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Lecture

# Current and resistance, sources of potential, energy and power

4 Pages
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Department
Physics
Course Code
PH 122
Professor
Brian Jones

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Description
10 February Current & Resistance Current is the flow of charge. Such a flow is produced by a potential difference. The current depends on the potential difference and the resistance. The resistance is a property of the material and the dimensions. A current dissipates energy in a resistor. This is the basis of many common devices. Circuit rules let us simplify complex circuits. We’ll look at series and parallel combinations. Electric Fish The electric eel and the torpedo ray both use bioelectricity to stun and capture prey. The eel can produce a larger voltage, but the ray produces a larger current. Black electrode at higher potential – negative reading Red electrode at higher potential – positive reading Sources of Potential Work is done to raise charges in a battery to a higher potential. In the cell (another source of potential) this work is done by ion pumps. With emf, energy is replenished Definition of current ΔQ I = Δt 1 C/s = 1 amp = 1 A The discharge of an electric eel can transfer a charge of 2.0 mC in a time of 2.0 ms. What current, in A, does this correspond to? ΔQ I = Δt −3 2.0x10 C I = 2.0x10 sec I = 1.0 A How much is too much? Physiological effect AC current (rms) (mA) DC current (mA) Threshold of sensation 1 3 Paralysis of respiratory 15 60 muscles Heart fibrillation, likely fat>100 >500 What determines the current? Adding a second battery increases the current. Changing wire dimensions – short thick wire → large current; long, thin wire → small current Changing wire material – a copper wire will carry a larger current than an iron wire of the same dimensions. Our basic circuit ΔV I = R Ohm’s law for a conductor of resistance R Human Circuits Hand-to-hand resistance of a human with dry skin is quite variable, but we can assume a typical value of about 50 kΩ. If the “energy ball” produces a potential difference of about 1.5 V, what is the current through your body? Is this enough to feel? ΔV I = R 1.5V I = 50x10 Ω I = 30 μA Determining r
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