Electric and Magnetic Fields Flashcards
What is an electric field
A region in which a charged particle experiences a non-contact force, attraction or repulsion. Like a force field.
What is electric charge measured in
Coulombs
What happens when a charged object is placed in an electric field
The object will experience a force
If the charged object is a uniformly charged sphere, what do you assume
Assume all of its charge is at its centre, it behaves like a point charge with a radial field.
How are electric fields represented
With field lines
What does coulombs law tell you
The force of attraction or repulsion between 2 point charges in a vacuum.
What is coulombs law
F = Q1Q2/4piE_0r^2
What is the relationship between Q1 and Q2 in coulombs law
The FORCE on Q1is always equal and opposite to the force on Q2, the direction of the force depends on the charges. This means regardless of their attraction, the force on Q1 and Q2 will always be equal and opposite
What happens to force between charged particles as they get further apart
Coulombs law is an inverse square law, the further apart the charges, the weaker the force between them.
What happens to Coulombs law if the particles are not in a vacuum
E_0, permittivity in vacuum, is replaces by E, permittivity of the material they are in. Air is treated as a vacuum.
How do you treat the permittivity of air when using coulombs law
Permittivity of air is same as permittivity in vacuum. Keep it as E_0
How do you experimentally measure the force between 2 charges
Use an electronic balance. Fix a charged sphere to a mass balance and zero the balance. Clamp another charged sphere carrying the same charge directly above the first sphere (make sure the 2 don’t touch). The spheres will repel each other, causing the lower sphere to push down on the scale, so the scales will register a mass. Convert mass reading on the scales into a force using F=W=mg
If you vary the distance between spheres, r, then you should find F is directly proportional to 1/r^2
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How do you charge spheres in the experiment to find force between 2 charges
By connecting the spheres to a power supply
What is electric field strength, E
Force per unit positive charge. The force that a charge of +1C would experience if placed in the electric field.
What is the equation for electric field strength
E = F/Q
What are the units for electric field strength
N C^-1
How does electric field strength vary in a radial field and in a uniform field
In a radial field, electric field strength depends on where you are in the field. In uniform field, electric field strength is the same everywhere
What direction is electric field strength pointing in
Electric field strength is a vector pointing in the direction that a positive charge would move
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What type of field does a point charge have
Radial field
Where is electric field the strongest in a radial field
The area where the field lines are closest together, highest field line density area
Which direction would the field lines on a radial field point for a negatively charged particle
Point towards the particle. Field lines point in the direction a proton would travel, and protons are attracted to negative charge, so they would go towards negative charge.
Which direction would the field lines on a radial field point for a positively charged particle
Away from the particle
What does the electric field strength depend on in a radial field
The distance from the point charge and magnitude of charge on point charge
How does electric field strength decrease as you get further away from the point charge in a radial field
Inverse squarely. E is inversely proportional to r^2. The further away the field lines are from each other, the weaker the electric field strength on a radial field.
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How is a uniform electric field produced
By connecting 2 parallel plates to the opposite poles of a battery
How are the field lines drawn in a uniform electric field
The field lines are parallel to each other and point from the positive plate to the negative plate
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How far apart are the field lines in a uniform electric field
They are the same distance apart from each other, equidistant. This means the field strength is the same everywhere in the field.
What does the electric field strength depend on in a uniform electric field
The potential difference between parallel plates, and the distance between the plates.
What is the equation for electric field strength of a uniform electric field
E = V/d
E is measured in Vm^-1
PRAC, how do you investigate charged drops in a uniform electric field
An atomiser creates a fine mist of oil drops that are charged by friction as they leave the atomiser. When the circuit is witched off, a drop falls from the top plate to the bottom plate due to its weiht. When the circuit is switched on, the pd between the plates creates a uniform electrci field, which exerts a force on the oil drop. A negatively charged oil drop can be made to float between the plates bu balancing the upaward fromce from the electric field with the downaward force of the oil drops weight by adjusting the voiltage between plates.
EQUATIONS
If you increase the pd while a drop is floating between plates, i.e the forces on the drop are balanced, you increase field stregnth. This means the forces on the oil willno longer be balanced so it will accelerate towards the positive top p;ate because the force due to the electric field is greater than its weight. If you increase the distance between the plates or decrease the pd, you reduce the field strength and therefore the electric force on the oil drop. The oil drop will acceleerate towards the bottom due to its weight being larger than the force due to the electric field.
If theres no electric field, the drop will accelerate until it reaches terminal velocity, when drag = weight of the drop
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What is an electric potential
The electric potential (voltage) a particle with +1C would have at a specific point in an electric field.
What does electric potential of a point depend on
How far it is from the charge, and the size/magnitude of the charge
What is the equation for electric potentials IN A RADIAL FIELD
V = Q/4piE_0r
V - electric potential (V)
Q - size of charge (C)
r - distance from charge (m)
What does the sign of V depend on in an electric potential
The sign of Q. If Q is positive, the force is repulsive and V is positive. If Q is negative, the force is attractive, and V is negative.
How does V change in electric potentials as you get further away from the point charge
Magnitude of V is greatest directly next to the charge, and decreases as the distance from the charge increases. V will be zero at an infinite distance from the charge.
What do the graphs of V against r look like for repulsive and attractive forces, for electric potentials
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Repulsive force, V is initially positive and tends towards zero as r increases towards infinity
Attractive force, V is initially negative and tends to zero as r increases towards infinity.
What does the gradient of a tangent of a V against r graph tell you, V being electric potential
Field strength at that point. E = ΔV/Δr
How can you find ΔV (change in electric potential) from a graph of E, electric field strength, against r, distance from charge, between 2 points. Change in electric potential between 2 points
Find the area under the graph between your 2 chosen points
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What is an equipotential
A line highlighting the area where the electric potential is equal.
What do equipotentials look like in radial fields
Wherever the radius of 2 points is equal, that is where the equipotential is. Both points have to be the same distance from the centre to have the same electric potential
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What do equipotentials look like in uniform fields
They are flat planes perpendicular to the direction of travel by charge. For example, each area that is within the +200V range between the 2 parallel plates will have the same electric potential.
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How does charge move along an equipotential
No work is done when you are traveling along an equipotential. An electric charge can travel along an equipotential without any energy being transferred.
What does a capacitor do
Store electrical charge
How are capacitors made
Two conducting plates separated by an air gap or insulating material.
What is the circuit symbol for capacitor
–||–
How does a capacitor work
When a capacitor is connected to a power source, positive and negative charge builds up on opposite plates. The insulating material stops charge moving between the 2 plates so a potential difference is created. This creates a uniform electric field between plates.
What is the unit to measure charge stored by a capacitor
Capacitance, the charge per unit voltage stored by a capacitor
What is the equation for capacitance, and units
C = Q/V
C is measured in Farads, F. Usually expressed as microfarads or nanofarads
Q - Coulombs
V - Voltage
What are farads usually expressed as in size
Farad is a huge unit, so capacitance is often in microfarads or nanofarads.
How does a capacitor work in a circuit
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When the switch is flicked to the left, and the circuit is split up, charge builds up on the plates of the capacitor. Electrical energy provided by the battery is stored by the capacitor. If the switch is flicked right, and the circuit is connected, the charge stored on the plates will discharge from the capacitor to the light bulb. Electrical energy turns to heat and light energy.
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Why is some energy lost when charging the capacitor
Work is done removing negative charge from one plate and depositing it onto the other plate to charge the capacitor. This comes from the electrical energy of the battery. This is given by
charge x average p.d
How do you find work done in a Voltage against Charge graph
You can find energy stored from the area under a graph of p.d against charge stored IN CAPACITOR
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p.d across capacitor and charge stored on capacitor is proportional to each other, so the graph will be a straight line through the origin. The energy stored is given by the yellow triangle
What is energy stored on capacitor equal to
Energy stored by capacitor is equal to work done
What is the equation for energy stored by capacitor
W = 1/2 QV
What are the other 2 forms of W = 1/2 QV
W = 1/2 CV^2
W = 1/2 Q^2/C
These are done by substituting variations of C = Q/V into the original equation. Q = CV for the top one, V = Q/C for the bottom one
How do you investigate what happens when you charge a capacitor
Set up circuit, close switch and connect uncharged capacitor to power supply, let capacitor charge while data logger records p.d (from voltmeter) and current (from ammeter) over time. When current through ammeter is 0, capacitor is fully charged. You can use a computer to plot a graph of charge, p.d or current against time.
How do you know when a capacitor is fully charged in the experiment investigating what happens when you charge a capacitor
When the current on the ammeter is 0, capacitor is fully charged. This is also when the p.d across the capacitor is equal to the p.d across the power supply.
How do you generate a graph of Charge over time, Q-t graph, when you only have readings for current, voltage and time in the experiment where you charge a capacitor
ΔQ = IΔt. Charge transferred in a given time is equal to the area under the I-t graph up to that point. This is how you generate the Q-t graph
What is Q_0, V_0, I_0 in capacitor graphs
Q_0 - The charge on the capacitor when its fully charged,
V_0 - the potential difference across the capacitor when its fully charged
I_0 - the maximum current flowing through the circuit
How do you set up the circuit in the investigation where you charge a capacitor
How do you explain the shape of the Q-t graph for the investigation of what happens when you charge a capacitor
As soon as switch is closed, current starts to flow. The electrons flow onto the plate connected to the negative terminal of the power supply, so a negative charge builds up. The build up of negative charge repels electrons off the plate connected to the positive terminal of the power supply, making that plate positive. Electrons are attracted to the positive side.
How do you explain the shape of the V-t graph for the investigation of what happens when you charge a capacitor
An equal but opposite charge builds on each plate, causing a p.d between plates. No charge can flow between plates because they are separated by an insulator
How do you explain the shape of the I-t graph for the investigation of what happens when you charge a capacitor
Initially the current through the circuit is high. At the start, the CURRENT IS HIGH. But as charge builds up on the plates, electrostatic repulsion makes it harder and harder for more electrons to be deposited. When p.d across capacitor is equal to p.d across the power supply, the current falls to 0. Capacitor is fully charged.
What happens to charge, potential difference, and current as a capacitor is charged
Charge increases, potential difference increases, current decreases
What happens to current, charge and p.d as the capacitor discharges
Current decreases, charge decreases, p.d decreases
How does an oscilloscope work
Basically, screen is divided into divisions. Y-axis is volts, x-axis is seconds. You use Gain to adjust scale of y-axis and timebase to adjust scale of x-axis. This helps you read measurements easier.
The screen of an oscilloscope is split into squares called divisions. Vertical axis is in volts. Volts per division shown on this axis is controlled by gain dial. This is the scale of the y-axis. The horizontal axis is in seconds. Seconds per division controlled by timebase dial. (scale of x-axis). Time base can be turned on and off. You can alter gain and timebase to make it easy to read off measurements. There’s also a button to freeze whatever’s on screen.
What do you get when you turn the timebase off, on an oscilloscope when you connect it across a charging capacitor
You just get a dot in the middle of the display which tells you the p.d across the capacitor. If you take regular readings of this p.d for a charging and discharging capacitor, you can plot V-t graphs
What do you get when you turn the timebase on, on an oscilloscope when you connect it across a charging capacitor
If you turn timebase on and adjust the dial to fiddle with the resolution, you can get the oscilloscope to plot the voltage-time graphs for you on the display. Once capacitor is fully charged, you can press stop (freeze) button and copy the information from the display.
What is the equation to calculate the charge left on the plates after a capacitor begins discharging from being fully charged.
The equation for the charge graph.
What are the equations of voltage and current in a discharging capacitor
Why is the time constant, 𝜏, so specific at 37%
If you take the value of time as 𝜏, which 𝜏=RC and put it into the p.d, current or charge equations of a dicharging capacitor, you get an answer of 1/e. 𝜏 is basically the value of t which will turn the power of e to -1.
How long does it normally take for a capacitor to charge or discharge fully
About 5 RC
How do you find the time constant with log-linear graphs
Rearrange any discharging capacitor equation by taking natural logs (ln) of both sides and rearranging. You get an equation in the form of y=mx+c, so plot that graph of lnQ against time. To get time constant from the graph, divide -1 by the gradient of the line.
How do you rearrange the equation of charge for a discharging capacitor to find time constant through log-linear graphs
Take natural logs of both sides then rearrange
Which equations do you use to plot log-linear graphs for current and p.d of a discharging capacitor
How do magnetic field lines work
They go from the north to the south pole of a magnet. The closer the lines are, the stronger the field.
What do field lines around a bar magnet, and between 2 bar mangets with N N and N S look like
What do field lines around a wire look like
Concentric circles centred on the wire. This is the wire if you looked at it from a 2-D plane. Just the top circle of the cylinder
How do you use the right hand rule
Make a thumbs up with your right hand. The thumb shows you the direction of conventional current. People are conventionally right handed and you are currently holding your right thumb up because life is good. And the direction which your other fingers are curling is the direction of the field. The 3 fingers look like a sideways ‘m’ for magnetic field.
How do you use the right hand rule
Make a thumbs up with your right hand. The thumb shows you the direction of conventional current. People are conventionally right handed and you are currently holding your right thumb up because life is good. And the direction which your other fingers are curling is the direction of the field. The 3 fingers look like a sideways ‘m’ for magnetic field.
What is the magnetic field shape of a wire in a coil shape, and the magnetic field of a solenoid, a long spring of wire
What happens to the magnetic fields when you put a current carrying wire between 2 magnets (an external magnetic field)
The field around the wire and the field from the magnets are added together. This causes a resultant field – the lines closer together show where the magnetic field is stronger. The bunched lines cause a ‘pushing’ force on the wire.
How do you use flemings left hand rule to find the relationship between field, force, and current
First finger Field. SeCond finger Current. ThuMb is the direction of motion, which is the direction of force.
How does force on a wire change in a magnetic field as you change the angle of the wire
q
The more perpendicular the wire to the magnetic field, the larger the force it will experience. The more parallel, the less force it will experience.
What is the equation linking the angle between the wire, magnetic field, and flux density
F = BIℓ sinθ
Which component of magnetic field causes a current-carrying wire to experience a force
The force on a current-carrying wire in an external magnetic field is caused by the component of magnetic field perpendicular to the wire – Bsinθ
What is magnetic flux
Think of it like the total number of field lines
How does a conductor cutting through magnetic flux induce emf
If there is relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force, causing them to accumulate at one end of the rod. This induces an electromotive force (emf) across the ends of the rod.
How is an emf induced in a solenoid or flat coil
When the move the magnet towards or away from the coil, or when you move the coil towards or away from the poles of the magnet.
What happens when you change the direction you are moving the magnet through a solenoid
The change in direction will mean the change in voltage. If moving forward for example, a positive emf may be induced, but moving it backward induces a negative emf
What is the equation to find magnetic flux for a single loop of wire when B is not perpendicular to the plane of the loop.
For a single loop of wire, when B is not perpendicular to the plane of the loop, you can find the magnetic flux using equation:
Φ = BA cosθ
θ is the angle between the field and the normal to the plane of the loop. cosθ is the perpendicular component of the magnetic field.
How do you get change in flux linkage from an emf against time graph
Take the area under the graph
How do you get the emf from a flux linkage against time graph
Take the gradient of the graph. Remember emf is also rate of change of flux over time.
How do you get the emf from a flux linkage against time graph
Take the gradient of the graph. Remember emf is also rate of change of flux over time.
How do yoyu find the direction of the current/induced emf in electromagnetic induction
Lenz’s Law says induced emf will produce a force to oppose the motion of the conductor, a resistance. Use Fleming’s left hand rule and point your thumb in the direction of the resistance, so opposite direction of conductors motion. And then point your first finger in the direction of the magnetic field. Your second finger will give you direction of the induced emf, or if conductor is part of a circuit, a current will be induced in the same direction as the induced emf.
