23, Magnetic Fields Flashcards
What direction does the arrow point on a magnetic field line?
The direction a free north pole would move, i.e. from north to south.
How are magnetic fields created?
From the movement of charge.
How do bar magnets have magnetic fields?
From the electrons moving around the atoms, each atom acts like a magnet and a bar magnet is made from the atoms all pointing in the same direction.
How can you create a close uniform magnetic field?
A coiled current carrying wire (solenoid) has a close uniform field through the middle of the coils.
How can you find the magnetic field lines of a magnet?
Use a plotting compass and move it around the magnet.
How can you find the direction of a magnetic field in a wire carrying a current?
Right-hand grip rule, point thumb upwards in the direction of the current, your fingers curl in the direction of the field.
What is the unit of magnetic flux density?
Tesla (T) (Nm^-1 A^-1).
How can you find the direction a force is applied on a current carrying wire in a magnetic field?
Fleming’s left-hand rule, First finger in direction of Field, seCond finger in direction of Current and thuMb in direction of Motion (or force).
What does the equation F = BIL sinθ mean?
Force = magnetic flux density * current * length of wire * sin (angle between current and the field).
How is the direction of current denoted when it is moving in or out of the page you’re looking at?
Circle with a dot represents current towards you, circle with cross represents current away from you.
How can magnetic flux density between two magnets be measured?
Place the magnet on a top-pan balance and run a wire through the magnets, apply a current and the change in “weight” shows the force applied on the wire.
How can the path a charge takes through a magnetic field be investigated?
Use an electron deflection tube, an electron gun fires electrons through a screen which has a magnetic field through it, electrons interact with the path showing the path taken.
How can the path of a charge through a magnetic field be predicted?
The charge will feel a constant perpendicular force, therefore circular motion can be used.
What does the equation F = BQv show?
The force applied to a charge Q moving through a magnetic field of flux density B at velocity v.
What is a velocity selector and how does it work?
A device which only lets through charges with certain velocities by providing an electric and magnetic field and only letting through the charges which go straight, i.e. forces are equal so EQ = BQv and therefore v = E/B.
What does the equation ϕ = BA cosθ show?
Magnetic flux = Magnetic flux density * cross sectional area * cos (angle between the perpendicular to the area and the magnetic field).
What is the unit of magnetic flux?
Weber (Wb or Tm^2 or NmA^-1).
What is magnetic flux linkage?
Denoted by Nϕ it is the product of the number of coils (or turns of a coil) and the magnetic flux through the coil.
How can an E.M.F be induced in a circuit?
When there is a change in the magnetic flux linking the circuit, since ϕ = BA cosθ, any of these variables changing induces an E.M.F.
State Faraday’s law.
The E.M.F. is directly proportional to the rate of change of magnetic flux linkage.
State Lenz’s law.
The direction of the induced E.M.F. or current is always such as to oppose the change producing it.
What does the equation ε = - Δ(Nϕ)/Δt mean and where does it come from?
E.M.F. is equal to (in the opposite direction of) the rate of change in flux linkage, comes from combining Faraday’s and Lenz’s laws.
How does an AC generator work?
By spinning a coil around inside a magnetic field, from Faraday’s law and the definition of flux, ε = - Δ(BANcos(θ))/Δt, since area of coil, B and N are constant the E.M.F. is proportional to Δcos(θ)/Δt giving a sinusoidal curve for E.M.F. over time.
Given a graph of flux linkage against time how can you derive an ε-t graph?
ε equal to rate of change of flux linkage so draw the graph of the differential.
What is a transformer?
An electrical component which changes the P.D. and current through a wire.
Why can’t transformers work on a D.C. power supply?
Because in a D.C. power supply there is no change in current so there is no Δ(Nϕ).
What does the equation n_s/n_p = V_s/V_p = I_p/I_s show?
The ratio of coils in the primary and secondary coils is equal to the ratio of P.D.’s in an ideal transformer.
