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1 Effects of Electric Current – Learning Outcomes Explain the heating, chemical, and magnetic effects. State Joule’s Law. Demonstrate each effect. Solve problems about the heating effect. Recognise examples of the effects in everyday life. Discuss EHT (Extra High Tension). Give uses of the chemical effect. 2 Heating Effect Joule’s Law states that the rate at which heat is produced in a conductor is directly proportional to the square of the current, provided its resistance is constant. Formula: 𝑃 ∝ 𝐼 2 (P = Power, I = current) As usual, we can convert this proportionality to an equation: 𝑃 = 𝑘𝐼 2 In this case, k is the resistance of the conductor, so: 𝑃 = 𝐼2 𝑅 3 Heating Effect We can get two more equations from this formula: If 𝑃 = 𝐼 2 𝑅, Using 𝑃 = 𝑊 𝑡 𝑊 , 𝑡 (W = work, t = time) = 𝐼2 𝑅 ⇒ 𝑊 = 𝐼 2 𝑅𝑡 If 𝑃 = 𝐼 2 𝑅, Using 𝑉 = 𝐼𝑅, 𝑃 = 𝑉𝐼 (V = voltage) 4 Heating Effect e.g. Find the heat produced in one second in a 5Ω resistor by a current of: i. 1A ii. 2A iii. 15A iv. 100A 5 Heating Effect e.g. A 3.5 kW heater operates on the mains at 230 V. Find: i. the current flowing in the heater ii. the resistance of the heater iii. the time taken to produce 100 MJ of heat 6 Extra High Tension 𝑃 = 𝐼 2 𝑅 tells us that the heat lost in the wire scales with current squared. Given 𝑃 = 𝑉𝐼 for the power transferred in the wire, companies can decrease the current if they increase the voltage while supplying the same power. increase the voltage to decrease the current while supplying the same power. Thus, electricity is transformed to 220 kV or more when sending over large distances, then transformed back to ~230 V when it is close to where it will be used. 7 Extra High Tension 8 Chemical Effect Electricity may cause a chemical reaction as it passes through a liquid. This is called electrolysis. The liquid is called the electrolyte. The connection to the positive terminal is the anode. The connection to the negative terminal is the cathode. Charge is carried by positive and negative ions. 9 Chemical Effect – Example Anode: copper loses 2 electrons and dissolves into the electrolyte. 𝐶𝑢 ⇒ 𝐶𝑢2+ + 2𝑒 − Cathode: copper in the electrolyte gains 2 electrons and adheres to the cathode. 𝐶𝑢2+ + 2𝑒 − ⇒ 𝐶𝑢 10 Magnetic Effect A current in a wire causes a magnetic field around it which can deflect a nearby compass needle (waaaaaay more on this later)