Download Effects of Electric Current * Learning Outcomes

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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.
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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
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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)
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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
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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
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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.
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Extra High Tension
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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.
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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
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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)