7p1 Electric and Magnetic Fields

Question 1

What is an electric field?

Answer 1

A region in which a charged particle experiences a non-contact force of attraction or repulsion

Question 2

Describe the field lines of a uniformly charged sphere (point charge) for positive and negative?

Answer 2

Positive - radial field with charge concentrated at the centre and field lines pointing away from the centre , negative - radial field with charge concentrated at the centre and field lines pointing towards the centre

Question 3

What does Coulomb’s Law state?

Answer 3

The magnitude of the force of attraction or repulsion between two point charges in a vacuum is directly proportional to the product of their charges and inversely proportional to the quare of the distance between them

Question 4

What is electric field strength and the equation to find it with units (write equation in words or write what each symbol in the equation means) ?

Answer 4

The force per unit positive charge, E = F/Q, E - electric field strength (NC-1), F - force(N), Q - charge

Question 5

What is electric potential at a point?

Answer 5

It is the potential energy per unit of positive point charge at that point in the field

Question 6

What’s the electric potential and force (attractive or repulsive) when the charge is negative?

Answer 6

The potential is negative and the force is attractive

Question 7

What’s the difference between a uniform field and radial field?

Answer 7

Uniform field exerts the same electric force everywhere in the field, radial field exerts a force depending on the distance between the two charges

Question 8

What equation is used to calculate the electric field strength with two parallel plates?

Answer 8

E = V/d - E is electric field strength, v is pd across plates and d is distance between plates

Question 9

What equation is used to find the electric potential difference in a radial field?

Answer 9

V = Q / 4πε0r -V is electric potential difference, r is distance between charges and Q is charge

Question 10

Draw parallel plates with field lines and equipotential lines?

Answer 10

parallel plates - top is positive and bottom is negative - arrows going from top plate to bottom plate and equipotential lines are parallel to the plates

Question 11

What is the acceleration of an electron at the point in an electric field where the electric field strength is 7 x 105 NC-1?

Answer 11

F = EQ, F=ma (1 - equations), ma=EQ 9.11x10-31 x a = 7x105 x 1.6x10-19 = 1.23 x 1017

Question 12

What is a capacitor

Answer 12

A capacitor is an electrical device consisting of two metal plates with an insulating material between - the insulator is known as a dielectric normally ceramic, paper and air

Question 13

What is the purpose of a capacitor

Answer 13

Capacitors are used to store charge - one plate becomes positively charged and the other plate becomes negatively charged

Question 14

How does charging a capacitor work

Answer 14

When a capacitor is connected to a power supply such as a battery - electrons flow from the battery and are deposited on one plate of the capacitor becoming negative - the other side loses electrons becoming positive - both plates have same magnitude of charge but opposite signs - creating a potential difference across the plates

Question 15

How does discharging a capacitor work

Answer 15

When a charged capacitor is connected to a circuit it becomes a power source - the excess electrons flow from the negative plate around the circuit to the positive plate - this causes the charge of the capacitor to decrease

Question 16

When charging a capacitor when does the circuit current become zero

Answer 16

When the emf is equal to the pd across the capacitor plates meaning the capacitor is fully charged

Question 17

What happens when you add two capacitors in parallel

Answer 17

The total capacitance is equal to the sum of the individual capacitance of the capacitance - the voltage is the same in each branch and the total charge and current is shared

Question 18

What happens when you add two capacitors in series

Answer 18

The total capacitance is 1/Ct = 1/C1 + 1/C2 where Ct is the total capacitance and C1 and C2 are the individual capacitance - the voltage is shared between components and the current and charge are the same

Question 19

What is capacitance

Answer 19

Capacitance is the amount of charge stored per unit of potential difference

Question 20

Whats the equation for capacitance

Answer 20

C = Q/V

Question 21

What is the unit for capacitance

Answer 21

Farads

Question 22

How to find the energy stored by a capacitor

Answer 22

using graph and equation It is the area under a graph of charge against potential difference - since they are proportionate the graph is a straight line though the origin so W = ½ QV - W is energy stored by capacitor Q is charge stored and V is pd across

Question 23

How to derive formula W = ½ CV^2

Answer 23

Sub Q = CV into W = ½ QV

Question 24

How to derive formula W = ½ Q^2 / C

Answer 24

Sub V = Q/C into W = ½ QV

Question 25

How is the time constant calculated

Answer 25

The time constant is the product of resistance and capacitance (RC)

Question 26

What is the time constant

Answer 26

It is the time take the discharge a capacitor to 1/e or 0.37 of initial value or the charge a capacitor to 1-1/e or 0.63 of its maximum value

Question 27

How to calculate time constant from graph

Answer 27

Find the time where the values are 0.37 of initial value for discharging or 0.63 of max value id charging

Question 28

How to derive equation for current and pd exponential discharging

Answer 28

Use charge equation (Q = Q0e-1/RC) and sub in Q=CV to get the one for V and then sub in V=IR to that one to get the one for I

Question 29

Describe an experiment to analyse the pd across a capacitor as it charges and discharges

Answer 29

Set up a circuit with DC power supply, high resistance resistor, switch capacitor , ammeter and voltmeter around the discharged capacitor. Set the capacitor only connected to the power supply and start the timer - record pd and current every 10 seconds - repeat three times and calculate mean V and I values - plot graph of current against time and pf against time - when discharging capacitor set it to the discharge position and record pd and current every 10s as it discharges - repeat three times calculate mean V and I and plot graph of current against time and pd against time

Question 30

What is magnetic flux density and the units

Answer 30

It is a measure of the strength of the magnetic field and its measured in Tesla

Question 31

What is magnetic flux and the equation to find it

Answer 31

It describes the magnetic field or field lines passing through a given area - when the field is perpendicular to the area then Magnetic flux(Ф) = Magnetic flux density (B) x Area (A)

Question 32

What is magnetic flux linkage

Answer 32

It is the magnetic flux multiplied by the number of turns of a coil

Question 33

Equation to find the magnitude of force exerted on a particle moving in a magnetic field

Answer 33

F = BQv sinθ - B is magnetic flux density, Q is charge and v is velocity and θ is the angle between the velocity of the particle and the direction of the magnetic field

Question 34

What does each finger represent in fleming's left hand rule

Answer 34

Thumb - motion, first finger - field direction and second finger is conventional current

Question 35

What shape does the magnetic field form when there is a current passing through a wire

Answer 35

Concentric rings around the wire - use right hand to figure out direction thumb points to negative

Question 36

What equation is used to find the force exerted on a current carrying wire

Answer 36

F = BIL sinθ - B is magnetic flux density, I is current and L is length and θ is the angle between the current and direction of the magnetic field

Question 37

What happens when a permanent magnet moves relative to a coil of wire

Answer 37

The electrons in the coil will experience a force and build up on one side - causing emf to be induced in the coil known as electromagnetic induction

Question 38

Describe faraday’s law

Answer 38

Faraday's law says the magnitude of induced emf is equal to the rate of change of flux linkage

Question 39

Describe lenz’s law

Answer 39

Lenz's law says the direction of induced current is such to oppose the motion causing it

Question 40

What factors affect the emf induced in a coil when there is a change of current in another coil linked to this coil

Answer 40

Magnetic flux density is directly proportional to induced emf, distance between two coils is inversely proportional induced emf, number of turns is directly proportional induced emf, area of cross section is directly proportional to the induced emf and time take for charge in current is inversely proportional to induced emf

Question 41

How to demonstrate lenz's law

Answer 41

Drop a magnet into a coil - as the magnet approaches the coil there is a rate of change of magnetic flux so the change in flux linkage induces an emf causing a current to be induced - due to the law the direction of current is acting opposite to the change in magnetic flux linkage so the force exerted on the magnet opposes the motion so the same pole of the magnet will be induced to repel the magnet - causing it to slow down - as the magnet passes through the centre of the coil there is not change in flux so no emf induced - as the magnet leaves the coil the same thing occurs except to oppose magnetic flux linkage the pole will be induced to attract the magnet

Question 42

How does lenz law relate to energy conservation

Answer 42

If the opposite were true then the magnet would speed up in the same process so kinetic energy increases causing energy to have been created from nothing which is a violation of the conservation of energy

Question 43

What is peak value and root mean square applied to ac current

Answer 43

Peak value is the max value from equilibrium, root mean square is the average of all the squares of the possible values which gives the effective value of current from an alternating current source

Question 44

How to calculate rms values for current and voltage

Answer 44

I(rms) = I/(sqrt 2) V(rms) = V/(sqrt 2) where I and V are peak values of current and voltage

Question 45

What happens if a magnet dropped through a copper tube with a slit through it

Answer 45

The slit will limit the size of the induced current, hence a smaller force will oppose the motion of the magnet, so the time taken will be less

Question 46

Is using a timer suitable to measure times

Answer 46

Manual timing will be affected by reaction time so the shorter the time being measured the greater the effect the reaction time will have