Electron Flow

• The lamp in a circuit lights up because there are electrons moving from one terminal of the battery to the other terminal

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• There must be a conducting path between the lamp and the battery

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• Although the free electrons flow from the negative terminal to the positive terminal of the battery

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• We use the conventional current direction (from positive to negative terminal of the battery).

Electron Flow

Electric current

• Electric current is the rate of flow of charge. In wires ,these changes are electrons.

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• The charge is measured in Coulombs(C) therefore current is measured in C/S or Amperes(A).

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Current(I)=Charge(Q)/Time(t)

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Potential Difference

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• It is the energy transferred per unit and it is measured in Volt(V) where 1 Volt is 1 J/C (joule per coulomb)

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Voltage(V)=energy transferred(J)/Charge(Q)

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Resistance

• The resistance of a component is what causes a voltage drop (potential difference ) across the component.
• If there is a P.D across a conductor , a current flows through it.
• Resistance is measured in Ohms .
• The resistance of a conductor is the ratio of the P>D applied across it , to current passing through it .

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Resistance(R)=Volts(V)/Current(I)

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Resistance of a Wire

1. The Resistance of a wire :
2. Increases with the length(L)
3. Increase as the cross-section area(A) decreases
4. Depends on the Resistivity(p) of the material (which varies from one to another )
5. Depends on the temperature such that :

1. Metallic conductors show increase in resistance as

temperature increases.

2.Non-Metalic is vice-versa

How Resistance s measured

• To find the resistance of a wire you must place it in a circuit .
• The voltmeter tells you the P.D across the wire .
• The Ammeter tell you he current across it .
• Calculate the resistance using R=V/I .

Series and Parallel circuits

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• Series Circuits:
• The same current flows through all parts of the circuit.
• The size of the current is determined by the P.D of the cells and the total resistance of the circuit(I=V/R)
• The bigger the resistance of a component , the bigger its share of the total P.D
• The total P.D of the supply is shared between the various components , so the voltages round a series circuit always add up to equal the total voltage of supply.

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Series and Parallel circuits

• Parallel Circuits:
• The voltage is the same across all components.
• The current through each component depends on the resistance
• The total current flowing around the circuit is equal to the total of all currents in the separate branches.

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