Basic ElectricityBasic Electricity

All the Electricity You Need to All the Electricity You Need to Know for PhysicsKnow for Physics

Measured QuantitiesMeasured Quantities

• CurrentCurrent is measured with an ammeter.

• It is measured in amps.• It is a flow of charge.• 1 A = 1 coulomb per

second.• 1 C = 6.2 × 1018

electrons.

• Potential differencePotential difference is measured with a voltmeter.

• It is measured in volts.• It is the energy

contained per unit charge.

• 1 V = 1 J/C.

Calculated QuantitiesCalculated Quantities

• PowerPower is the rate of changing electrical energy into other forms.

• Power is measured in watts (W).

• Power = voltage × current (P = VI).

• ResistanceResistance is the opposition of a conductor to the flow of electricity.

• Resistance is measured in Ohms )

• Resistance = voltage ÷ current (R = V/I).

Heating Effect of a CurrentHeating Effect of a Current

• If P = VI and V = IR, we can see that P = I2R.• This means that if the current doubles, the power

through a component goes up four times.• Therefore heavy currents need thick wires, otherwise

the wires get hot.• It is important to check the power rating of a

resistor; a ¼ watt resistor will burn out if it has to dissipate 5 watts.

The heating effect can be dramatic!

1 2 3

This was a 33 resistor connected to a 20 V supply. The current would be 20 V ÷ 33 = 0.61 A

The power would be 0.61 × 20 V = 12 watts. Plenty enough to fry a 1 watt resistor.

It is important that we ensure that any current limiting resistors can dissipate the power through them. The above situation could be highly dangerous.

Short Circuits can be DramaticShort Circuits can be Dramatic

You can see the effects of the intense heating effect on a circuit breaker, and the bogie of an electric locomotive.

Pick-up shoe for third rail

Short Circuit in a TransformerShort Circuit in a Transformer

Wire wound resistors can dissipate more heat

Voltage – Current Voltage – Current CharacteristicsCharacteristics

• These are graphs plotted from measurements of voltage and current.

• We usually put current on the horizontal axis and voltage on the vertical axis.

• This allows us to work out the resistance by working out the gradient.

• For the resistor and the lamp, the graph is symmetrical about the origin. If the voltage is positive, the current is positive.

VA

Basic CircuitBasic Circuit

We then put a lamp in instead of a resistor.

Ex 1335 Voltage vs Current

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4.00

6.00

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12.00

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00

Current (mA)

Vo

lta

ge

(V

)

Current (mA)

Characteristic Graph for a ResistorCharacteristic Graph for a Resistor

It is a straight line as Ohm’s Law is obeyed

Characteristic Graph for a LampCharacteristic Graph for a Lamp

Current (A)

Voltage (V)

It is a curve as the resistance increase as the filament gets hotter

Characteristic for a DiodeCharacteristic for a Diode

V

mA

5 k6 V

150

This circuit is more complex, allowing us to control the voltage more effectively.

Voltage Current Graph for DiodeVoltage Current Graph for DiodeVoltage vs Current for a Diode

-5.0

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30.0

0.000 0.100 0.200 0.300 0.400 0.500 0.600 0.700 0.800

Voltage (V)

Cu

rre

nt

(mA

)