5 + P6 + P7 - Electricity and Magnetism Flashcards
The basic phenomena of magnetism
Opposite poles attract; like poles repel
Ends of magnets
Poles
* magnets have two poles: noth and south
What are magnets?
Magnets are objects which experience attraction and repulsion
* Like poles repel (push each other apart)
* Unlike poles attract (move towards each other)
Properties of magnetic materials (3)
- Experience a force when placed in a magnetic field
- Are attracted to a magnet when unmagnetised
- Can be magnetised to form a magnet
Permanent magnets
A type of magnet that retains its magnetic properties indefinitely, even after removing the external magnetic field
* usually made from steel
Uses of permanent magnets (name 3)
- Compasses - navigation
- School lab experiments
- Toys - toy trains and trucks with magnet attached the carriages to the engine
- Fridge magnets
Electromagnets
A type of nagnet in which the magnetic field is produced by electric current
* Made up of a coil of wire (solenoid) wrapped around an iron core
* They can be switched on and off
Uses of electromagnets (name 3)
- MRI scanners
- Speakers and earphones
- Recycling
- Mag-Lev trains - hover above tracks to increase speed
Difference between magnetic and non-magnetic materials
Magnetic materials are attracted to a magnet; non-magnetic materials are not
Metals on the Periodic Table which are magnetic (3)
- Iron
- Cobalt
- Nickel
Two types of magnets and their definitions
- Permanent magnets: made out of permanent magnetic materials and will produce its own magnetic field
- Temporary (Induced) magnets: when a magnetic material is placed in a magnetic field, the material can temporarily be turned into a magnet
Magnetic field
The region around a magnet where a force acts on another magnet or on a magnetic material (such as iron, steel, cobalt and nickel)
Uniform magnetic field
A uniform field is created when two opposite poles are held close together. Magnetic fields are always directed from North to South
* has the same strength and direction at all points
Magnetic field lines
Magnetic field lines are used to represent the strength and direction of a magnetic field
Rules of drawing magnetic field lines (3)
- The direction of the magnetic field is shown using arrows
- Always go from north to south (indicated by an arrow midway along the line)
- Must never touch or cross other field lines
Plotting magnetic field lines using iron filings (3 steps)
- Place a piece of paper on top of the magnet
- Gently sprinkle iron filings on top of the paper
- Now carefully tap the paper to allow the iron filings to settle on the field lines
Plotting magnetic field lines using a compass (8 steps)
- Place the magnet on top of a piece of paper
- Draw a dot at one end of the magnet (near its corner)
- Place a plotting compass next to the dot so that one end of the needle of the compass points towards the dot
- Use a pencil to draw a new dot at the other side of the compass needle
- Now move the compass so that it points towards the new dot, and repeat the above process
- Keep repeating until you have a chain of dots going from one end of the magnet to the other. Then remove the compass, and link the dots using a smooth curve – the magnetic field line
- The direction of the field line is the same as the direction of the plotting compass
- You can now repeat the whole process several times to create several other magnetic field lines
Two types of electric charge
Positive and Negative
Electrostatic repulsion
Caused by the force between two charges of the same kind
Demonstrating electrostatic charge experiment
- Suspend one of the insulating materials using a cradle and a length od string so that the material can rotate freely
- Rub one end of the materials using a cloth (to give it a charge)
- Take a second piece of insulating material and charge that by rubbing with a cloth
- Hold the charged end of the second piece close to the charged end of the first piece
* If the first piece rotates away then the materials have the same charge
* If the first piece moved towards then they have opposite charges
Electric field
A region in which an electric charge experiences a force
Rule for drawing electric field lines
Fields lines always point away from positive charges and towards negative charges
What do the field lines in an electric field represent?
The direction of the force on a positive charge at that point
* Field lines show the direction that a positive or negative charge would experience if it was at that point
Describe the field lines between to oppositely charged parallel conducting plates (4)
- It is a uniform electric field
- Directed from the positive to the negative plate
- Parallel
- Straight lines
Difference between conductors and insulators
Conductors allow charge carriers to freely move
Insulators do not allow charge carriers to move
Examples of conductors (3)
(usually metals)
* Silver
* Copper
* Aluminium
* Steel
Examples of insulators (3)
- Rubber
- Plastic
- Glass
- Wood
Current
The current is the amount of charge passing a point in a circuit every second (charge per second)
Formula for charge
Charge = Current x Time
(Q = I x t)
What is charge measured in?
Coulombs
What is current measured in?
Amps
Electric current
When two oppositely charged conductors are connected together (by a length of wire), charge will flow between the two conductors. This flow is called an electric current
* The greater the flow, the greater the current
Direct current
The electrons flow in one direction only, from the negative terminal to the positive terminal
* Produced when using dry cells and batteries
Alternating current
The direction of electron flow changes direction regularly
* Comes from mains electricity and generators
Mains electricity
The electricity that is delivered to homes and businesses through an electric grid
What device measures current?
Ammeter
How to use an ammeter?
Should always be connected in series with the part of the circuit you wish to measure the current through
Two types of ammeter
- Digital (with an electronic read out)
- Analogue (with a needle and scale)
Zero error
When the measuring system gives a false reading when the true value of a measured quantity is zero
Parallax error
An error in reading caused by not reading the measurement at eye level
Range for an analogue ammeter
0.1 - 1.0 Amps
1.0 - 5.0 Amps
How to use an analogue ammeter and voltmeter to avoid errors? (2)
- Always double check exactly where the marker is before an experiment, if not at zero, you will need to subtract this from all your measurements. They should be checked for zero errors before using
- Always read the meter from a position directly perpendicular to the scale
Advantages of using digital ammeters and voltmeters (3)
- Digital ammeters can measure very small currents, in mA or µA
- Digital displays show the measured values as digits and are more accurate than analogue displays
- They’re easy to use because they give a specific value and are capable of displaying more precise values
How to use a digital ammeter to avoid errors?
- Displays may ‘flicker’ back and forth between values and a judgement must be made as to which to write down
- You should check for zero errors: make sure the reading is zero before starting an experiment, or subtract the “zero” value from the end results
Electron flow/current
Electrons flow from negative to positive since they are negatively charged
Conventional current
Flow from positive to negative
Electromotive force
The electrical work done by a source in moving a unit charge around a complete circuit (measured in volts)
* the total voltage supplied by a power supply
Formula for electromotive force
E = W / Q
Work / Charge
Potential difference
The work done by a unit charge passing through a component (measured in volts)
* the difference in electric potential between two points
Potential difference formula
V = W / Q
Work / Charge
What device measures potential difference?
Voltmeter
Two types of voltmeter
- Digital (electronic readout)
- Analogue (needle and scale
How to use a voltmeter?
Should be connected in parallel with the component being tested
* has to be connected to two points in the circuit
Range for an analogue voltmeter
0.1 - 1.0 V
0 - 5.0 V
Resistance
The opposition to current
* the higher the resistance, the lower the current
The unit for resistance
Ohms Ω
Resistance formula
R = V / I
Potential difference / Current
Ohm’s law
Current is directly proportional to potential difference as long as the temperature remains constant
What do resistors control in a circuit? (2)
- The current in branches of the circuit (through certain components)
- The potential difference across certain components
Consequences of Ohm’s Law (2)
- The current in an electrical conductor decreases as its resistance increases (for a constant p.d.)
- The p.d. across an electrical conductor increases as its resistance increases (for a constant current)
Rule for drawing IV graph for a resistor
The current is proportional to the potential difference
Rule for drawing IV graph for a lamp
The current increases at a proportionally slower rate than the potential difference
Diode
A non-ohmic conductor that allows current to flow in one direction only
Forward bias (diode)
The direction of the current flow as dictated by a diode
Reverse bias
The opposite direction of the diode activity which has very high resistance causing no current to flow
Rule for IV graph of a diode
- When the diode is in forward bias, the graph shows a sharp increase in voltage and current (on the right side of the graph)
- When the diode is switched around, in reverse bias, the graph shows a flat line where current is zero at all voltages (on the left side of the graph)
Explain the IV graph for a lamp (3 points)
- The current causes the filament in the lamp to heat up
- As the filament gets hot, its resistance increases
- This opposes the current, causing it to increase at a slower rate
What happens as electrons pass through a wire?
- As electrons pass through a wire, they collide with the metal ions in the wire which resists their flow
Two conditions for resistance in a wire
- The longer a wire, the greater its resistance (since electrons will collide with more ions)
- The thicker a wire, the smaller its resistance (more space for electrons)
Relationship between resistance and length
Resistance is directly proportional to length
R - L
Relationship between cross-sectional area and resistance
Resistance is inversely proportional to cross-sectional area (width, or thickness)
R - 1/A
Electrical energy formula
E = VIt
Power
The rate of doing work
Power formula
P = E/t = W/t
Energy / time = Work done / time
Power formula (using current and voltage)
P = IV
Power in terms of resistance and current
P = I^2 x R
Power in terms of potential difference and resistance
P = V^2 / R
Kilowatt hour
A unit of energy equivalent to one kilowatt of power expended for one hour
What do power ratings on appliances tell consumers?
The amount of energy transferred (by electrical work) to the device every second
Kilowatt hour equation
E = Pt
Power x time
Cell (symbol and definition)
Required to push electrons around a circuit
Battery (symbol and definition)
Consists of two or more cells
Wire (symbol and definition)
Connects cell and receiver
Wire junction (symbol)
Switch (symbol and definition)
Enables the current in a circuit to be turned on or off
Indicator (symbol and definition)
Often a light bulb - show wether circuit is on or not
Ammeter (symbol and definition)
Measures electric current in amperes
Voltemeter (symbol and definition)
Measures voltage in volts
Fixed resistor (symbol and definition)
Used to lomit the current in a circuit at a fixed value
Variable resistor (symbol and definition)
Resistance can be changed
Thermistor (symbol and definition)
A device whose resistance decreases with temperature
Light dependent resistor (LDR) (symbol and definition)
A device whose resistance decreases with brightness
Diode (symbol and definition)
Only allows current to flow in one direction (indicated by the arrow)
Light emitting diode (LED) (symbol and definition)
Emits light when it allows the flow of current
Fuse (symbol and definition)
Designed to melt and so break the electric circuit when too much electric current flows
Heater (symbol and definition)
Convert electrical energy to heat
Series circuit
If the same current passes through each device in turn with no junction between them
What does increasing the voltage of the power source do in a series circuit?
Drives more current around the circuit
What does increasing the number of components in a series circuit do?
Increases the total resistance
Parallel circuit
A parallel circuit consists of two or more components attached along separate branches of the circuit
Advantage of parallel circuits (2)
- The components can be individually controlled, using their own switches
- If one component stops working the others will continue to function
Current, voltage and resistance equation in series circuit when two or more resistors are connected in series
I = I1 = I2 = I3
V = V1 + V2 +V3
R = R1 + R2 + R3
Current, voltage and resistance equation in parallel circuit when two or more resistors are to wires, side by side to one another
I = I1 + I2 + I3
V = V1 = V2 = V3
1/R = 1/R1 + 1/R2 + 1/R3
Advantages of parallel-connected lamps (name 2)
- Safer because a single failure in one lamp does not affect the rest of the lighting
- You can switch individual lamps on and off without affecting the others, allowing you to customise lighting
- Generally parallel circuits are more reliable because they can isolate individual components
Thermistor IV diagram (drawing and explanation)
- Resistance decreases as temperature increases
- The greater the slope of the line, the higher the temperature
Examples of electric hazards (name 3)
- Damaged insulation - subjection to lethal shock if someone touches an exposed wire
- Overheating of cables - passing too much current through a small wire can lead to the wire overheating causing a fire
- Damp conditions - if moisture comes into contact with live wires the moisture could conduct electricity wither causing a short circuit or electrocution
- Excess current from overloading of plugs, extension leads, single and multiple sockets when using mains supply - overloaded use can cause overheating and a fire
What safety features are built into domestic appliances like plugs? (name 3)
- Double insulation
- Earthing
- Fuses
- Circuit breakers
Reason for wire insulation
The conducting part of a wire is usually made of copper or some other metal
* If this comes into contact with a person, this poses a risk of electrocution
* For this reason, wires are covered with an insulating material, such as rubber
What are the two layers in double insulation?
- Insulation around the wires themselves
- A non-metallic case that acts as a second layer of insulation
How does the earth wire provide additional safety against electrocution?
If the wire is touched:
* The earth wire provides a low resistance path to the earth
* It causes a surge of current in the earth wire and hence also in the live wire
* The high current through the fuse causes it to melt and break
* This cuts off the supply of electricity to the appliance, making it safe
The 3 wires and their colours
- Live wire - brown insulation
- Neutral wire - blue
- Earth wire - yellow-green insulation
Trip switch
(Found in the Consumer Box, where the electricity enters a building) it does the same job as a fuse:
* When the current is too high the switch ‘trips’ (automatically flicks to the off position)
* This stops current flowing in that circuit
How to choose the correct fuse to use?
- Fuses come in different sizes: 3A, 5A, 13A
- To choose you need to calculate how much current the appliance needs (using I = P/V)
- The fuse should always have a current rating higher than the current needed without being too high - always choose the next size up
How is e.m.f induced?
- A conductor, such as a wire, cuts through a magnetic field (uniform magnetic field)
- The direction of a magnetic field through a coil changes (solenoid)
Where is electromagnetic induction used?
- Electrical generators which convert mechanical energy to electrical energy
- Transformers which are used in electrical power transmission
Electromagnetic induction
The creation of an electro-motive force (EMF) by way of a moving magnetic field around an electric conductor
Experiment for e.m.f induction - moving a magnet through coil
When a coil is connected to a sensitive voltmeter, a bar magnet can be moved in and out of the coil to induce an EMF
What happens as the bar magnet moves into the coil? (e.m.f coil experiment)
- As the bar magnet moves, its magnetic field lines ‘cut through’ the coil
- This induces an EMF within the coil, shown momentarily by the reading on the voltmeter
What happens as the bar magnet is not moving? (e.m.f coil experiment)
When the bar magnet is held still inside, or outside, the coil, there is no cutting of magnetic field lines, so, there is no EMF induced
What happens as the bar magnet is taken back out of the coil? (e.m.f coil experiment)
- As the magnet changes direction, the direction of the current changes
- The voltmeter will momentarily show a reading with the opposite sign
Lenz’s law
The direction of an induced potential difference always opposes the charge that produces it
What happens as the bar magnet’s speed increases? (e.m.f coil experiment)
Increasing the speed of the magnet induces an e.m.f with a higher magnitude
Experiment for e.m.f induction - moving a wire through a magnet
- When a long wire is connected to a voltmeter and moved between two magnets, an EMF is induced
- The pattern of a magnetic field in a wire can be investigated using this set up
What happens when the wire is not moving? (e.m.f wire experiment)
When the wire is held still inside, or outside, the magnets, the rate of change of flux is zero, so, there is no EMF induced
What happens when the wire is moved? (e.m.f wire experiment)
- As the wire is moved through between the magnets, an EMF is induced within the wire, shown momentarily by the reading on the voltmeter
- As the wire moves, it ‘cuts through’ the magnetic field lines of the magnet, generating a change in magnetic flux
What happens when the wire is taken back out? (e.m.f wire experiment)
As the wire changes direction, the direction of the current changes
The voltmeter will momentarily show a reading with the opposite sign
Factors affecting EM induction (name 3)
- The speed at which the wire, coil or magnet is moved
- The number of turns on the coils of wire
- The size of the coils
- The strength of the magnetic field
- The orientation of the poles of the magnet
How does the speed of the wire, coil or magnet movement affect EM induction?
- Increasing the speed will increase the rate at which the magnetic field lines are cut
- This will increase the induced potential difference
How does the number of turns on the coils in the wire affect EM induction?
Increasing the number of turns on the coils in the wire will increase the potential difference induced
* This is because each coil will cut through the magnetic field lines and the total potential difference induced will be the result of all of the coils cutting the magnetic field lines
How does the size of the coils affect EM induction?
- Increasing the area of the coils will increase the potential difference induced
This is because there will be more wire to cut through the magnetic field lines
How does the strength of the magnetic field affect EM induction?
Increasing the strength of the magnetic field will increase the potential difference induced
How does the orientation of the poles of the magnet affect EM induction?
Reversing the direction in which the wire, coil or magnet is moved
Generator effect
Occurs whenever a potential difference is induced across a conductor which is experiencing a change in external magnetic field
Alternator
An alternator is a rotating coil in a magnetic field connected to slip rings
Explain how an A.C. generator is set up
A rectangular coil is forced to spin in a uniform magnetic field
* The coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings
* The slip rings and brushes provide a continuous connection between the coil and the meter
How is the pointer effected when the coil turns in one direction? (A.C. generator)
- The pointer defects first one way, then the opposite way, and then back again
- This is because the coil cuts through the magnetic field lines and an EMF, and therefore current, is induced in the coil
Why does the pointer deflect in both directions? (AC generator)
Because the current in the circuit repeatedly changes direction as the coil spins
* This is because the induced EMF in the coil repeatedly changes its direction
* This continues as long as the coil keeps turning in the same direction
What kind of graph depicts the results of an a.c. generator?
A sine graph
Lenz’s law
The direction of an induced potential difference always opposes the change that produces it
Lenz’s left hand rule
Thumb: force
Index finger: magnetic field (denoted by the arrows on the graph)
Middle finger: current
REMEMBER LEFT HAND ONLY
Conducting wire
Any wire that has current flowing through it
Why does the diagram for the magnetic field around a current-carrying wire use concentric circles?
A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles
What does the proximity of circles around a conducting wire suggest about its magnetic field?
- As the distance from the wire increases the circles get further apart: magnetic field is strongest closest to the wire
- No current: no circles
- More current: greater strength, closer together
Drawing current carrying wires
- Dot in centre: current is flowing out of the plane (towards you)
- Cross in centre: current is flowing into the plane (away from you)
Right-hand thumb rule
- Round your fingers around your thumb like a loose fist
- Point your thumb upwards
- Point you thumb in the direction of the current
- The direction of your fingers suggests the direction of the magnetic field
Properties of a magnetic field around a solenoid
It is strong and uniform (same strength and direction at every point)
How to work out the polarity of each end of a solenoid
- If the current is travelling around in a clockwise direction at one end then it is the south pole
- If the current is travelling around in an anticlockwise direction at one end then it is the north pole
How can a solenoid be used as an electromagnet?
By adding a soft iron core
* This will become an induced magnet when current is flowing
* The magnetic field produced will create a much stronger magnet overall
* Changing the direction of current will change the direction of the magnetic field produced by the iron core
Factors affecting the strength of the magnetic field produced around a solenoid (increase) (3)
- Size of the current
- Number of coils
- Adding an iron core
Two applications of electromagnets
- Relay circuits (used in electric bells, electronic locks etc.)
- Loudspeakers and headhones
Electromagnet
A soft metal (iron) core made into a magnet by the passage of electric current through a coil surrounding it
How do electric bells work?
When the button K is pressed:
* A current passes through the electromagnet E creating a magnetic field
* This attracted the iron armature A, causing the hammer to strike the bell B
* The movement of the armature breaks the circuit at T
* This stops the current, destroying the magnetic field and so the armature returns to its previous position
* This re-establishes the circuit, and the whole process starts again
Relays
Switches that open and close via the action of an electromagnet
What do relay circuits consist of?
- An electrical circuit containing an electromagnet
- A second circuit with a switch which is near to the electromagnet in the first circuit
Motor effect
A current carrying wire experiences a force in a magnetic field
How can the force on a current-conducting wire, be increased (3)
- The current is increased
- A stronger magnet is used
- The length of the wire in the field is increased
Describe the operation of a DC motor
- When the current is flowing in the coil at 90 degrees to the direction of the magnetic field:
- The current creates a magnetic field around the coil
- The magnetic field produced around the coil interacts with the field produced by the magnets
- This results in a force being exerted on the coil
- The direction of the force can be determined using Fleming’s left-hand rule
- As current will flow in opposite directions on each side of the coil, the force produced will push one side of the coil up and the other side of the coil down
- This will cause the coil to rotate, and it will continue to rotate until it is in the vertical position
In the vertical position momentum keeps the coil turning until the magnetic force takes over again
How do the split ring commutators work in a d.c. motor?
The split ring commutator swaps the contacts of the coil
* This reverses the direction in which the current is flowing every half turn
* This keeps the current leaving the motor in the same direction (d.c)
Reversing the direction of the current will also reverse the direction in which the forces are acting
* As a result, the coil will continue to rotate
How can the speed at wich the coil rotates in a d.c. motor be increased? (2)
- Increasing the current
- Use a stronger magnet
How can the direction of rotation of coil in the d.c. motor be changed? (2)
- Reversing the direction of the current
- Reversing the direction of the magnetic field by reversing the poles of the magnet
How can the force supplied by the motor in a d.c. motor be increased?
- Increasing the current in the coil
- Increasing the strength of the magnetic field
- Adding more turns to the coil
Transformer
A transformer is an electrical device that can be used to increase or decrease the potential difference of an alternating current (voltage transformations) through the generator effect
Generator effect
Occurs whenever a potential difference is induced across a conductor which is experiencing a change in external magnetic field
Parts of a basic transformer
- a primary coil
- a secondary coil
- a soft iron core (used because it is easily magnetised)
Operation of a transformer (5 steps
- Alternating current is supplied to the primary coil
- It will produce a changing magnetic field around the primary coil
- As the iron core is easily magnetised, the changing magnetic field will pass through it
- Therefore, there is now changing magnetic field inside the secondary coil
- This cuts through the secondary coil and induces a potential difference
- this p.d. will be alternating and have the same frequency as the current supplied
Step-up transformer
A step-up transformer increases the potential difference of a power source and has more turns on the secondary coil than the primary coil
Step-down transformer
A step-down transformer decreases the potential difference of a power source and has fewer turns on the secondary coil
What does the output potential difference of a transformer depend on? (2)
- The number of turns on the primary and secondary coils
- The input potential difference (voltage)
Equation for output potential difference
Vp / Vs = Np / Ns
where V = p.d. or voltage in primary or secondray coil and N is the number of turns on the two coils
Equation for trandfromer effeciency if it is 100% efficient
Input power = output power
Vp x Ip = Vs x Is
where I is the current through both coils
Uses of transformers (name 2)
- They are used to increase the potential difference of electricity before it is transmitted across the national grid
- They are used to lower the high voltage electricity used in power lines to the lower voltages used in houses
- They are used in adapters to lower mains voltage to the lower voltages used by many electronic devices
Explain the advantage of high voltage transmission
- When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss
- To keep same poer: potential difference at which the electricity is transmitted should be increased
- This will result in a smaller current being transmitted through the power lines
- A smaller current flowing through the power lines results in less heat being produced in the wire - reducing energy loss
Power loss equation
P = I2 x R
