Today4/9 Lab “Current Balance” Right Hand Rule for: Magnetic Force Magnetic field North Pole of a Current Loop Start reading Ch. 21 HW: 4/9 “Magnetism” Due Friday, 4/11

Magnetic Forces RHR A positive charge moves in a B-field that points to the right. What direction is the force on the charge for each case? F = qvB sin  Fingers with B, thumb with v, force points in the direction of your palm. v F big, out F = 0 F = 0 F smaller, in F smallest, in F big, in B  is the angle between v and B.

Magnetic Forces RHR A positive charge moves to the right in a B-field that points in the direction shown. What direction is the force on the charge? Fingers with B, thumb with v, force points in the direction of your palm. F = qvB sin   B F v F is out of the page, still perpendicular to the velocity.

Magnetic Forces, Wires Find the direction of the force on the wire placed in a B field. F = qvB sin F = IlB sin F qv becomes Il I The unit for B is “tesla” ot T eg: B = 4T B

Motion in B Field (for HW) A positive charge moves to the right in a B-field that points into the page. What is the acceleration? F = ma F What kind of motion? Uniform, circular B into the page v How big is the circle? a = v2/r (see circular motion) F = qvB sin

B Fields caused by ? The discussion so far has been about how charges respond to Magnetic fields, but what causes the Magnetic field? B-Fields are caused by moving charges, or currents. Wrap a wire around a nail-push current through the wire-produce a magnetic field

B Fields caused by Currents Compass needle and a wire RHR #2 for B-field due to current: Grip wire with thumb in direction of current-fingers wrap around wire. Can only give CW or CCW See fig. 21.25 in text

B Fields caused by Currents Two currents 1 2 Due to wire 2 Due to wire 1 Forces equal and opposite, 3rd law

B Fields caused by Currents Two currents 1 2 Due to wire 2 Due to wire 1 Forces equal and opposite, 3rd law

B Fields - Current Loop 1 1 Current loops 3 1 4 1 Now add vectors 4 2

B Fields - Current Loop The net field points one way inside the loop and the opposite way outside the loop. Current loops

B Fields - Current Loop Current loops

B Fields - Current Loop Current loops are magnets!!! Behave like magnets!!! N S

B Fields - Current Loop Current loops Phantom Bar Magnet N S

Magnetic Forces, Wires What happens to loop? Very complicated, turn the loop into a compass needle. Loop is a magnet (needle) centered in the loop with N facing away from you. I2 S

Magnetic Forces, Wires What happens to loop? Very complicated, turn the loop into a compass needle. Loop is a magnet (needle) centered in the loop with N facing away from you. S Which way does the field point at the magnet? The loop (magnet) will rotate so it lines up with the field. (S up and out of the page)

Magnetic Forces, Wires What happens to loop? Very complicated, turn the loop into a compass needle. Loop is a magnet (needle) centered in the loop with N facing away from you. S Which way does the field point at the magnet due to the wire? N The loop (magnet) will rotate so it lines up with the field. (S up and out of the page)