6.3 Electromagnetism

Question 1

Magnetic fields are due to what?

Answer 1

moving charges or permanent magnets

Question 2

What direction do field lines go in?

Answer 2

north to south

Question 3

How do you know a field is uniform?

Answer 3

If field lines are equally spaced and in the same direction the field is uniform

Question 4

What is the Magnetic field patterns for a long straight current-carrying conductor

Answer 4

Field lines are concentric circles centred on the wire

Question 5

What is the Magnetic field patterns for a flat

coil?

Answer 5

Single coil-looks like: wire into page and out of page (circle with cross and dot) use right hand rule to know the field line directions

Question 6

What is the Magnetic field patterns for a long solenoid?

Answer 6

Row of wires into page and out, with uniform looking field lines between. Use right hand rule to draw field lines.

Question 7

What is Fleming’s left-hand rule?

Answer 7

Shows the direction of the force on a conductor carrying a current in a magnetic field.

Thumb: Force
First finger: Field
Second finger: Current

Question 8

Techniques and procedures used to determine the uniform magnetic flux density between the poles of a magnet using a current-carrying wire and digital balance

Answer 8

Place the horseshoe magnet on a set of digital balance scales and zero the scales.

Then, connect a straight wire to a DC power supply, a variable resistor and an ammeter in series.

Align the wire so that the force produced on the wire
is upwards using Fleming’s left-hand rule. This will ensure that the force on the magnet is downwards by Newton’s third law of motion i.e. the force on the wire will produce a force equal in magnitude but opposite in direction of the magnet.

Also, measure the length of the wire that is in the field using a ruler. The scales will then register the magnetic force as weight and give a value for the extra mass it assumes has been place on the scales.

Record a series of values for the current and the mass registered by the scales. The current can be varied by altering the resistance of the circuit using the variable resistor.

Finally plot a graph of the mass registered versus the current in the wire. By balancing the forces
the magnetic flux density can be determined from the gradient. BL/g (g and L constant)

Question 9

Define Magnetic flux density (B)

Answer 9

A measure of the strength of the magnetic field, defined by the equation for the force on a current carrying conductor in a magnetic field, F=BILsin(x)

Question 10

How do you find the Force on a charged particle travelling at right angles to a uniform magnetic
field

Answer 10

F=BQv

Question 11

What does Fleming left hand rule tell us about the motion of a charge particle in a uniform magnetic field?

Answer 11

magnetic field is always perpendicular to its direction of travel (mathematically a condition for circular motion), creates circular path

Question 12

how do you derive r=mv/BQ

Answer 12

f=mv^2/r and F=BQv

Question 13

What is a velocity selector?

Answer 13

Separate out particles of a certain velocity from a stream of accelerated charged particles at a range of speeds.

Question 14

How do velocity selectors work?

Answer 14

By applying both a magnetic and electric field at the same time perpendicular to each other, while a stream of particles is fired perpendicular to both fields at a device with a collimator.

Question 15

What are velocity selectors used for?

Answer 15

Mass spectrometer

Question 16

Only particles with what velocity can pass through the collimator in a velocity selector?

Answer 16

v=E/B

Question 17

What must you remember when looking at a velocity selector diagram?

Answer 17

Fleming left had rule
Like charges repel/opposites attract
Magnetic field from positive to negative

Question 18

What is magnetic flux symbol and unit?

Answer 18

φ

unit: Wb

Question 19

Define Weber

Answer 19

A change in flux linkage of one weber per second will induce an electromotive force of 1 volt in a loop of wire

Question 20

Define Magnetic flux linkage

Answer 20

the product of the magnetic flux and the number of turns on the coil. Unit = Weber turns

Question 21

Define Faraday’s law of electromagnetic induction

Answer 21

The induced e.m.f. is directly proportional to the rate of change of flux linkage

Question 22

Define Lenz’s law

Answer 22

The induced e.m.f. is always in such a direction as to oppose the change that caused it.

Question 23

What does lenz’s law say?

Answer 23

• If the og magnetic field is getting stronger, the induced magentic field will be in the opposite direction to try weaken it
• if the og magentic field is getting weaker, the induced magnetic field will be in the same direction to maintain it

Question 24

Techniques and procedures used to investigate magnetic flux using search coils

Answer 24

1. 2 bar magnets a small distance apart, opposite poles facing
2. Get search coil, connect to data recorder to record induced emf with small time interval between
3. place search coil in middle of magnetic field, so area is parallel to surface of magnet, immediately move coil out of field
4. e.m.f. induced changing from max to zero.
5. Plot graph of induced e.m.f against time
6. Area under curve=total flux linkage change, so can find B by dividing by A & N
7. repeat and find mean B.

Question 25

What is a search coil?

Answer 25

Small coil of wire with a known number of turns (N) and known area (A)

Question 26

How do generator, or dyanmos induce current?

Answer 26

Convert KE into electrical energy by rotating a coil in a magnetic field

Question 27

when does the output of pd and current change for a generator?

Answer 27

with every half rotation of the coil, producing ac current

Question 28

What do slip ring and brushes do?

Answer 28

Slip ring and brushes connect the coil to an external circuit

Question 29

What is a transformer?

Answer 29

Devices that use electromagnetic induction to change the size of the voltage for an alternating current

Question 30

Why should you use a laminated core?

Answer 30

reduces energy lost

Question 31

does the national gird transmit current at high or low value?

Answer 31

Low, P=I^2R, so doubling current, quadruples power loss.

Question 32

What contains transformers?

Answer 32

plug, to lower pd for appliance

Question 33

How do you investigate the number of turns, voltage across the coils?

Answer 33

1. 2 C-cores together, wrap wire around each to make coils. 5 tuns in primary, 10 turns in secondary (1:2) connected to low voltage ac supply (with voltmeter attached to each coil)
2. turn on ac supply to primary coil, record voltage across each coil
3. keep Vp the same, try 1:1 ratio, 2:1 ratio.
   Divide ns by np and Vs

Question 34

How do you investigate the number of turns, voltage and current of transformers in a circuit?

Answer 34

1. 2 C-cores together, wrap wire around each to make coils. 5 tuns in primary, 10 turns in secondary (1:2) connected to low voltage ac supply (with voltmeter attached to each coil)
   but also add variable resistor around primary coil and ammeter to both coils,
2. turn on power supply and record currents and voltages
3. leave number of turns constant, adjust variable resistor to change input current. record voltages and currents, repeate for range of input current
4. should find ns/np=vs/vp=is/ip

Question 35

Define electric potential

Answer 35

Work done per unit charge in bringing a positive charge from infinity to the point

Question 36

Define magnetic flux

Answer 36

Magnetic flux density * area normal to the field