Physics 4 – Feb 5, 2019 P3 Challenge – Determine the amount of a) charge and b) energy stored across a 4.0 F capacitor when connected to a 12 V battery. Get out 11.3 p470 #22-33 for HMK check

Objectives/Agenda/Assignment Ch 5.4 Magnetism Assignment: p241 #37-46 Agenda: Homework Review for 11.3 Sources of magnetism Magnetic field due to a wire Magnetic flux density Right hand rule for force Magnetic force on current carrying wire Force between two current carrying wires

Magnetism Magnetism is another fundamental property of matter. Demonstrated by electrons, protons and neutrons. Has a polarity – a north and south pole, gives a direction Exists as a field – a directional vector field (like an electric field) Sources include Particles Bar magnets (all particles aligned in agreement) Current carrying wires and solenoids (coiled cylinder of wire)

Magnetic fields around sources

Direction of magnetic field Fixed magnet: From south to north (like on a map, up in north) Due to current in wire: Right hand grip rule – Thumb points in direction of current, fingers curl around wire in direction of circular magnetic field

Direction of magnetic field From a Solenoid: Right hand grip rule – Fingers curl in direction of wire current around the solenoid, thumb points in direction of the magnetic field Can be determined in lab using a simple compass.

Magnetic Flux Density Magnetic flux density – measures the strength of a magnetic field. Symbol: B Unit: Tesla, T A 1 T magnetic flux density produces a 1 N force on a 1 C charge moving at 1 m/s at right angles to the field.

Magnetic Force F = qvBsin Magnetic fields produce a force on moving charges. F = qvBsin Where q is the charge, v is the speed of the charge, B is the magnetic flux density, and  is the angle between the direction of the charge and the direction of the magnetic field.

Direction of the magnetic force The Right Hand Rule: 3 equivalent versions: Flat hand Fingers = mag. field Thumb = charge velocity Palm = force

Direction of the magnetic force The Right Hand Rule: 3 equivalent versions: Perpendicular 3 fingers Thumb = charge velocity First = mag. field Middle = force

Direction of the magnetic force The Right Hand Rule: 3 equivalent versions: Finger curl motion Fingers point = charge velocity, Palm/Curl fingers = mag. field Thumb = force

Magnetic force on a current wire F = BILsin Where B is the magnetic flux density, I is the current, L is the length of wire in the field, and  is the angle between the current and the magnetic field.

Magnetic force on a current wire Direction of force given by the same right hand rule as for moving charges, a current is a moving charge. Force directions are defined for a positive moving charge or current Recall that electrons move in the opposite direction to a current and will experience the magnetic force in the opposite direction.

Force between parallel wires Two sources can combine their magnetic fields in regular vector field addition, just like we saw with electric fields.

Force between parallel wires Two wires can be parallel if the currents flow in the same direction or anit- parallel if the currents flow in opposite directions. Parallel currents create an attractive force Antiparallel create a repulsive force

Definition of the Ampere Remember we use the ampere as the fundamental SI base units for all things electrical. Why? Because it can be standardized through the magnetic force between current carrying wires: If the force on a 1 m length of two wires that are 1 m apart and carrying equal currents is 2 x 10-7 N, then the current in each wire is defined to be 1 Ampere.

Exit slip and homework Exit Slip – A fixed bar magnet is aligned so that it’s north pole is due north. What is the direction of the force of a charge that is traveling due east past this magnet? What’s due? (homework for a homework check next class) p241 #38-44 What’s next? (What to read to prepare for the next class) Read 11.1 Electromagnetic induction, p434 - 441