Physics 2: eview for Celebration #2 Chapter 22: Current and esistance Current: q Current: I [I] amps (A) 1 A 1 C/s t Current flows because a potential difference across a conductor creates an electric field which exerts a force on free electrons in the circuit. Conventional current: the hypothetical flow of positive charge (it is really electrons flowing in opposite direction) Law of conservation of current: The current is the same at all points ion a current-carrying wire. Kirchhoff s junction law: Iin Iout atteries and EMF: Electromotive force or emf: the maximum difference in electrical potential between the terminals of a battery A battery creates a potential difference by doing work on charges (charge escalator model). The energy to move charges comes from chemical reactions. W q chem Vbat ε esistance and esistivity esistance is a measure of how hard it is to push charges through a wire or through an electronic device such as a resistor. V [] ohm (Ω) 1 Ω 1 V/A I esistivity is a property of materials such as copper or gold that measures how hard it is for charges to flow through the material. [ ρ ] Ω m The resistance of a wire is given by: ρl A Ohm s Law: V I or V I
Power: Power: P Iε rate at which battery supplies energy to the circuit 2 2 ( V ) P I V I rate at which a resistor dissipates energy
Chapter 23: Circuits Kirchhoff s Laws: Kirchhoff s junction law: Iin Iout Sign conventions for Kirchhoff s loop law: V V bat bat Kirchhoff s loop law: Vloop Vi 0 + ε going through a battery from the to the + terminal ε going through a battery from the + to the - terminal V I going through a resistor in the same direction as the current V + I going through a resistor in the opposite direction as the current i esistors in series: Current is the same through each resistor Voltage is split among individual resistors esistors in parallel: Voltage is the same across each resistor Current is split among the individual resistors + + + 1 2 3... 1 1 1 1 + + +... 1 2 3 1 1 1 + + +... 1 2 3 1 esistors in series have the same current as their uivalent resistance. esistors in parallel have the same voltage as their uivalent resistance. Voltmeters and Ammeters: Ammeters have a very low resistance and must be connected in series with the circuit element whose current is to be measured. Voltmeters have a very high resistance and must be connected in parallel with the circuit element whose voltage is to be measured.
Chapter 24: Magnetic Fields and Forces Magnetic Fields: all magnets have both a north pole and a south pole like poles repel, opposite poles attract a magnetic field surrounds every magnet or moving electric charge (magnetic field points from north to south pole) Magnetic Force on a Charge Moving in a Magnetic Field: magnitude of the force is given by F q vsinα direction of the force is given by the H for magnetic force: point the fingers of right hand along and thumb along v ; your palm points in the direction of F on a positive charge a charge moving in a magnetic field will travel in a circular path of radius: Magnetic Fields due to Currents: r mv q long wire center of loop insidesolenoid 0I 2π r 0I 2 N 0I L 7 0 4π 10 Tm A the direction of the magnetic field is given by the H for magnetic fields: point the thumb of your right hand in the direction of conventional current; your fingers curl in the direction of the magnetic field Current Carrying Wire: the force on a current carrying wire in a magnetic field is given by: F ILsinα wire parallel currents attract, anti-parallel currents repel
Magnetic flux: Chapter 25: Electromagnetic Induction and EM Waves Φ Acosθ θ is the angle between and A [Φ] Webber (Wb) 1 Wb 1 Tm 2 The magnetic flux changes if: the magnetic field changes the area of the coil within changes the angle between and A changes the current induced in a coil arises from an induced electromotive force or emf Lenz s law: Lenz s law: an induced current has a direction such that the induced magnetic field from the current opposes the change in the magnetic flux that induced the current * if Φ is decreasing, the induced current produces a field in the same direction as the original field (the induced magnetic field reinforces the original field) * if Φ is increasing, the induced current produces a field in the opposite direction as the original field (the induced magnetic field opposes the original field) Joe s H for current loops: curl the fingers of your right hand in the direction of conventional current; your thumb points in the direction of (within the loop) Faraday s law: ε N Φ t the induced current is given by: i Electromagnetic Waves: ε All EM waves travel through a vacuum at the same speed: c 3.00 10 8 m/s the electromagnetic spectrum consists of EM waves of all fruencies for all EM waves in vacuum: λ f c EM waves consist of discrete, massless units called photons. The energy of a photon is given by: 34 Ephoton hf h 6.63 10 J s 1 ev 1.60 10-19 J