Edexcel A Level Physics:復(fù)習(xí)筆記3.3 Ohm's Law
Defining Resistance
Resistance
- Resistance is defined as the?opposition?to current
- It is further defined by Ohm's Law, which says that the resistance of a conductor is given by the ratio of potential difference across it to the current flowing in it
- For a given potential difference, then,?the higher the resistance the lower the current
Resistance of a component is the ratio of the potential difference and current
- Resistance is measured in?Ohms (Ω)
- An Ohm is defined as?one volt per ampere
- The resistance controls the size of the current in a circuit
- A?higher?resistance means a?smaller?current
- A?lower?resistance means a?larger?current
- All electrical components, including wires, have some value of resistance
Measuring Resistance
- To find the resistance of a component, a simple circuit can be used, containing:
- A power supply
- A component (such as a lamp or resistor)
- An ammeter in series with the component
- A voltmeter in parallel with the compenent
A circuit to determine the resistance of a component
- The power supply should be set to a low voltage to avoid heating the component, typically 1-2 V
- Measurements of the potential difference and current should then be taken from the voltmeter and ammeter respectively
- Finally, these readings should be substituted into the resistance equation
Worked Example
A charge of 5.0 C passes through a resistor of resistance R Ω at a constant rate in 30 s.
If the potential difference across the resistor is 2.0 V, calculate the value of R.
Ohm's Law
- Ohm’s law is defined as:
The current through a component is directly proportional to the potential difference across it, providing the temperature is constant
- Constant temperature implies constant resistance
- This is shown the equation below:
Ohm’s law
- By adjusting the resistance on the variable resistor, the current and potential difference will vary in the circuit.
- The variation of current with potential difference through the fixed resistor can be plotted on a pair of axes
- This will produce a straight-line graph
Circuit for plotting graphs of current against voltage
- Since the gradient is constant, the resistance?R?of the resistor can be calculated by using?1 ÷ gradient?of the graph
- An electrical component obeys Ohm’s law if its graph of current against potential difference is a?straight line?through the origin
- A resistor?does?obey Ohm’s law
- A filament lamp does?not?obey Ohm’s law
- Any metal wires will follow Ohm's Law, provided that the current isn’t large enough to increase their temperature
Worked Example
The current flowing through a component varies with the potential difference?V?across it as shown
Which graph best represents how the resistance?R?varies with?V?
Exam Tip
Resistance is used to control current. Increasing the resistance in a circuit will reduce the current. Don't get the cause and effect mixed up here. Reducing current does not increase resistance - it's the other way round!
Using graphs;
- In maths, the gradient is the?slope?of the graph
- The graphs below show a summary of how the slope of the graph represents the gradient
Graphs showing varying gradients
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