Unit 9 Lesson 4 Magnetic Flux Flux magnetic

Unit 9, Lesson 4: Magnetic Flux

Flux = magnetic field X perpendicular area Φ=BAsinƟ where Ɵ is the angle between the plane and the magnetic field Unit: weber = tesla x square meter 1 Wb = 1 Tm 2

Try This! Find the flux through a circular loop of wire of radius 25 cm in a vertical magnetic field of 5 x 10 -5 T if: a) the loop is laying flat on the ground, b) the loop makes a right angle with the ground, c) the plane of the loop makes a 30 O angle with the ground.

Brain Break!

Faraday’s Law and Lenz’s Law • A changing magnetic field induces a current and voltage in a coil of wire. • The magnitude of the voltage (EMF) = the number of turns X the rate of change of flux (Faraday) • The direction of the induced current always counteracts the change that produced it (Lenz) • ε = −N(∆Φ/∆t) where Φ=BA • Note: the negative sign symbolizes Lenz's law but should not be included in calculations.

Lenz`s Law • Determines the direction of induced current. • The induced current will create its own magnetic field (an induced magnetic field). • The induced magnetic field will act to cancel out the change in the original flux and will oppose any movement that created a change in flux.

Examples – Lenz`s Law • Find the direction of the current in the resistor in each case: Answer: Left to right Answer: Right to left

No Homework! • We will practice Lenz’s and Faraday’s Laws more next class.

Unit 9, Lesson 5: Understanding Faraday and Lenz

Warm Up – Lenz`s Law Find the direction of the current in the resistor: Answer: Left to right

Practice – Lenz`s Law Find the direction of the current in the resistor: a) immediately after the switch is turned on b) a while after the switch is turned on c) immediately after the switch is turned off a) Right to left b) 0 c) Left to right

Practice - Faraday`s Law A circular coil of wire with 100 turns and a cross-sectional area of 25 cm 2 is placed in a perpendicular magnetic field of 2. 5 T as shown. a) The field is increased to 5. 0 T in 0. 20 seconds. Find the induced EMF and the direction of the induced current. b) The loop is then moved out of the area of constant magnetic field (5. 0 T) in 0. 3 seconds. Find the induced EMF and the direction of the induced current. a) 3. 1 V counterclockwise b) 4. 2 V clockwise

Brain Break!

Motional EMF • A conductive bar cutting across a perpendicular magnetic field experiences an induced EMF that opposes its motion! B=magnetic field strength (T) l = length of bar (m) v = velocity (m s) ε = Blv

Motional EMF – Example • A passenger jet with a wing span of 50 m approaches the north pole at 1000 km h. Find the voltage induced across the wings and state which wing is positive. The Earth`s magnetic field is 5 x 10 -5 T.

Homework: • Pg. 243 #1 -4 • Pg. 251 #1 -3