Basic Electricity
All the Electricity You Need to
Know for Physics
Measured Quantities
• Current 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 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 Quantities
• Power is the rate of
changing electrical
energy into other
forms.
• Power is measured in
watts (W).
• Power = voltage ×
current (P = VI).
• Resistance is the
opposition of a
conductor to the flow
of electricity.
• Resistance is
measured in Ohms
(W).
• Resistance = voltage ÷
current (R = V/I).
Heating 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 W resistor connected to a 20 V supply. The current
would be 20 V ÷ 33 W = 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 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 Transformer
Wire wound resistors can dissipate more heat
Voltage – Current
Characteristics
• 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.
Basic Circuit
A
V
We then put a lamp in instead of a resistor.
Characteristic Graph for a Resistor
Ex 1335 Voltage vs Current
It is a
straight
line as
Ohm’s
Law is
obeyed
12.00
10.00
Voltage (V)
8.00
6.00
Current (mA)
4.00
2.00
0.00
0.00
5.00
10.00
15.00
20.00
25.00
Current (mA)
30.00
35.00
40.00
45.00
Characteristic Graph for a Lamp
Voltage
(V)
Current (A)
It is a curve as the resistance increase as the filament gets hotter
Characteristic for a Diode
150 W
6V
5k
V
m
A
This circuit is more complex, allowing us to control the
voltage more effectively.
Voltage Current Graph for Diode
Voltage vs Current for a Diode
30.0
25.0
Current (mA)
20.0
15.0
10.0
5.0
0.0
0.000
0.100
0.200
0.300
0.400
-5.0
Voltage (V)
0.500
0.600
0.700
0.800