How to investigate transformers
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
Efficiency of transformers
Soft iron easy to magnetise and demagnetise, laminated to reduce eddy currents, low resistance windings to reduce power loss due to heat
Equation for an ideal transformer
n(s) / n(p) = V(s) / V(p) = I(p) / I(s)
Simple laminated core transformer
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
Simple a.c. generator
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
Faraday’s Law
The magnitude of the e.m.f is directly proportional to the rate of change of magnetic flux linkage
Lenz’s Law
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
Magnetic Flux Linkage
Product of magnetic flux and number of turns in the coil used, N ⏀ = N B A costheta
Magnetic Flux
The product of the component of the magnetic flux density perpendicular to the area and the cross-sectional area, ⏀ = B A costheta
Velocity selector
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
How to determine the magnetic flux density between the poles of a magnet
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
Fleming’s left-hand rule
First finger FIELD, Second finger CURRENT, Thumb FORCE (or MOVEMENT)
How to investigate magnetic flux using search coils How to investigate magnetic flux using search coils
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
Electromagnetic induction
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
Investigating magnetic flux
Search coil: small coil of wire with known number of turns and known area
- Connect search coil to data logger and set data logger to measure p.d. with a small time interval between readings
- Keeping coil perpendicular to field, move it from the middle of the field to outside the field
- Plot induced e.m.f. against time and find area under graph. Take the negative and divide by number of coils to find
A.C. generator
- 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
Transformers - power loss and reduction
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
