Electromagnetism

Question 1

How to investigate transformers

Answer 1

Setup transformer circuit, voltmeter and ammeter on each side, alternating power supply, vary input voltage and record output voltage and current along with input current

Question 2

Efficiency of transformers

Answer 2

Soft iron easy to magnetise and demagnetise, laminated to reduce eddy currents, low resistance windings to reduce power loss due to heat

Question 3

Equation for an ideal transformer

Answer 3

n(s) / n(p) = V(s) / V(p) = I(p) / I(s)

Question 4

Simple laminated core transformer

Answer 4

Laminated iron core, primary and secondary coils, alternating current applied, varying magnetic flux in core links primary coil to secondary coil, varying e.m.f produced in secondary coil

Question 5

Simple a.c. generator

Answer 5

Consists of rectangular coil rotating in a magnetic field connected to slip rings and brushes connecting it to a circuit, maximum e.m.f occurs when coil is parallel to field because its direction of movement is perpendicular to the field and the rate of change of flux linkage is maximum

Question 6

Faraday’s Law

Answer 6

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

Question 7

Lenz’s Law

Answer 7

The direction of the induced e.m.f or current is always such as to oppose the change producing it

• An expression of conservation of energy

Question 8

Magnetic Flux Linkage

Answer 8

Product of magnetic flux and number of turns in the coil used, N ⏀ = N B A costheta

Question 9

Magnetic Flux

Answer 9

The product of the component of the magnetic flux density perpendicular to the area and the cross-sectional area, ⏀ = B A costheta

Question 10

Velocity selector

Answer 10

Two parallel horizontal plates connected to a power supply, magnetic field also applied, forces cancel out for particles of a certain speed and they move straight through the device, other particles are deflected

Question 11

How to determine the magnetic flux density between the poles of a magnet

Answer 11

Magnets on top pan balance with wire between them connected to ammeter and variable power supply, length of wire measured, current recorded, change in mass recorded, wire experiences upward force so magnets experience downward force, F = m g, B = F / I L

Question 12

Fleming’s left-hand rule

Answer 12

First finger FIELD, Second finger CURRENT, Thumb FORCE (or MOVEMENT)

Question 13

How to investigate magnetic flux using search coils How to investigate magnetic flux using search coils

Answer 13

Search coil placed between poles of a strong magnet at right angles to the magnetic field and then rapidly removed, connected to oscilloscope, calculate desired value using Faraday’s law

Question 14

Electromagnetic induction

Answer 14

Relative motion* between a conducting rod and magnetic field causes:
- electrons in rod to experience a force (BQv)
- so electrons accumulate at one end of rod
- and thus an e.m.f. is induced across the ends of the rod

• Some of the work done to move the conductor is transferred into electrical energy

Question 15

Investigating magnetic flux

Answer 15

Search coil: small coil of wire with known number of turns and known area

1. Connect search coil to data logger and set data logger to measure p.d. with a small time interval between readings
2. Keeping coil perpendicular to field, move it from the middle of the field to outside the field
3. Plot induced e.m.f. against time and find area under graph. Take the negative and divide by number of coils to find

Question 16

A.C. generator

Answer 16

• Converts K.E. into electrical energy
• Rotate coil uniformly to induce an alternating e.m.f.
• Flux linkage and e.m.f. vary sinusoidally
• Can change e.m.f. induced by changing speed of rotation or magnetic flux density
• Have slip rings and brushes to connect coil to external circuit

Question 17

Transformers - power loss and reduction

Answer 17

Eddy currents:
- Looping currents induced by changing magnetic flux in the core
- Their fields reduce the external field strength
- Dissipate energy by heat generation

Laminated cores:
- Have thin layers of insulator separating layers of core
- Minimise current flow to reduce power loss

Soft iron:
- Easy to magnetise and demagnetise so reduces power loss

Low-resistance metal for coils:
- Reduces power loss