1. Circuits are drawn using standard circuit symbols.
2 Electricity Topic overview
Circuits
Circuits are drawn using standard circuit symbols.
A cell is shown as with the long end positive.
A battery is a combination of cells, connected positive to negative.
Circuits can be connected in series or parallel or a combination of both.
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2. Electrical charge and current
2 Electricity Topic overview
Electrical charge and current
Charge, Q, is measured in Coulombs (C).
Potential difference or p.d., V, is the electrical work done per coulomb in a circuit component.
Potential difference is measured in Volts (V).
Current, I, is the flow of charge, Q, per second in a circuit.
Current, I, is measured in Amperes or Amps (A)
𝑄=𝐼×𝑡 or 𝐼= 𝑄 𝑡 where t is the time in seconds.
Current is the same at all points in a series circuit.
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3. Current and potential difference
2 Electricity Topic overview
Current and potential difference
Current in a circuit can be changed by changing the p.d. or the resistance.
Current is measured by adding an ammeter in series in a circuit.
P.d. is measured by adding a voltmeter in parallel with cells or other components.
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4. Resistance, R, is measured in Ohms (Ω)
2 Electricity Topic overview
Resistance
Resistance, R, is measured in Ohms (Ω)
Potential difference = current x resistance
𝑉=𝐼×𝑅
Resistance is sometimes measured in kilohms (kΩ) or megohms (MΩ)
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5. Resistance 2 Electricity Topic overview
In ohmic conductors, the resistance is always the same.
For some conductors, e.g. a filament lamp, the resistance increases with temperature.
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6. Resistance 2 Electricity Topic overview
In a diode, the resistance depends on size and direction of the p.d.
For other resistors, e.g. a LDR or most thermistors, the resistance changes with an external property.
LDR resistance decreases with increasing light intensity
Thermistor resistance decreases with increasing temperature
LDRs and thermistors are used in switching circuits, e.g. automatic lights and thermostats.
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7. Series Circuits 2 Electricity Topic overview
In a series circuit the p.d. is shared between components in the same proportions as their resistances.
Total p.d., 𝑉= 𝑉 1 + 𝑉 2
Current is the same at all places, 𝐴 1 = 𝐴 2
Total resistance, 𝑅= 𝑅 1 + 𝑅 2
Current in a series circuit reduces since total resistance is greater
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8. Parallel Circuits 2 Electricity Topic overview
In a parallel circuit the p.d. across every component in parallel is the same.
The total current is equal to the sum of the currents through the separate components.
The current is shared between the
parallel components in inverse proportion to their resistance.
𝐴 1 = 𝐴 2 + 𝐴 3
The total resistance of resistors in parallel is always lower than that of the lowest resistor.
Current in a parallel circuit increases since the total resistance is smaller.
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9. Direct and alternating potential difference
2 Electricity Topic overview
Direct and alternating potential difference
Cells have a steady p.d. and cause direct current (d.c.) in a circuit. Direct current is always in
the same direction.
An alternating current (a.c.) is
caused by an alternating p.d. which
changes direction many times per
second.
In the UK mains electricity has a p.d. of about 230V and changes direction at 50 Hz.
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10. Domestic uses and safety Mains cables have three wires
2 Electricity Topic overview
Domestic uses and safety
Mains cables have three wires
Brown – live
Blue – neutral
Green/Yellow – Earth
The live wire changes potential compared to the neutral
The neutral is at or near Earth potential
The Earth wire is for safety and only carries a current in the case of a fault.
This large current will cause a fuse to melt or a circuit-breaker to trip so preventing fire or other danger.
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11. Power 2 Electricity Topic overview
Electrical power, P, is measured in Watts (W) or kilowatts (kW).
Electrical power = p.d. x current
𝑃=𝑉×𝐼 and since 𝑉=𝐼×𝑅 then 𝑃= 𝐼 2 ×𝑅
Energy, E, is transferred in an appliance or component
𝐸=𝑃×𝑡 where t is the time in seconds.
and 𝐸=𝑄×𝑉 where Q is the charge that has flowed.
Mains appliances give their power in kW
A kettle of power 2.2 kW = 2200 W therefore 𝐼= 𝑃 𝑉 = =9.6 A
The National grid transfers electrical power from where it is generated to the consumer.
Transformers are used to make this transfer of electrical power more efficient by reducing power loss in the transmission lines.
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12. Static electricity 2 Electricity Topic overview
This is stationary charge on the surface of an object.
Often achieved by rubbing an object which adds or removes negative charge (electrons).
unlike charges attract
like charges repel
Charge can jump from one charged object to another oppositely charged object causing a spark, e.g. lightning.
A charged object has an electric field around it.
A charge experiences a force in an electric field.
The direction of the field is the direction of the force on a positive charge.
A charged sphere has a radial electric field around it.
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