Electricity and magnetism Flashcards
What are the two poles a magnet has?
North and south (poles are the ends of the magnet)
What happens when two opposite poles are facing eachother?
North and south poles are attracted to each other (unlike poles attract).
What happens when two of the same poles (north and north or south and south) are facing eachother?
They will repel each other (like poles repel)
Characteristics of magnetic materials
- Experience aforcewhen placed in a magnetic field
- Are attracted to a magnet whenunmagnetised
- Can bemagnetisedto form a magnet
- Contain iron, nickel or cobalt
Characteristics of non-magnetic materials
- Not attracted or repelled by magnets and cannot be magnetised
- All metals that don’t contain iron, nickel or cobalt and all non-metals
- Don’texperience a force when placed in a magnetic field
How to determine if a material is a magnet, magnetic material or non-magnetic material
Bring it close to a known magnet
- If the material is a magnet when you bring the known magnet close to one end it will attract and when you bring the known magnet close to the other end it will repel (Only amagnetcanrepelanother magnet)
- If the material is magnetic when you bring the known magnet close to either end it will attract (only attracts)
- If the material is non-magnetic, it will be unaffected either end you bring it to
What are the two types of magnet?
Permanent magnets and temporary magnet
Characteristics of permanent magnet
- Made of steel (also known as hard iron)
- Difficult to magnetise
- Produces their own magnetic field
- Don’t lose their magnetism even in the absence of magnets
- They are used in compasses, school lab experiments, toys and fridge magnets
Characteristics of temporary magnet
- Made of soft iron
- Easy to magnetise
- Easy to lose magnetism (its magnetism can be turned on and off by moving the magnet closer and farther from it)
- Magnetism is only temporary
- Used in cores of electromagnet and transformers
How to induce magentism?
Magnetism can be induced (produced) in a metal which has magnetic properties (example: iron, steel) by simply placing the magnetic metal in a magnetic field (close to a strong magnet without touching). The iron will also have a south and north pole and will attract the opposite pole of the magnet. The end of the material closest to the magnet will have theoppositepole to magnets pole closest to the material.
What is magnetization
the process of inducing magnetism in magnetic materials
What is the most effective way to magnetise magentic materials?
By using a direct current
1. A steel nail (or iron) is inserted in a long coil of wire
2. D.C power supply is turned on to pass a large d.c current through the coil
3. Current generates a magnetic field which magnetises the nail
Another way to produce strong magnetism is to stroke the steel with the magnet
What is demagnetisation?
Process that destroys the magnetism of a magnet
3 ways to demagnetise
Demagnetisation by heating
- Place a magnet along the west-east of the earth
- Heat the magnet with fire until it demagnetises
Demagnetisation by hitting
- Place a magnet along the west-east of the earth
- Hit a magnet with a hammer using a moderate amount of force until it demangetises
Demagnetisation by alternate current
- Insert a magnet into a long coil
- Turn on the a.c power supply
- Slowly pull the magnet from the coil until it is completely outside of the coil
- The large alternating current generates the changing magnetic field to demagnetise the magnet
What is an electromagnet and what is often used to make it?
Electromagnets use electricity to create a magnet from a current carrying wire (They have the advantage that they can be magnetised and demagnetised, literally at the flick of a switch). Soft iron is normally used for this
Uses for electromagnets
The uses for electromagnets are in MRI scanners, speakers and earphones, recycling and Maglev trains
What is a magnetic field?
A magnetic field is a space or region around a magnet in which a force is experienced by a magnetic pole
Rules for magnetic field lines around a bar magnet
- Lines run from North to South pole
- Lines cannot cross each other
- The relative strength of a magnetic field is represented by the spacing of the field lines
- The direction of a magnetic field at a point is the direction of the force on the N pole of a magnet at that point
- The magnetic field isstrongest at the poles and becomesweaker as the distance from the magnet increases. This is because the magnetic field lines are gettingfurther apart
Why are there magnetic forces?
Magnetic forces are due to interactions between magnetic fields. Two bar magnets can attract or repel
What happens in the gap where two opposing poles are placed near each other?
A uniform magnetic field is produced in the gap where two opposing poles are placed a few centimetres apart
What is a uniform magnetic field?
- A uniform magnetic field is one that has thesame strength and direction at all points
- The lines are all parallel and equal distance apart to show magnetic field strength remains constant
- To show that the magnetic field is acting in the same direction at all points there must be an arrow on each magnetic field line going from thenorthpole to thesouthpole
What happens when two of the same poles are placed near each other?
When two of the same poles are placed near each other, they repel. In the centre there is something called a neutral point where no magnetic field exists
What are the two ways you can plot magnetic field lines?
Using iron filings, using compasses
How do you plot magnetic field lines using iron filings?
- Place a piece of paper or glass on top of the magnet
- Gently sprinkle iron filings on top of the paper/glass
- Carefully tap the paper/glass to allow the iron filings to settle on the field lines
How do you plot magnetic field lines using compasses?
- Place a magnet on top of a piece of paper
- Place a compass at one end of the magnet
- Draw a dot at the tip of the needle
- Move the compass so the needle lines up with the previous dot and so on
- When the dots are joined up the result is a magnetic field line
- More lines can be drawn by starting with the compass in different positions
What is charge measured in?
Coulombs
What happens when two charged objects are brought near eachother?
When two charged objects are brought near each other there is a force between the two objects
- Like charges repel (positive charges repel positive charges, negative charges repel negative charges)
- Opposite charges attract (positive charges attract negative charges)
What is an electric field?
An electric field is a region in which an electric charge experiences a force
Rules for field lines around a point charge
- The direction of an electric field at a point is the direction of the force on a positive charge at that point
- Field lines always point from positive charges to negative charges
- Strength of an electric field increases where the field lines are close together (the field is strongest close to the charged object)
- Field lines cannot cross together
Where do the arrows around a positive point charge face?
Positive charge the arrows face away
Where do the arrows around a negative point charge face?
Negative charge the arrows face inwards
What does the relationship between the strength of the force and distance apply to?
The relationship between the strength of the force and the distance applies to both the force of attraction and force of repulsion. Two negative charges brought close together will have astrongerrepulsive force than if they were far apart
What are the field lines around a charge conducting sphere?
The field lines around a charge conducting sphere are symmetrical, as with a point charge
- This is because the charges on the surface of the sphere will be evenly distributed, the charges are the same, so they repel and the surface is conducting allowing them to move
- This field line pattern can be demonstrated using a Van der Graaff Generator
What are the field lines between two parallel conducting plates?
- It creates a uniform electrical field, where lines are parallel and same space between each line
- The field lines are always directed from the positive plate to the negative plate
What is a conductor and what does it tend to be?
A conductor is a material that allows charge (usually electrons) to flow through it easily. This is because it has free moving electrons. Conductors tend to be metals. Ex: copper, aluminum, gold and silver
What is an insulator? give some examples
An insulator is a material that doesn’t allow the flow of charge through it easily because it has no free moving charges. Ex: rubber, plastic, glass and wood
Why do metals conduct electricity well?
Metals conduct electricity very well because current is the rate of flow of charged particles and metals have free moving electrons that have escaped from the outer shell of metal atoms, they are free to move throughout the structure and carry the charge
Why do some insulators conduct?
Some non-metals, such as wood, allow some charge to pass through them. Although they are not very good at conducting, they do conduct a little in the form ofstaticelectricity. For example, two insulators can build up charge on their surfaces. If those surfaces touch, this would allow that charge to be conducted away
How to distinguish between conductors and insulators?
To distinguish between conductors and insulators agold-leaf electroscope (GLE)can be used
The GLE consists of
- Ametal plateattached to one end of ametal rod
- At the other end of theroda verythin leaf of gold foilis attached
- The rod is held by aninsulating collarinside a box withglass sides, allowing the gold leaf to both be seen and protected from draughts
When the GLE is charged, the plate, rod and gold leaf have the same charge (either positive or negative) - Since the rod and leaf have thesame charge, theyrepel, and the leaf sticks out to the side
- When the rod and leaf are discharged (areneutral) the leafhangs down
Charge the plate of the GLE so that the gold leaf stands clear of the rod
What are the observations of whether an item is a conductor or an insulator using gold leaf electroscope?
- Record the observations each time- Leaf falls: material is agoodconductor
Leafremainsin place: object is apoorconductor (good insulator)
Leaffalls slowly: material is apoorconductor
What is static electricity?
Static electricityrefers to the accumulation of charge on an object, which then attracts other objects or can even produce sparks.
What is charging by friction?
When certain insulating materials are rubbed against each other they becomeelectrically charged. This is calledcharging by friction and the charges remain on the insulators and cannot immediately flow away
Give two examples of charging by friction
- Ex: When a polythene rod is rubbed with a woolen cloth, the friction causes the electron transfer from the wool to the polythene. This causes the woolen cloth to become positively charged and the polythene rod to become negatively charged
- Ex 2: Inflate a balloon and rub it quickly on any dry surface e.g. a carpet. Then open a tap and hold the balloon next to it (without touching the water). You should see that the water bends towards the balloon. This is because the negatively charged balloon attracts the positive charges in a stream of water, it bends towards the balloon because the attractive forces between unlike charges is greater than the repulsive force between like charges
How does charging by friction work?
When two different materials are rubbed together, friction occurs. This friction provides energy that can cause electrons to move from one material to the other. The material that loses electrons becomes positively charged because it now has more protons than electrons. The material that gains electrons becomes negatively charged because it now has more electrons than protons. When charging by friction, it is only theelectronsthat can move, not any ‘positive’ charges
What is current?
Current is the amount of charge passing a point per second (rate of flow of charge)
Formula for current
I = Q/t (current= charge/time)
Unit for current
The unit for current is ampere (A) or coulomb per second (C/s)
What is conventional current in a circuit?
Conventional current is from positive to negative
What is the flow of electrons in a circuit?
The flow of electrons is negative to positive
What are ammeters used for and how are they connected?
Ammeters are used to measure current and they must be connected in series (meaning in line with the circuit)
What are the two ways ammeters can be?
Digital (with an electronic read out) or Analogue (with a needle and scale)
Features of analogue ammeters
- Typical ranges are 0.1-1.0 A and 1.0-5.0 A for analogue ammeters
- They should be checked forzero errorsbefore using, if not at zero, you will need to subtract this from all your measurements.
- They are also subject toparallax error
- Always read the meter from a position directly perpendicular to the scale
Features of digital ammeters
- Digital ammeters can measure very small currents, in mA or µA, they 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. However digital displays may ‘flicker’ back and forth between values and a judgement must be made as to which to write down
- Digital ammeters should be checked forzero error before using, if not at zero, you will need to subtract this from all your measurements.
Uses of each range of ammeters
Low Range Ammeters (e.g., 0-1mA, 0-10mA):
- Applications: Used for measuring small currents in sensitive electronic circuits, such as in laboratory experiments and in testing electronic components.
Medium Range Ammeters (e.g., 0-1A, 0-10A):
- Applications: Suitable for general-purpose current measurements in electrical circuits, household appliances, and automotive diagnostics.
High Range Ammeters (e.g., 0-100A, 0-1000A):
- Applications: Used for measuring high currents in industrial settings, power distribution systems, and large machinery.
What is the current caused by in a metal conductor?
In a metal conductor the current is caused by the flow of the free moving delocalised electrons. The greater the flow of charge, the greater the current.
What are the two types of current?
direct current (DC) and alternating current (AC)
What happens in DC?
In DC the electrons flow steadily in a single direction from negative to positive
What happens in AC?
Electrons keep switching directions, going forward and then backwards in AC
When is DC and AC produced?
DC is produced when using dry cells and batteries while AC is produced from mains electricity of house and generators
What does the graph for DC look like?
X-axis: Time
Y-axis: Current (+ and -)
A straight line above the x axis
What does the graph for AC look like?
Going up and down like a sine graph
What is the electromotive force?
Electromotive force is the electrical work done by a source to drive a unit charge around a complete circuit (basically the voltage supplied by a power supply).
What are the units for electromotive force?
It is measured in volts (V) or joules per coulomb (J/C)
What is the equation for electromotive force?
The equation for electromotive force is E=W/Q (E=electromotive force (V), W=work done (J), Q=charge (C))
What is potential difference and what is it measured in?
Potential difference is the work done by a unit charge passing through a component. It is measured in volts (V)
What is the equation for potential difference?
The equation for potential difference is V=W/Q (V=potential difference (V), W=work done (J), Q=charge (C))
What is potential difference measured with, how is it connected and what are the two types?
Potential difference is measured using a voltmeter, which can be either: Digital (with an electronic read out) or Analogue (with a needle and scale). Voltmeters are connected inparallelwith the component being tested
Features of analogue voltmeters
- Analogue voltmetersare subject toparallax error, always read the meter from a position directly perpendicular to the scale
- Typical ranges are 0.1-1.0 V and 0-5.0 V for analogue voltmeters although they can vary and check for zero error
Features of digital voltmeters
- Digital voltmeterscan measure very small potential differences, in mV or µV. They 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. However digital displays may ‘flicker’ back and forth between values and a judgement must be made as to which to write down
- Digital voltmeters should be checked forzero error
Uses of each range of voltmeters
Low Range Voltmeters (e.g., 0-1V, 0-10V):
- Applications: Used for measuring small voltages in sensitive electronic circuits, such as in laboratory experiments and in testing electronic components.
Medium Range Voltmeters (e.g., 0-100V, 0-300V):
- Applications: Suitable for general-purpose voltage measurements in electrical circuits, household appliances, and automotive diagnostics.
High Range Voltmeters (e.g., 0-1000V, 0-5000V):
- Applications: Used for measuring high voltages in industrial settings, power distribution systems, and large machinery.
What is resistance?
Resistance is the ratio of potential difference to current
What is the equation for resistance?
The equation is R=V/I (R= resistance (ohms, Ω or volts per ampere V/A), V= potential difference (volts, V), I= current (amperes, A))
Unit for resistance
ohms, Ω
What is Ohm’s law?
Ohm’s law states that current is directly proportional to potential difference as long as the temperature remains constant. Therefore: resistance and current are inversely proportional, resistance and voltage are directly proportional)
What is a resistor and variable resistor used for?
A resistor is the compound in a circuit that is used to control the current, while a variable resistor is used to vary the current in the circuit
Experiment to find resistance in a circuit
Voltmeter measured voltage/p.d. and ammeter measures current, so you can use these devices to determine the resistance between two points in a circuit.
- Set up an ammeter somewhere in the series circuit; this will give you the amount of current flowing in the circuit.
- Next set up a voltmeter in parallel to the object, in this case a light bulb, to find the potential difference across it.
- Using the equation R = V/I , we can find the resistance.
What does the current-voltage graph look like for a resistor of constant resistance?
For a resistor of constant resistance, the current is proportional to potential difference so the graph will be a straight line
What does the current-voltage graph look like for a filament lamp?
For a filament lamp, the current increases at a proportionally slower rate than the potential difference. This is because the current causes the filament in the lamp toheat up and as the filament gets hot, itsresistance increases. This opposes the current, causing it to increase at aslower rate. This causes the graph to curve slowly
What does the current-voltage graph look like for a semiconductor diode?
For a diode which is a non-ohmicconductor that allows current to flow inonedirection only, the direction is shown by the triangular arrow called forward bias, in the reverse direction the diode has very high resistance, and thereforenocurrent flows which is called the reverse bias. This is shown in the graph as when the diode is inforward bias,the graph shows a sharp increase in voltage and current (on the right side of the graph), when the diode is switched around, inreverse bias, the graph shows a flat line where current is zero at all voltages (on the left side of the graph). The left side is on the x-axis and the right side is a sharp upward curve
What factors affect resistance?
Length, cross sectional area
What is the relationship between length and resistance?
If the wire is longer, each electron will collide with more ions and so there will be more resistance. Resistance and length are directly proportional
What is the relationship between thickness and resistance?
Resistance and cross-sectional area are inversely proportional. When the area of the cross-section of the conductor is increased, space between charged particles will also increase. So the probability of collision of electrons will also decrease and more electrons can flow.
