Physics 2B: Review for Celebration #2. Chapter 22: Current and Resistance

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1 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

2 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

3 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 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.

4 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

5 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 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 J s 1 ev J