Current and Resistance
An introduction to Ohm’s Law
Recall: Potential Difference
Potential Difference
• Provided by the battery
– AKA voltage measured in volts
• It is a “push” for the electrons in the wire that
causes them to flow.
• The battery DOES NOT supply the electrons,
they are already in the wire.
Current:
The rate at which electrons flow through a conductor
• 𝐼=
𝑞
𝑡
• Current: I measured in C/s (amp)
• The battery supplies the potential difference
needed for the electrons to flow. THE
BATTERY DOES NOT SUPPLY THE ELECTRONS
THEMSELVES!
• Current only flows if there is a complete
circuit!
Conservation of charge
Total current into a junction must equal
the total current out.
3 Amps
Magnitude and direction
of current flow?
6 Amps
4 Amps
In
out
6 amps
4 amps
3 amps
Total in: 9 amps
Total out: 4 amps
The unknown current
is 5 amps out of the
junction (to the right)
Simple Circuit
• Every Circuit must have
– A source of potential difference (battery)
– A conducting material (wires)
– An electrical load (resistor/lightbulb)
Circuit Symbols
• Draw a circuit with a battery, a lamp and a switch
using the symbols above
Resistance
• A resistor is any electrical device that you plug in.
• It impedes the flow of electrons
• The resistance of an object is DIRECTLY related to
temperature.
– The hotter the wires, the more resistance they have.
• The resistance of a wire depends on
– Its length
– The material its made of (resistivity)
– The cross-sectional area of the wire
Calculating Resistance
Resistance
is measured
in Ohms (Ω)
Circuits Measurements
Ammeter
• Measures current (in Amps)
• Wired IN SERIES with the
resistor
• Has a very LOW resistance
Voltmeter
• Measures potential
difference (in Volts)
• Wired IN PARALLEL with the
resistor
• Has a very HIGH resistance
Types of Circuits
Series
Parallel
One complete loop
Multiple paths
Ohm’s Law
𝑽 = 𝑰𝑹
Slope = Resistance
V
Slope = 1/Resistance
I
I
V
R
I
Resistance and
current are
INVERSES of each
other. As one
increases the other
decreases!
Electrical Power
When Potential
Difference is
constant
When resistance is constant
- “ohmic”
- “Follows ohm’s law”
- “constant temperature”
P
P
I
I
Electrical Power
When current is
constant
When resistance is constant
- “ohmic”
- “Follows ohm’s law”
- “constant temperature”
P
P
V
V
Electrical Power
When Voltage is
constant
When current is
constant
P
P
R
R
Electrical Energy (AKA work)
Measured in Joules