Electric Circuits

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• Consists of converting electrical potential energy into other useful types of energy

• 3 parts to a circuit (minimum)

  1. Source of potential energy
     1. Battery
     2. Electrical cells
     3. Power points
  2. Wires
  3. Device/Component/Resistor

• Current: conventional current, i.e. flow of positive charge

• Electron current refers to flow of electrons

Circuit Diagrams

• Ammeter: measures current (I), unit ampere (A)
• Voltmeter: measures potential difference (pd)
  • Voltage (V)
  • e.m.f., electromotive force
  • Measured in volts (V)
• Galvanometer: a very sensitive ammeter

Why do electrons move in a circuit?

• Electrical potential energy: results from a separation of charge, i.e. potential difference i.e. voltage i.e. e.m.f.
• $V=\frac{W}{q}$
• V = voltage/pd/emf measured in volts (V)
• W = ($\Delta E$) = work done on charges (J)
• q = charge (C)

Current

• Current: rate of flow of charge, a.k.a. flow of electrons $I=\frac{q}{\Delta t}$
• I = current in amperes (A)
• q = charge (C)
• t = time elapsed (s)

Series Circuits

• When components in a circuit are connected in the same path, they are said t be in series
• When components are connected in different branches of a circuit, they are said to be in parallel
• Ammeters must be connected in series
• Voltmeters must be connected in parallel

Things we should have noticed in practical

Series

• For a single component in series with emf, current was the same in every part of the circuit
• If more components are added in series, the current is the same in every part of the circuit, but less than before
  • $I_{total}=I_1=I_2=I_3...$
• The voltage for a single component, V ~ emf
• If more components are added, their individual Voltage < emf (decreases), but sum of voltage = total voltage
  • $V_{total}=V_1+V_2+V_3...$

Parallel

• For components in parallel, the current is divided between them
  • $I_{total}=I_1+I_2+I_3...$
• Voltage is the same in each branch
  • $V_{total}=V_1=V_2=V_3...$

Ohm’s Law

• Resistance: measure of the difficulty with which charge flows in a medium
• $V=IR$
  • V: voltage (in volts)
  • I: current (in amps)
  • R: resistance (in ohms)

Resistance in a series component:

• $V=I{R}_{total}=I{R}_1+I{R}_2+I{R}_3...$
• $R_{total}=R_1+R_2+R_3...$
• $V_k=I{R}_k$

Resistance in a parallel component

• $V_{total}=V_1=V_2=V_3...$
• $I=\frac{V}{R_{total}}=\frac{V}{R_1}+\frac{V}{R_2}+\frac{V}{R_3}...$
• $\frac{1}{R_{total}}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}...$