23, Magnetic Fields

Question 1

What direction does the arrow point on a magnetic field line?

Answer 1

The direction a free north pole would move, i.e. from north to south.

Question 2

How are magnetic fields created?

Answer 2

From the movement of charge.

Question 3

How do bar magnets have magnetic fields?

Answer 3

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.

Question 4

How can you create a close uniform magnetic field?

Answer 4

A coiled current carrying wire (solenoid) has a close uniform field through the middle of the coils.

Question 5

How can you find the magnetic field lines of a magnet?

Answer 5

Use a plotting compass and move it around the magnet.

Question 6

How can you find the direction of a magnetic field in a wire carrying a current?

Answer 6

Right-hand grip rule, point thumb upwards in the direction of the current, your fingers curl in the direction of the field.

Question 7

What is the unit of magnetic flux density?

Answer 7

Tesla (T) (Nm^-1 A^-1).

Question 8

How can you find the direction a force is applied on a current carrying wire in a magnetic field?

Answer 8

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).

Question 9

What does the equation F = BIL sinθ mean?

Answer 9

Force = magnetic flux density * current * length of wire * sin (angle between current and the field).

Question 10

How is the direction of current denoted when it is moving in or out of the page you’re looking at?

Answer 10

Circle with a dot represents current towards you, circle with cross represents current away from you.

Question 11

How can magnetic flux density between two magnets be measured?

Answer 11

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.

Question 12

How can the path a charge takes through a magnetic field be investigated?

Answer 12

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.

Question 13

How can the path of a charge through a magnetic field be predicted?

Answer 13

The charge will feel a constant perpendicular force, therefore circular motion can be used.

Question 14

What does the equation F = BQv show?

Answer 14

The force applied to a charge Q moving through a magnetic field of flux density B at velocity v.

Question 15

What is a velocity selector and how does it work?

Answer 15

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.

Question 16

What does the equation ϕ = BA cosθ show?

Answer 16

Magnetic flux = Magnetic flux density * cross sectional area * cos (angle between the perpendicular to the area and the magnetic field).

Question 17

What is the unit of magnetic flux?

Answer 17

Weber (Wb or Tm^2 or NmA^-1).

Question 18

What is magnetic flux linkage?

Answer 18

Denoted by Nϕ it is the product of the number of coils (or turns of a coil) and the magnetic flux through the coil.

Question 19

How can an E.M.F be induced in a circuit?

Answer 19

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.

Question 20

State Faraday’s law.

Answer 20

The E.M.F. is directly proportional to the rate of change of magnetic flux linkage.

Question 21

State Lenz’s law.

Answer 21

The direction of the induced E.M.F. or current is always such as to oppose the change producing it.

Question 22

What does the equation ε = - Δ(Nϕ)/Δt mean and where does it come from?

Answer 22

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.

Question 23

How does an AC generator work?

Answer 23

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.

Question 24

Given a graph of flux linkage against time how can you derive an ε-t graph?

Answer 24

ε equal to rate of change of flux linkage so draw the graph of the differential.

Question 25

What is a transformer?

Answer 25

An electrical component which changes the P.D. and current through a wire.

Question 26

Why can’t transformers work on a D.C. power supply?

Answer 26

Because in a D.C. power supply there is no change in current so there is no Δ(Nϕ).

Question 27

What does the equation n_s/n_p = V_s/V_p = I_p/I_s show?

Answer 27

The ratio of coils in the primary and secondary coils is equal to the ratio of P.D.’s in an ideal transformer.