Correct answer 3 When a current ows in a wire of length L and cross sectional area A the resistance of the wire is proportional to length and inversely proportional to area Week 6 Day 1 Concept Question Resistance Current Flow of Charge Direction of the current Ohm s Law V V B V A IR Why does Current Flow Drift Velocity Current and Current Density Conductivity and Resistivity conductivity resistivity Microscopic Ohm s Law Why does current ow Ohm s Law V IR and R l A Concept Question Resistance Average current is equal to the charge Q owing across area A at time t Instantaneous current is the differential limit of the average current Direction of current is in the direction of the ow of positive charge You can also think of it as the opposite of ow of negative charge What is the relationship between electric potential difference and current If an electric eld is set up in a conductor charge will move making a current in the direction of E Drift velocity is the average velocity forced by applied electric eld in the present of collisions The current density is current per unit area Current is the ow ux of current density through an open surface Special case J uniform and perpendicular to the surface Concept Question Current Density Correct answer A current I 200 mA ows in the wire with width of 10cm length of 20cm and height of 5cm The magnitude of the current density J is Ability of current to ow depends on density of charges and rate of scattering E J and J E so we know that 1 Instead of thinking of electric eld think of potential difference across the conductor V V B V A E s El RI Correct answer 1 Two resistors are made out of the same material but have different dimensions as shown in the gures The current through these two resistors is in the directions shown If the resistor on the left has a resistance of 1 00 ohm the resistor on the right will have a resistance of 1 50 This is because the areas are the same and the block on the right has a length that is 3 2 the one on the right Worked Example Calculating Resistance Consider a hollow cylinder of length L and inner radius a and outer radius b The material has resistivity Suppose a potential difference is applied between the ends of the cylinder and produces a current owing parallel to the axis What is the resistance measured Moving Charges Feel Magnetic Force Magnetic force perpendicular both to velocity of charge and magnetic eld Concept Question Force Direction Correct answer
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